diff options
Diffstat (limited to 'erts/doc/src')
| -rw-r--r-- | erts/doc/src/Makefile | 2 | ||||
| -rw-r--r-- | erts/doc/src/erl.xml | 47 | ||||
| -rw-r--r-- | erts/doc/src/erlang.xml | 775 | ||||
| -rw-r--r-- | erts/doc/src/time_correction.xml | 923 |
4 files changed, 1447 insertions, 300 deletions
diff --git a/erts/doc/src/Makefile b/erts/doc/src/Makefile index e8b856c3ff..a83aa9b875 100644 --- a/erts/doc/src/Makefile +++ b/erts/doc/src/Makefile @@ -177,6 +177,8 @@ release_docs_spec: docs $(INSTALL_DIR) "$(RELSYSDIR)/doc/html" $(INSTALL_DATA) $(HTMLDIR)/* \ "$(RELSYSDIR)/doc/html" + $(INSTALL_DATA) $(ERL_TOP)/erts/example/time_compat.erl \ + "$(RELSYSDIR)/doc/html" $(INSTALL_DATA) $(INFO_FILE) "$(RELSYSDIR)" $(INSTALL_DIR) "$(RELEASE_PATH)/man/man3" $(INSTALL_DATA) $(MAN3DIR)/* "$(RELEASE_PATH)/man/man3" diff --git a/erts/doc/src/erl.xml b/erts/doc/src/erl.xml index c25c9eeedb..ea94a4e82b 100644 --- a/erts/doc/src/erl.xml +++ b/erts/doc/src/erl.xml @@ -495,24 +495,35 @@ <c><![CDATA[werl]]></c>, not <c><![CDATA[erl]]></c> (<c><![CDATA[oldshell]]></c>). Note also that <c><![CDATA[Ctrl-Break]]></c> is used instead of <c><![CDATA[Ctrl-C]]></c> on Windows.</p> </item> - <tag><marker id="+c"><c><![CDATA[+c]]></c></marker></tag> - <item> - <p>Disable compensation for sudden changes of system time.</p> - <p>Normally, <c><![CDATA[erlang:now/0]]></c> will not immediately reflect - sudden changes in the system time, in order to keep timers - (including <c><![CDATA[receive-after]]></c>) working. Instead, the time - maintained by <c><![CDATA[erlang:now/0]]></c> is slowly adjusted towards - the new system time. (Slowly means in one percent adjustments; - if the time is off by one minute, the time will be adjusted - in 100 minutes.)</p> - <p>When the <c><![CDATA[+c]]></c> option is given, this slow adjustment - will not take place. Instead <c><![CDATA[erlang:now/0]]></c> will always - reflect the current system time. Note that timers are based - on <c><![CDATA[erlang:now/0]]></c>. If the system time jumps, timers - then time out at the wrong time.</p> - <p><em>NOTE</em>: You can check whether the adjustment is enabled or - disabled by calling - <seealso marker="erlang#system_info_tolerant_timeofday">erlang:system_info(tolerant_timeofday)</seealso>.</p> + <tag><marker id="+c"><c><![CDATA[+c true | false]]></c></marker></tag> + <item> + <p>Enable or disable + <seealso marker="time_correction#Time_Correction">time correction</seealso>:</p> + <taglist> + <tag><c>true</c></tag> + <item><p>Enable time correction. This is the default if + time correction is supported on the specific platform.</p></item> + + <tag><c>false</c></tag> + <item><p>Disable time correction.</p></item> + </taglist> + <p>For backwards compatibility, the boolean value can be omitted. + This is interpreted as <c>+c false</c>. + </p> + </item> + <tag><marker id="+C_"><c><![CDATA[+C no_time_warp | single_time_warp | multi_time_warp]]></c></marker></tag> + <item> + <p>Set + <seealso marker="time_correction#Time_Warp_Modes">time warp mode</seealso>: + </p> + <taglist> + <tag><c>no_time_warp</c></tag> + <item><p><seealso marker="time_correction#No_Time_Warp_Mode">No Time Warp Mode</seealso> (the default)</p></item> + <tag><c>single_time_warp</c></tag> + <item><p><seealso marker="time_correction#Single_Time_Warp_Mode">Single Time Warp Mode</seealso></p></item> + <tag><c>multi_time_warp</c></tag> + <item><p><seealso marker="time_correction#Multi_Time_Warp_Mode">Multi Time Warp Mode</seealso></p></item> + </taglist> </item> <tag><c><![CDATA[+d]]></c></tag> <item> diff --git a/erts/doc/src/erlang.xml b/erts/doc/src/erlang.xml index 6daa4b68a8..880c294c73 100644 --- a/erts/doc/src/erlang.xml +++ b/erts/doc/src/erlang.xml @@ -58,7 +58,71 @@ </datatype> <datatype> <name name="timestamp"></name> - <desc><p>See <seealso marker="#now/0">now/0</seealso>.</p> + <desc><p>See <seealso marker="#timestamp/0">erlang:timestamp/0</seealso>.</p> + </desc> + </datatype> + <marker id="type_time_unit"/> + <datatype> + <name name="time_unit"></name> + <desc><p>Currently supported time unit representations:</p> + <taglist> + <tag><c>PartsPerSecond :: integer() >= 1</c></tag> + <item><p>Time unit expressed in parts per second. That is, + the time unit equals <c>1/PartsPerSecond</c> second.</p></item> + + <tag><c>seconds</c></tag> + <item><p>Symbolic representation of the time unit + represented by the integer <c>1</c>.</p></item> + + <tag><c>milli_seconds</c></tag> + <item><p>Symbolic representation of the time unit + represented by the integer <c>1000</c>.</p></item> + + <tag><c>micro_seconds</c></tag> + <item><p>Symbolic representation of the time unit + represented by the integer <c>1000000</c>.</p></item> + + <tag><c>nano_seconds</c></tag> + <item><p>Symbolic representation of the time unit + represented by the integer <c>1000000000</c>.</p></item> + + <tag><c>native</c></tag> + <item><p>Symbolic representation of the native time unit + used by the Erlang runtime system.</p> + + <p>The <c>native</c> time unit is determined at + runtime system start, and will remain the same until + the runtime system terminates. If a runtime system + is stopped and then started again (even on the same + machine), the <c>native</c> time unit of the new + runtime system instance may differ from the + <c>native</c> time unit of the old runtime system + instance.</p> + + <p>One can get an approximation of the <c>native</c> + time unit by calling <c>erlang:convert_time_unit(1, + seconds, native)</c>. The result equals the number + of whole <c>native</c> time units per second. In case + the number of <c>native</c> time units per second does + not add up to a whole number, the result will be + rounded downwards.</p> + + <note> + <p>The value of the <c>native</c> time unit gives + you more or less no information at all about the + quality of time values. It sets an upper bound for + the resolution as well as for the precision, but it + gives absolutely no information at all about the + accuracy.</p> + </note> + </item> + + </taglist> + + <p>The <c>time_unit/0</c> type may be extended. Use + <seealso marker="#convert_time_unit/3"><c>erlang:convert_time_unit/3</c></seealso> + in order to convert time values between time units.</p> + </desc> </datatype> </datatypes> @@ -585,6 +649,22 @@ </desc> </func> <func> + <name name="convert_time_unit" arity="3"/> + <fsummary>Convert time unit of a time value</fsummary> + <desc> + <p>Converts the <c><anno>Time</anno></c> value of time unit + <c><anno>FromUnit</anno></c> to the corresponding + <c><anno>ConvertedTime</anno></c> value of time unit + <c><anno>ToUnit</anno></c>. The result is rounded + using the floor function.</p> + + <warning><p>You may lose accuracy and precision when converting + between time units. In order to minimize such loss, collect all + data at <c>native</c> time unit and do the conversion on the end + result.</p></warning> + </desc> + </func> + <func> <name name="crc32" arity="1"/> <fsummary>Compute crc32 (IEEE 802.3) checksum</fsummary> <desc> @@ -2205,14 +2285,15 @@ os_prompt% </pre> </func> <func> <name name="make_ref" arity="0"/> - <fsummary>Return an almost unique reference</fsummary> + <fsummary>Return a unique reference</fsummary> <desc> - <p>Returns an almost unique reference.</p> - <p>The returned reference will re-occur after approximately 2^82 - calls; therefore it is unique enough for practical purposes.</p> - <pre> -> <input>make_ref().</input> -#Ref<0.0.0.135></pre> + <p>Return a <seealso marker="doc/efficiency_guide:advanced#unique_references">unique + reference</seealso>. The reference is unique among + connected nodes.</p> + <warning><p>Known issue: When a node is restarted multiple + times with the same node name, references created + on a newer node can be mistaken for a reference + created on an older node with the same node name.</p></warning> </desc> </func> <func> @@ -2513,97 +2594,178 @@ os_prompt% </pre> </desc> </func> <func> - <name name="monitor" arity="2"/> + <name name="monitor" arity="2" clause_i="1"/> + <name name="monitor" arity="2" clause_i="2"/> + <type name="registered_name"/> + <type name="registered_process_identifier"/> + <type name="monitor_process_identifier"/> <fsummary>Start monitoring</fsummary> <desc> - <p>The calling process starts monitoring <c><anno>Item</anno></c> which is - an object of type <c><anno>Type</anno></c>.</p> - <p>Currently only processes can be monitored, i.e. the only - allowed <c><anno>Type</anno></c> is <c>process</c>, but other types may be - allowed in the future.</p> - <p><c><anno>Item</anno></c> can be:</p> - <taglist> - <tag><c>pid()</c></tag> - <item> - <p>The pid of the process to monitor.</p> - </item> - <tag><c>{RegName, Node}</c></tag> - <item> - <p>A tuple consisting of a registered name of a process and - a node name. The process residing on the node <c>Node</c> - with the registered name <c>RegName</c> will be monitored.</p> - </item> - <tag><c>RegName</c></tag> - <item> - <p>The process locally registered as <c>RegName</c> will be - monitored.</p> - </item> - </taglist> - <note> - <p>When a process is monitored by registered name, the process - that has the registered name at the time when - <c>monitor/2</c> is called will be monitored. + <p>Send a monitor request of type <c><anno>Type</anno></c> to the + entity identified by <c><anno>Item</anno></c>. The caller of + <c>monitor/2</c> will later be notified by a monitor message on the + following format if the monitored state is changed:</p> + <code type="none">{Tag, <anno>MonitorRef</anno>, <anno>Type</anno>, Object, Info}</code> + <note><p>The monitor request is an asynchronous signal. That is, it + takes time before the signal reach its destination.</p></note> + <p>Currently valid <c><anno>Type</anno></c>s:</p> + <taglist> + <tag><marker id="monitor_process"/><c>process</c></tag> + <item> + <p>Monitor the existence of the process identified by + <c><anno>Item</anno></c>. Currently valid + <c><anno>Item</anno></c>s in combination with the + <c>process <anno>Type</anno></c>:</p> + <taglist> + <tag><c>pid()</c></tag> + <item> + <p>The process identifier of the process to monitor.</p> + </item> + <tag><c>{RegisteredName, Node}</c></tag> + <item> + <p>A tuple consisting of a registered name of a process and + a node name. The process residing on the node <c>Node</c> + with the registered name <c>{RegisteredName, Node}</c> will + be monitored.</p> + </item> + <tag><c>RegisteredName</c></tag> + <item> + <p>The process locally registered as <c>RegisteredName</c> + will become monitored.</p> + </item> + </taglist> + <note><p>When a process is monitored by registered name, the + process that has the registered name at the time when the + monitor request reach its destination will be monitored. The monitor will not be effected, if the registered name is - unregistered.</p> - </note> - <p>A <c>'DOWN'</c> message will be sent to the monitoring - process if <c><anno>Item</anno></c> dies, if <c><anno>Item</anno></c> does not exist, - or if the connection is lost to the node which <c><anno>Item</anno></c> - resides on. A <c>'DOWN'</c> message has the following pattern:</p> - <code type="none"> -{'DOWN', MonitorRef, Type, Object, Info}</code> - <p>where <c>MonitorRef</c> and <c>Type</c> are the same as - described above, and:</p> - <taglist> - <tag><c>Object</c></tag> - <item> - <p>A reference to the monitored object:</p> - <list type="bulleted"> - <item>the pid of the monitored process, if <c><anno>Item</anno></c> was - specified as a pid.</item> - <item><c>{RegName, Node}</c>, if <c><anno>Item</anno></c> was specified as - <c>{RegName, Node}</c>.</item> - <item><c>{RegName, Node}</c>, if <c><anno>Item</anno></c> was specified as - <c>RegName</c>. <c>Node</c> will in this case be the - name of the local node (<c>node()</c>).</item> - </list> - </item> - <tag><c>Info</c></tag> - <item> - <p>Either the exit reason of the process, <c>noproc</c> - (non-existing process), or <c>noconnection</c> (no - connection to <c><anno>Node</anno></c>).</p> - </item> - </taglist> - <note> - <p>If/when <c>monitor/2</c> is extended (e.g. to - handle other item types than <c>process</c>), other - possible values for <c>Object</c>, and <c>Info</c> in the - <c>'DOWN'</c> message will be introduced.</p> - </note> - <p>The monitoring is turned off either when the <c>'DOWN'</c> - message is sent, or when - <seealso marker="#demonitor/1">demonitor/1</seealso> - is called.</p> - <p>If an attempt is made to monitor a process on an older node - (where remote process monitoring is not implemented or one - where remote process monitoring by registered name is not - implemented), the call fails with <c>badarg</c>.</p> - <p>Making several calls to <c>monitor/2</c> for the same - <c><anno>Item</anno></c> is not an error; it results in as many, completely - independent, monitorings.</p> + unregistered, or unregistered and later registered on another + process.</p></note> + <p>The monitor is triggered either when the monitored process + terminates, is non existing, or if the connection to it is + lost. In the case the connection to it is lost, we do not know + if it still exist or not. After this type of monitor has been + triggered, the monitor is automatically removed.</p> + <p>When the monitor is triggered a <c>'DOWN'</c> message will + be sent to the monitoring process. A <c>'DOWN'</c> message has + the following pattern:</p> + <code type="none">{'DOWN', MonitorRef, Type, Object, Info}</code> + <p>where <c>MonitorRef</c> and <c>Type</c> are the same as + described above, and:</p> + <taglist> + <tag><c>Object</c></tag> + <item> + <p>equals:</p> + <taglist> + <tag><c><anno>Item</anno></c></tag> + <item>If <c><anno>Item</anno></c> was specified by a + pid.</item> + <tag><c>{RegisteredName, Node}</c></tag> + <item>If <c><anno>Item</anno></c> was specified as + <c>RegisteredName</c>, or <c>{RegisteredName, Node}</c> + where <c>Node</c> corresponds to the node that the + monitored process resides on.</item> + </taglist> + </item> + <tag><c>Info</c></tag> + <item> + <p>Either the exit reason of the process, <c>noproc</c> + (non-existing process), or <c>noconnection</c> (no + connection to the node where the monitored process + resides).</p></item> + </taglist> + <p>The monitoring is turned off either when the <c>'DOWN'</c> + message is sent, or when + <seealso marker="#demonitor/1">demonitor/1</seealso> + is called.</p> + <p>If an attempt is made to monitor a process on an older node + (where remote process monitoring is not implemented or one + where remote process monitoring by registered name is not + implemented), the call fails with <c>badarg</c>.</p> + <note> + <p>The format of the <c>'DOWN'</c> message changed in the 5.2 + version of the emulator (OTP release R9B) for monitor + <em>by registered name</em>. The <c>Object</c> element of + the <c>'DOWN'</c> message could in earlier versions + sometimes be the pid of the monitored process and sometimes + be the registered name. Now the <c>Object</c> element is + always a tuple consisting of the registered name and + the node name. Processes on new nodes (emulator version 5.2 + or greater) will always get <c>'DOWN'</c> messages on + the new format even if they are monitoring processes on old + nodes. Processes on old nodes will always get <c>'DOWN'</c> + messages on the old format.</p> + </note> + </item> + <tag><marker id="monitor_time_offset"/><c>time_offset</c></tag> + <item> + <p>Monitor changes in + <seealso marker="#time_offset/0">time offset</seealso> + between + <seealso marker="time_correction#Erlang_Monotonic_Time">Erlang + monotonic time</seealso> and + <seealso marker="time_correction#Erlang_System_Time">Erlang + system time</seealso>. There is only one valid + <c><anno>Item</anno></c> in combination with the + <c>time_offset <anno>Type</anno></c>, namely the atom + <c>clock_service</c>. Note that the atom <c>clock_service</c> is + <em>not</em> the registered name of a process. In this specific + case it serves as an identifier of the runtime system internal + clock service at current runtime system instance.</p> + + <p>The monitor is triggered when the time offset is changed. + This either if the time offset value is changed, or if the + offset is changed from preliminary to final during + <seealso marker="#system_flag_time_offset">finalization + of the time offset</seealso> when the + <seealso marker="time_correction#Single_Time_Warp_Mode">single + time warp mode</seealso> is used. When a change from preliminary + to final time offset is made, the monitor will be triggered once + regardless of whether the time offset value was changed due to + the finalization or not.</p> + + <p>If the runtime system is in + <seealso marker="time_correction#Multi_Time_Warp_Mode">multi + time warp mode</seealso>, the time offset will be changed when + the runtime system detects that the + <seealso marker="time_correction#OS_System_Time">OS system + time</seealso> has changed. The runtime system will, however, + not detect this immediately when it happens. A task checking + the time offset is scheduled to execute at least once a minute, + so under normal operation this should be detected within a + minute, but during heavy load it might take longer time.</p> + + <p>The monitor will <em>not</em> be automatically removed + after it has been triggered. That is, repeated changes of + the time offset will trigger the monitor repeatedly.</p> + + <p>When the monitor is triggered a <c>'CHANGE'</c> message will + be sent to the monitoring process. A <c>'CHANGE'</c> message has + the following pattern:</p> + <code type="none">{'CHANGE', MonitorRef, Type, Item, NewTimeOffset}</code> + <p>where <c>MonitorRef</c>, <c><anno>Type</anno></c>, and + <c><anno>Item</anno></c> are the same as described above, and + <c>NewTimeOffset</c> is the new time offset.</p> + + <p>When the <c>'CHANGE'</c> message has been received you are + guaranteed not to retrieve the old time offset when calling + <seealso marker="#time_offset/0"><c>erlang:time_offset()</c></seealso>. + Note that you may observe the change of the time offset + when calling <c>erlang:time_offset()</c> before you + get the <c>'CHANGE'</c> message.</p> + + </item> + </taglist> + <p>Making several calls to <c>monitor/2</c> for the same + <c><anno>Item</anno></c> and/or <c><anno>Type</anno></c> is not + an error; it results in many, completely independent, + monitorings.</p> + <p>The monitor functionality is expected to be extended. That is, + other <c><anno>Type</anno></c>s and <c><anno>Item</anno></c>s + are expected to be supported in the future.</p> <note> - <p>The format of the <c>'DOWN'</c> message changed in the 5.2 - version of the emulator (OTP release R9B) for monitor <em>by registered name</em>. The <c>Object</c> element of - the <c>'DOWN'</c> message could in earlier versions - sometimes be the pid of the monitored process and sometimes - be the registered name. Now the <c>Object</c> element is - always a tuple consisting of the registered name and - the node name. Processes on new nodes (emulator version 5.2 - or greater) will always get <c>'DOWN'</c> messages on - the new format even if they are monitoring processes on old - nodes. Processes on old nodes will always get <c>'DOWN'</c> - messages on the old format.</p> + <p>If/when <c>monitor/2</c> is extended, other + possible values for <c>Tag</c>, <c>Object</c>, and + <c>Info</c> in the monitor message will be introduced.</p> </note> </desc> </func> @@ -2654,6 +2816,51 @@ os_prompt% </pre> </desc> </func> <func> + <name name="monotonic_time" arity="0"/> + <fsummary>Current Erlang monotonic time</fsummary> + <desc> + <p>Returns the current + <seealso marker="time_correction#Erlang_Monotonic_Time">Erlang + monotonic time</seealso> in <c>native</c> + <seealso marker="#type_time_unit">time unit</seealso>. This + is a monotonically increasing time since some unspecified point in + time.</p> + + <note><p>This is a + <seealso marker="time_correction#Monotonically_Increasing">monotonically increasing</seealso> time, but <em>not</em> a + <seealso marker="time_correction#Strictly_Monotonically_Increasing">strictly monotonically increasing</seealso> + time. That is, consecutive calls to + <c>erlang:monotonic_time/0</c> may produce the same result.</p> + + <p>Different runtime system instances will use different + unspecified points in time as base for their Erlang monotonic clocks. + That is, it is <em>pointless</em> comparing monotonic times from + different runtime system instances. Different runtime system instances + may also place this unspecified point in time different relative + runtime system start. It may be placed in the future (time at start + will be a negative value), the past (time at start will be a + positive value), or the runtime system start (time at start will + be zero). The monotonic time as of runtime system start can be + retrieved by calling + <seealso marker="#system_info_start_time"><c>erlang:system_info(start_time)</c></seealso>.</p></note> + </desc> + </func> + <func> + <name name="monotonic_time" arity="1"/> + <fsummary>Current Erlang monotonic time</fsummary> + <desc> + <p>Returns the current + <seealso marker="time_correction#Erlang_Monotonic_Time">Erlang + monotonic time</seealso> converted + into the <c><anno>Unit</anno></c> passed as argument.</p> + + <p>Same as calling + <seealso marker="#convert_time_unit/3"><c>erlang:convert_time_unit</c></seealso><c>(</c><seealso marker="#monotonic_time/0"><c>erlang:monotonic_time()</c></seealso><c>, + native, <anno>Unit</anno>)</c> + however optimized for commonly used <c><anno>Unit</anno></c>s.</p> + </desc> + </func> + <func> <name name="nif_error" arity="1"/> <fsummary>Stop execution with a given reason</fsummary> <desc> @@ -2748,6 +2955,13 @@ os_prompt% </pre> <type name="timestamp"/> <fsummary>Elapsed time since 00:00 GMT</fsummary> <desc> + <warning><p><em>This function is deprecated! Do not use it!</em> + See the users guide chapter + <seealso marker="time_correction">Time and Time Correction</seealso> + for more information. Specifically the + <seealso marker="time_correction#Dos_and_Donts">Dos and Dont's</seealso> + section for information on what to use instead of <c>erlang:now/0</c>. + </p></warning> <p>Returns the tuple <c>{MegaSecs, Secs, MicroSecs}</c> which is the elapsed time since 00:00 GMT, January 1, 1970 (zero hour) on the assumption that the underlying OS supports this. @@ -2760,10 +2974,6 @@ os_prompt% </pre> <p>It can only be used to check the local time of day if the time-zone info of the underlying operating system is properly configured.</p> - <p>If you do not need the return value to be unique and - monotonically increasing, use - <seealso marker="kernel:os#timestamp/0">os:timestamp/0</seealso> - instead to avoid some overhead.</p> </desc> </func> <func> @@ -5510,6 +5720,35 @@ ok <p>Returns the old value of the flag.</p> </desc> </func> + <marker id="system_flag_time_offset"/> + <func> + <name name="system_flag" arity="2" clause_i="12"/> + <fsummary>Finalize the Time Offset</fsummary> + <desc> + <p>Finalizes the <seealso marker="#time_offset/0">time offset</seealso> + when the <seealso marker="time_correction#Single_Time_Warp_Mode">single + time warp mode</seealso> is being used. If another time warp mode than + the "single time warp mode" is used, the time offset state will be left + unchanged.</p> + <p>Returns the old state identifier. That is, if:</p> + <list> + <item><p><c>preliminary</c> is returned, finalization was + performed and the time offset is now final.</p></item> + + <item><p><c>final</c> is returned, the time offset was + already in the final state. This either due to another + <c>erlang:system_flag(time_offset, finalize)</c> call, or + due to the + <seealso marker="time_correction#No_Time_Warp_Mode">no + time warp mode</seealso> being used.</p></item> + + <item><p><c>volatile</c> is returned, the time offset + cannot be finalized due to the + <seealso marker="time_correction#Multi_Time_Warp_Mode">multi + time warp mode</seealso> being used.</p></item> + </list> + </desc> + </func> <func> <name name="system_info" arity="1" clause_i="1"/> <name name="system_info" arity="1" clause_i="2"/> @@ -5790,6 +6029,15 @@ ok <name name="system_info" arity="1" clause_i="53"/> <name name="system_info" arity="1" clause_i="54"/> <name name="system_info" arity="1" clause_i="55"/> + <name name="system_info" arity="1" clause_i="56"/> + <name name="system_info" arity="1" clause_i="57"/> + <name name="system_info" arity="1" clause_i="58"/> + <name name="system_info" arity="1" clause_i="59"/> + <name name="system_info" arity="1" clause_i="60"/> + <name name="system_info" arity="1" clause_i="61"/> + <name name="system_info" arity="1" clause_i="62"/> + <name name="system_info" arity="1" clause_i="63"/> + <name name="system_info" arity="1" clause_i="64"/> <fsummary>Information about the system</fsummary> <desc> <p>Returns various information about the current system @@ -6177,6 +6425,57 @@ ok documentation of versions in the system principles guide</seealso>.</p> </item> + <tag><marker id="system_info_os_monotonic_time_source"><c>os_monotonic_time_source</c></marker></tag> + <item> + <p>Returns a list containing information about the source of + <seealso marker="erts:time_correction#OS_Monotonic_Time">OS + monotonic time</seealso> that is used by the runtime system.</p> + <p>In case <c>[]</c> is returned, no OS monotonic time is + available. The list contains two-tuples with <c>Key</c>s + as first element, and <c>Value</c>s as second element. The + order if these tuples is undefined. Currently the following + tuples may be part of the list, but more tuples may be + introduced in the future:</p> + <taglist> + <tag><c>{function, Function}</c></tag> + <item><p><c>Function</c> is the name of the funcion + used. This tuple always exist if OS monotonic time is + available to the runtime system.</p></item> + + <tag><c>{clock_id, ClockId}</c></tag> + <item><p>This tuple only exist if <c>Function</c> + can be used with different clocks. <c>ClockId</c> + corresponds to the clock identifer used when calling + <c>Function</c>.</p></item> + + <tag><c>{resolution, OsMonotonicTimeResolution}</c></tag> + <item><p>Highest possible resolution of current + OS monotonic time source as parts per second. If + no resolution information can be retreived from + the OS, <c>OsMonotonicTimeResolution</c> will be + set to the resolution of the time unit of + <c>Function</c>s return value. That is, the actual + resolution may be lower than + <c>OsMonotonicTimeResolution</c>. Also note that + the resolution does not say anything about the + accuracy, and that the precision might not align + with the resolution. You do, however, know that the + precision won't be higher than + <c>OsMonotonicTimeResolution</c>.</p></item> + + <tag><c>{parallel, Parallel}</c></tag> + <item><p><c>Parallel</c> equals <c>yes</c> if + <c>Function</c> is called in parallel from multiple + threads. If it is not called in parallel, because + calls needs to be serialized, <c>Parallel</c> equals + <c>no</c>.</p></item> + + <tag><c>{time, OsMonotonicTime}</c></tag> + <item><p><c>OsMonotonicTime</c> equals current OS + monotonic time in <c>native</c> + <seealso marker="#type_time_unit">time unit</seealso>.</p></item> + </taglist> + </item> <tag><marker id="system_info_port_parallelism"><c>port_parallelism</c></marker></tag> <item><p>Returns the default port parallelism scheduling hint used. For more information see the @@ -6302,6 +6601,11 @@ ok <p>Returns <c>true</c> if the emulator has been compiled with smp support; otherwise, <c>false</c>.</p> </item> + <tag><marker id="system_info_start_time"/><c>start_time</c></tag> + <item><p>The <seealso marker="#monotonic_time/0">Erlang monotonic + time</seealso> in <c>native</c> + <seealso marker="#type_time_unit">time unit</seealso> at the + time when current Erlang runtime system instance started.</p></item> <tag><c>system_version</c></tag> <item> <p>Returns a string containing version number and @@ -6325,12 +6629,64 @@ ok (<seealso marker="erts:erl_driver#driver_async">driver_async()</seealso>) as an integer.</p> </item> + <tag><marker id="system_info_time_correction"/><c>time_correction</c></tag> + <item><p>Returns a boolean value indicating whether + <seealso marker="time_correction#Time_Correction">time correction</seealso> + is enabled or not. + </p></item> + <tag><marker id="system_info_time_offset"/><c>time_offset</c></tag> + <item><p>Returns the state of the time offset:</p> + <taglist> + <tag><c>preliminary</c></tag> + <item><p>The time offset is preliminary, and will be changed + at a later time when being finalized. The preliminary time offset + is used during the preliminary phase of the + <seealso marker="time_correction#Single_Time_Warp_Mode">single + time warp mode</seealso>.</p></item> + + <tag><c>final</c></tag> + <item><p>The time offset is final. This + either due to the use of the + <seealso marker="time_correction#No_Time_Warp_Mode">no + time warp mode</seealso>, or due to the time offset having + been finalized when using the + <seealso marker="time_correction#Single_Time_Warp_Mode">single + time warp mode</seealso>.</p></item> + + <tag><c>volatile</c></tag> + <item><p>The time offset is volatile. That is, it may + change at any time. This due to the + <seealso marker="time_correction#Multi_Time_Warp_Mode">multi + time warp mode</seealso> being used.</p></item> + </taglist> + </item> + <tag><marker id="system_info_time_warp_mode"/><c>time_warp_mode</c></tag> + <item><p>Returns a value identifying the + <seealso marker="time_correction#Time_Warp_Modes">time warp + mode</seealso> being used:</p> + <taglist> + <tag><c>no_time_warp</c></tag> + <item><p>The <seealso marker="time_correction#No_Time_Warp_Mode">no + time warp mode</seealso> is being used.</p></item> + + <tag><c>single_time_warp</c></tag> + <item><p>The <seealso marker="time_correction#Single_Time_Warp_Mode">single + time warp mode</seealso> is being used.</p></item> + + <tag><c>multi_time_warp</c></tag> + <item><p>The <seealso marker="time_correction#Multi_Time_Warp_Mode">multi + time warp mode</seealso> is being used.</p></item> + </taglist> + </item> <tag><marker id="system_info_tolerant_timeofday"><c>tolerant_timeofday</c></marker></tag> <item> - <p>Returns whether compensation for sudden changes of system - time is <c>enabled</c> or <c>disabled</c>.</p> - <p>See also <seealso marker="erts:erl#+c">+c</seealso> - command line flag.</p> + <p>Returns whether a pre erts-7.0 backwards compatible compensation + for sudden changes of system time is <c>enabled</c> or <c>disabled</c>. + Such compensation is <c>enabled</c> when the + <seealso marker="#system_info_time_offset">time offset</seealso> is + <c>final</c>, and + <seealso marker="#system_info_time_correction">time correction</seealso> + is enabled.</p> </item> <tag><c>trace_control_word</c></tag> <item> @@ -6609,7 +6965,44 @@ ok </note> </desc> </func> + <func> + <name name="system_time" arity="0"/> + <fsummary>Current Erlang system time</fsummary> + <desc> + <p>Returns current + <seealso marker="time_correction#Erlang_System_Time">Erlang system time</seealso> + in <c>native</c> + <seealso marker="#type_time_unit">time unit</seealso>.</p> + <p>Calling <c>erlang:system_time()</c> is equivalent to: + <seealso marker="#monotonic_time/0"><c>erlang:monotonic_time()</c></seealso><c> + + + </c><seealso marker="#time_offset/0"><c>erlang:time_offset()</c></seealso>.</p> + + <note><p>This time is <em>not</em> a monotonically increasing time + in the general case. For more information, see the documentation of + <seealso marker="time_correction#Time_Warp_Modes">time warp modes</seealso> in the + ERTS User's Guide.</p></note> + </desc> + </func> + <func> + <name name="system_time" arity="1"/> + <fsummary>Current Erlang system time</fsummary> + <desc> + <p>Returns current + <seealso marker="time_correction#Erlang_System_Time">Erlang system time</seealso> + converted into the <c><anno>Unit</anno></c> passed as argument.</p> + + <p>Calling <c>erlang:system_time(<anno>Unit</anno>)</c> is equivalent to: + <seealso marker="#convert_time_unit/3"><c>erlang:convert_time_unit</c></seealso><c>(</c><seealso marker="#system_time/0"><c>erlang:system_time()</c></seealso><c>, + native, <anno>Unit</anno>)</c>.</p> + + <note><p>This time is <em>not</em> a monotonically increasing time + in the general case. For more information, see the documentation of + <seealso marker="time_correction#Time_Warp_Modes">time warp modes</seealso> in the + ERTS User's Guide.</p></note> + </desc> + </func> <func> <name name="term_to_binary" arity="1"/> <fsummary>Encode a term to an Erlang external term format binary</fsummary> @@ -6686,6 +7079,88 @@ ok </desc> </func> <func> + <name name="time_offset" arity="0"/> + <fsummary>Current time offset</fsummary> + <desc> + <p>Returns the current time offset between + <seealso marker="time_correction#Erlang_Monotonic_Time">Erlang monotonic time</seealso> + and + <seealso marker="time_correction#Erlang_System_Time">Erlang system time</seealso> in + <c>native</c> <seealso marker="#type_time_unit">time unit</seealso>. + Current time offset added to an Erlang monotonic time gives + corresponding Erlang system time.</p> + + <p>The time offset may or may not change during operation depending + on the <seealso marker="time_correction#Time_Warp_Modes">time + warp mode</seealso> used.</p> + + <note> + <p>A change in time offset may be observed at slightly + different points in time by different processes.</p> + + <p>If the runtime system is in + <seealso marker="time_correction#Multi_Time_Warp_Mode">multi + time warp mode</seealso>, the time offset will be changed when + the runtime system detects that the + <seealso marker="time_correction#OS_System_Time">OS system + time</seealso> has changed. The runtime system will, however, + not detect this immediately when it happens. A task checking + the time offset is scheduled to execute at least once a minute, + so under normal operation this should be detected within a + minute, but during heavy load it might take longer time.</p> + </note> + </desc> + </func> + <func> + <name name="time_offset" arity="1"/> + <fsummary>Current time offset</fsummary> + <desc> + <p>Returns the current time offset between + <seealso marker="time_correction#Erlang_Monotonic_Time">Erlang monotonic time</seealso> + and + <seealso marker="time_correction#Erlang_System_Time">Erlang system time</seealso> + converted into the <c><anno>Unit</anno></c> passed as argument.</p> + + <p>Same as calling + <seealso marker="#convert_time_unit/3"><c>erlang:convert_time_unit</c></seealso><c>(</c><seealso marker="#time_offset/0"><c>erlang:time_offset()</c></seealso><c>, native, <anno>Unit</anno>)</c> + however optimized for commonly used <c><anno>Unit</anno></c>s.</p> + </desc> + </func> + <func> + <name name="timestamp" arity="0"/> + <type name="timestamp"/> + <fsummary>Current Erlang System time</fsummary> + <desc> + <p>Returns current + <seealso marker="time_correction#Erlang_System_Time">Erlang system time</seealso> + on the format <c>{MegaSecs, Secs, MicroSecs}</c>. This format is + the same that <seealso marker="kernel:os#timestamp/0"><c>os:timestamp/0</c></seealso> + and the now deprecated <seealso marker="#now/0"><c>erlang:now/0</c></seealso> + uses. The reason for the existence of <c>erlang:timestamp()</c> is + purely to simplify usage for existing code that assumes this timestamp + format. Current Erlang system time can more efficiently be retrieved in + the time unit of your choice using + <seealso marker="#system_time/1"><c>erlang:system_time/1</c></seealso>.</p> + + <p>The <c>erlang:timestamp()</c> BIF is equivalent to:</p><code type="none"> +timestamp() -> + ErlangSystemTime = erlang:system_time(micro_seconds), + MegaSecs = ErlangSystemTime div 1000000000000, + Secs = ErlangSystemTime div 1000000 - MegaSecs*1000000, + MicroSecs = ErlangSystemTime rem 1000000, + {MegaSecs, Secs, MicroSecs}.</code> + <p>It however use a native implementation which does + not build garbage on the heap and with slightly better + performance.</p> + + <note><p>This time is <em>not</em> a monotonically increasing time + in the general case. For more information, see the documentation of + <seealso marker="time_correction#Time_Warp_Modes">time warp modes</seealso> in the + ERTS User's Guide.</p></note> + </desc> + + </func> + <func> <name name="tl" arity="1"/> <fsummary>Tail of a list</fsummary> <desc> @@ -7452,6 +7927,100 @@ ok </desc> </func> <func> + <name name="unique_integer" arity="0"/> + <fsummary>Get a unique integer value</fsummary> + <desc> + <p>Generates and returns an + <seealso marker="doc/efficiency_guide:advanced#unique_integers">integer + unique on current runtime system instance</seealso>. The same as calling + <seealso marker="#unique_integer/1"><c>erlang:unique_integer([])</c></seealso>.</p> + </desc> + </func> + <func> + <name name="unique_integer" arity="1"/> + <fsummary>Get a unique integer value</fsummary> + <desc> + <p>Generates and returns an + <seealso marker="doc/efficiency_guide:advanced#unique_integers">integer + unique on current runtime system + instance</seealso>. The integer is unique in the + sense that this BIF, using the same set of + modifiers, will not return the same integer more + than once on the current runtime system instance. + Each integer value can of course be constructed + by other means.</p> + + <p>By default, i.e. when <c>[]</c> is passed as + <c><anno>ModifierList</anno></c>, both negative and + positive integers will be returned. This is order + to be able to utilize the range of integers that do + not need to be heap allocated as much as possible. + By default the returned integers are also only + guaranteed to be unique, i.e., any integer returned + may be either smaller, or larger than previously + returned integers.</p> + + <p>Currently valid <c><anno>Modifier</anno></c>s:</p> + <taglist> + + <tag>positive</tag> + <item><p>Return only positive integers.</p> + <p>Note that by passing the <c>positive</c> modifier + you will get heap allocated integers (big-nums) + quicker.</p> + </item> + + <tag>monotonic</tag> + <item><p>Return + <seealso marker="time_correction#Strictly_Monotonically_Increasing">strictly + monotonically increasing</seealso> integers + corresponding to creation time. That is, the integer + returned will always be larger than previously + returned integers on the current runtime system + instance.</p> + <p>These values can be used when ordering events + on the runtime system instance. That is, if both + <c>X = erlang:unique_integer([monotonic])</c> and + <c>Y = erlang:unique_integer([monotonic])</c> are + executed by different processes (or the same + process) on the same runtime system instance and + <c>X < Y</c> we know that <c>X</c> was created + before <c>Y</c>.</p> + <warning><p>Strictly monotonically increasing values + are inherently quite expensive to generate and scales + poorly. This since the values needs to be + synchronized. That is, do not pass the <c>monotonic</c> + modifier unless you really need strictly monotonically + increasing values.</p></warning> + </item> + + </taglist> + + <p>All currently valid <c><anno>Modifier</anno></c>s + can be combined. Repeated (valid) + <c><anno>Modifier</anno></c>s in the <c>ModifierList</c> + are ignored.</p> + + <note><p>Note that the set of integers returned by + <c>unique_integer/1</c> using diffrent sets of + <c><anno>Modifier</anno></c>s <em>will overlap</em>. + For example, by calling <c>unique_integer([monotonic])</c>, + and <c>unique_integer([positive, monotonic])</c> + repeatedly, you will eventually see some integers being + returned by both calls.</p></note> + + <p>Failures:</p> + <taglist> + <tag><c>badarg</c></tag> + <item>if <c><anno>ModifierList</anno></c> is not a + proper list.</item> + <tag><c>badarg</c></tag> + <item>if <c><anno>Modifier</anno></c> is not a + valid modifier.</item> + </taglist> + </desc> + </func> + <func> <name name="unlink" arity="1"/> <fsummary>Remove a link, if there is one, to another process or port</fsummary> <desc> diff --git a/erts/doc/src/time_correction.xml b/erts/doc/src/time_correction.xml index 7f7c28fc30..3bc3d04186 100644 --- a/erts/doc/src/time_correction.xml +++ b/erts/doc/src/time_correction.xml @@ -21,8 +21,8 @@ </legalnotice> - <title>Time and time correction in Erlang</title> - <prepared>Patrik Nyblom</prepared> + <title>Time and Time Correction in Erlang</title> + <prepared></prepared> <responsible></responsible> <docno></docno> <approved></approved> @@ -31,6 +31,176 @@ <rev>PA1</rev> <file>time_correction.xml</file> </header> + + <section> + <title>New Extended Time Functionality</title> + <note><p>As of OTP 18 (ERTS version 7.0) the time functionality of + Erlang has been extended. This both includes a + <seealso marker="#The_New_Time_API">new API</seealso> + for time, as well as + <seealso marker="#Time_Warp_Modes">time warp + modes</seealso> which alters the behavior of the system when + system time changes.</p> + <p>The <seealso marker="#No_Time_Warp_Mode">default + time warp mode</seealso> has the same behavior as before, and the + old API will still work, so you are not required to change + anything unless you want to. However, <em>you are strongly + encouraged to use the new API</em> instead of the old API based + on <seealso marker="erlang#now/0"><c>erlang:now/0</c></seealso>. + <c>erlang:now/0</c> has been deprecated since it is and forever + will be a scalability bottleneck. By using the new API you will + automatically get scalability and performance improvements. This + will also enable you to use the + <seealso marker="#Multi_Time_Warp_Mode">multi time warp mode</seealso> + which improves accuracy, and precision of time measurements.</p></note> + </section> + + <section> + <title>Some Terminology</title> + <p>In order to make it easier to understand this document we first + define some terminology. This is a mixture of our own terminology + (Erlang/OS system time, Erlang/OS monotonic time, time warp) + and globally accepted terminology.</p> + + <marker id="Monotonically_Increasing"/> + <section> + <title>Monotonically Increasing</title> + <p>In a monotonically increasing sequence of values, all values + that have a predecessor are either larger than, or equal to its + predecessor.</p> + </section> + + <marker id="Strictly_Monotonically_Increasing"/> + <section> + <title>Strictly Monotonically Increasing</title> + <p>In a strictly monotonically increasing sequence of values, + all values that have a predecessor are larger than its + predecessor.</p> + </section> + + <marker id="UT1"/> + <section> + <title>UT1</title> + <p>Universal Time. Based on the rotation of the earth. Conceptually + mean solar time at 0° longitude.</p> + </section> + + <marker id="UTC"/> + <section> + <title>UTC</title> + <p>Coordinated Universal Time. UTC almost align with + <seealso marker="#UT1">UT1</seealso>, however, UTC uses the + SI definition of a second which is not exactly of the same length + as the second used by UT1. This means that UTC slowly drifts from + UT1. In order to keep UTC relatively in sync with UT1, leap seconds + are inserted, and potentially also deleted. That is, an UTC day may + be 86400, 86401, or 86399 seconds long.</p> + </section> + + <marker id="POSIX_Time"/> + <section> + <title>POSIX Time</title> + <p>Time since + <url href="http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap03.html#tag_21_03_00_17">Epoch</url>. + Epoch is defined to be 00:00:00 <seealso marker="#UTC">UTC</seealso>, + January 1, 1970. + <url href="http://pubs.opengroup.org/onlinepubs/009604499/basedefs/xbd_chap04.html#tag_04_14">A day in POSIX time</url> + is defined to be exactly 86400 seconds long. Strangely enough + Epoch is defined to be a time in UTC, and UTC have another + definition of how long a day is. Quoting the Open Group + <url href="http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html#tag_21_04_15">"POSIX time is therefore not necessarily UTC, despite its appearance"</url>. The effect of this is that when an UTC leap second is + inserted, POSIX time either stops for a second, or repeats the + last second. If an UTC leap second would be deleted (has never + happened yet), POSIX time would make a one second leap forward.</p> + </section> + + <marker id="OS_System_Time"/> + <section> + <title>OS System Time</title> + <p>The operating systems view of + <seealso marker="#POSIX_Time">POSIX time</seealso>. It can be + retrieved by calling + <seealso marker="kernel:os#system_time/0"><c>os:system_time()</c></seealso>. + This may or may not be an accurate view of POSIX time. This time + may typically be adjusted both backwards and forwards without + limitation. That is, huge leaps both backwards and forwards in time + may be observed.</p> + </section> + + <marker id="OS_Monotonic_Time"/> + <section> + <title>OS Monotonic Time</title> + <p>A monotonically increasing time provided by the operating + system. This time does not leap and have a relatively steady + frequency although not completely correct. However, it is not + uncommon that the OS monotonic time stops if the system is + suspended. This time typically increase since some unspecified + point in time that is not connected to + <seealso marker="#OS_System_Time">OS system time</seealso>. Note that + this type of time is not necessarily provided by all operating + systems.</p> + </section> + + <marker id="Erlang_System_Time"/> + <section> + <title>Erlang System Time</title> + <p>The Erlang runtime systems view of + <seealso marker="#POSIX_Time">POSIX time</seealso>. It can be + retrieved by calling + <seealso marker="erlang#system_time/0"><c>erlang:system_time()</c></seealso>. + This time may or may not be an accurate view of POSIX time, and may + or may not align with <seealso marker="#OS_System_Time">OS system + time</seealso>. The <seealso marker="#Time_Warp_Modes">time + warp mode</seealso> determines how it behaves when OS system + time suddenly change.</p> + </section> + + <marker id="Erlang_Monotonic_Time"/> + <section> + <title>Erlang Monotonic Time</title> + <p>A monotonically increasing time provided by the + Erlang runtime system. The Erlang monotonic time increase since + some unspecified point in time. It can be retrieved by calling + <seealso marker="erlang#monotonic_time/0"><c>erlang:monotonic_time()</c></seealso>. + The accuracy, and precision of Erlang monotonic time heavily + depends on the accuracy and precision of + <seealso marker="#OS_Monotonic_Time">OS monotonic time</seealso>, + the accuracy and precision of + <seealso marker="#OS_System_Time">OS system time</seealso> as well + as on the + <seealso marker="#Time_Warp_Modes">time warp mode</seealso> + used. On a system that is lacking OS monotonic time, the Erlang + monotonic time can only guarantee monotonicity and can more or less + not give any other guarantees. The frequency adjustments made to + the Erlang monotonic time depends on the time warp mode + used.</p> + + <p>Internally in the runtime system the Erlang monotonic + time is the "time engine" that is used for more or less + everything that has anything to do with time. All timers + regardless of it is a <c>receive ... after</c> timer, BIF timer, + or a timer in the <c>timer</c> module are triggered + relative Erlang monotonic time. Even + <seealso marker="#Erlang_System_Time">Erlang system + time</seealso> is based on Erlang monotonic time. + By adding current Erlang monotonic time with current time + offset you get current Erlang system time. Current time + offset can be retrieved by calling + <seealso marker="erlang#time_offset/0"><c>erlang:time_offset/0</c></seealso>. + </p> + </section> + + <marker id="Time_Warp"/> + <section> + <title>Time Warp</title> + <p>A time warp is a leap forwards or backwards in time.</p> + </section> + + </section> + + <section> + <title>Introduction</title> + <p>Time is vital to an Erlang program and, more importantly, <em>correct</em> time is vital to an Erlang program. As Erlang is a language with soft real time properties and we have the possibility to express @@ -83,192 +253,587 @@ microsecond resolution or much less, but generally it has a drift that is not to be ignored.</p> - <p>So we have this monotonic ticking and we have the wall clock - time. Two unreliable times that together can give us an estimate of - an actual wall clock time that does not jump around and that - monotonically moves forward. If the tick counter has a high - resolution, this is fairly easy to do, if the counter has a low - resolution, it's more expensive, but still doable down to - frequencies of 50-60 Hz (of the tick counter).</p> - - <p>So the corrected time is the nearest approximation of an atomic - clock that is available on the computer. We want it to have the - following properties:</p> - <taglist> - <tag>Monotonic</tag> - <item>The clock should not move backwards</item> - <tag>Intervals should be near the truth</tag> - <item>We want the actual time (as measured by an atomic clock or - an astronomer) that passes between two time stamps, T1 and T2, to be as - near to T2 - T1 as possible.</item> - <tag>Tight coupling to the wall clock</tag> - <item>We want a timer that is to be fired when the wall clock - reaches a time in the future, to fire as near to that point in - time as possible</item> - </taglist> - <p>To meet all the criteria, we have to utilize both times in such a - way that Erlangs "corrected time" moves slightly slower or slightly - faster than the wall clock to get in sync with it. The word - "slightly" means a maximum of 1% difference to the wall clock time, - meaning that a sudden change in the wall clock of one minute, takes - 100 minutes to fix, by letting all "corrected time" move 1% slower - or faster.</p> - - <p>Needless to say, correcting for a faulty handling of daylight - saving time may be disturbing to a user comparing wall clock - time to for example calendar:now_to_local_time(erlang:now()). But - calendar:now_to_local_time/1 is not supposed to be used for presenting wall - clock time to the user.</p> - - <p>Time correction is not perfect, but it saves you from the havoc - of clocks jumping around, which would make timers in your program - fire far to late or far to early and could bring your whole system - to it's knees (or worse) just because someone detected a small error - in the wall clock time of the server where your program runs. So - while it might be confusing, it is still a really good feature of - Erlang and you should not throw it away using time functions which - may give you higher benchmark results, not unless you really know - what you're doing.</p> + </section> + <marker id="Time_Correction"/> <section> - <title>What does time correction mean in my system?</title> - <p>Time correction means that Erlang estimates a time from current - and previous settings of the wall clock, and it uses a fairly - exact tick counter to detect when the wall clock time has jumped - for some reason, slowly adjusting to the new value.</p> - - <p>In practice, this means that the difference between two calls - to time corrected functions, like erlang:now(), might differ up to - one percent from the corresponding calls to non time corrected - functions (like os:timestamp()). Furthermore, if comparing - calendar:local_time/0 to calendar:now_to_local_time(erlang:now()), - you might temporarily see a difference, depending on how well kept your - system is.</p> - - <p>It is important to understand that it is (to the program) - always unknown if it is the wall clock time that moves in the - wrong pace or the Erlang corrected time. The only way to determine - that, is to have an external source of universally correct time. If - some such source is available, the wall clock time can be kept - nearly perfect at all times, and no significant difference will be - detected between erlang:now/0's pace and the wall clock's.</p> - - <p>Still, the time correction will mean that your system keeps - it's real time characteristics very well, even when the wall clock - is unreliable.</p> + <title>Time Correction</title> + <p>If time correction is enabled, the Erlang runtime system + will make use of both + <seealso marker="#OS_System_Time">OS system time</seealso> + and <seealso marker="#OS_Monotonic_Time">OS monotonic time</seealso>, + in order to make adjustments of the frequency of the Erlang + monotonic clock. Time correction will ensure that + <seealso marker="#Erlang_Monotonic_Time">Erlang monotonic time</seealso> + will not warp, and that the frequency is relatively accurate. + The type of adjustments made to the frequency depends on the + time warp mode used. This will be discussed in more details in + the <seealso marker="#Time_Warp_Modes">time warp modes</seealso> + section below.</p> + + <p>By default time correction will be enabled if support for + it on the specific platform exist. Support for it includes + both an OS monotonic time provided by the OS, and an + implementation in the Erlang runtime system utilizing the + OS monotonic time. You can check if your system has support + for OS monotonic time by calling + <seealso marker="erlang#system_info_os_monotonic_time_source"><c>erlang:system_info(os_monotonic_time_source)</c></seealso>, + and you can check if time correction is enabled on your + system by calling + <seealso marker="erlang#system_info_time_correction"><c>erlang:system_info(time_correction)</c></seealso>.</p> + + <p>Time correction is enabled or disabled by passing the + <seealso marker="erl#+c"><c>+c [true|false]</c></seealso> + command line argument to <c>erl</c>.</p> + + <p>If time correction is disabled, Erlang monotonic time + may warp forwards, it may stop and even freeze for extended + periods of time, and there are no guarantees that the frequency + of the Erlang monotonic clock is accurate or stable.</p> + + <p><em>You typically never want to disable time correction</em>. + Previously there was a performance penalty associated with time + correction, but nowadays it is most often the other way around. + By disabling time correction you are likely to get bad scalability, + bad performance, and bad time measurements.</p> </section> + + + <marker id="Time_Warp_Safe_Code"/> <section> - <title>Where does Erlang use corrected time?</title> - <p>For all functionality where real time characteristics are - desirable, time correction is used. This basically means:</p> - <taglist> - <tag>erlang:now/0</tag> - <item>The infamous erlang:now/0 function uses time correction so - that differences between two "now-timestamps" will correspond to - other timeouts in the system. erlang:now/0 also holds other - properties, discussed later.</item> - <tag>receive ... after</tag> - <item>Timeouts on receive uses time correction to determine a - stable timeout interval.</item> - <tag>The timer module</tag> - <item>As the timer module uses other built in functions which - deliver corrected time, the timer module itself works with - corrected time.</item> - <tag>erlang:start_timer/3 and erlang:send_after/3</tag> - <item>The timer BIF's work with corrected time, so that they - will not fire prematurely or too late due to changes in the wall - clock time.</item> - </taglist> - - <p>All other functionality in the system where erlang:now/0 or any - other time corrected functionality is used, will of course - automatically benefit from it, as long as it's not "optimized" to - use some other time stamp function (like os:timestamp/0).</p> - - <p>Modules like calendar and functions like erlang:localtime/0 use - the wall clock time as it is currently set on the system. They - will not use corrected time. However, if you use a now-value and - convert it to local time, you will get a corrected local time - value, which may or may not be what you want. Typically older code - tend to use erlang:now/0 as a wall clock time, which is usually - correct (at least when testing), but might surprise you when - compared to other times in the system.</p> + <title>Time Warp Safe Code</title> + <p>Time warp safe code is code that is able to handle + a time warp of + <seealso marker="#Erlang_System_Time">Erlang system time</seealso>. + </p> + + <p><seealso marker="erlang#now/0"><c>erlang:now/0</c></seealso> + behaves very bad when Erlang system time warps. When Erlang + system time do a time warp backwards, the values returned + from <c>erlang:now/0</c> will freeze (if you disregard the + micro second increments made due to the actual call) until + OS system time reach the point of the last value returned by + <c>erlang:now/0</c>. This freeze might continue for very + long periods of time. It might take years, decades, + and even longer than this until the freeze stops.</p> + + <p>All uses of <c>erlang:now/0</c> are not necessarily + time warp unsafe. If you do not use it to get time, it + will be time warp safe. However <em>all uses of + <c>erlang:now/0</c> are suboptimal</em> from a performance + and scalability perspective. So you really want to replace + the usage of it with other functionality. For examples + of how to replace the usage of <c>erlang:now/0</c>, + see the <seealso marker="#Dos_and_Donts">Dos and Donts</seealso> + section.</p> </section> + + <marker id="Time_Warp_Modes"/> <section> - <title>What is erlang:now/0 really?</title> - <p>erlang:now/0 is a function designed to serve multiple purposes - (or a multi-headed beast if you're a VM designer). It is expected - to hold the following properties:</p> - <taglist> - <tag>Monotonic</tag> - <item>erlang:now() never jumps backwards - it always moves - forward</item> - <tag>Interval correct</tag> - <item>The interval between two erlang:now() calls is expected to - correspond to the correct time in real life (as defined by an - atomic clock, or better)</item> - <tag>Absolute correctness</tag> - <item>The erlang:now/0 value should be possible to convert to an - absolute and correct date-time, corresponding to the real world - date and time (the wall clock)</item> - <tag>System correspondence</tag> - <item>The erlang:now/0 value converted to a date-time is - expected to correspond to times given by other programs on the - system (or by functions like os:timestamp/0)</item> - <tag>Unique</tag> - <item>No two calls to erlang:now on one Erlang node should - return the same value</item> - </taglist> - <p>All these requirements are possible to uphold at the same - time if (and only if):</p> - <taglist> - <tag>The wall clock time of the system is perfect</tag> - <item>The system (Operating System) time needs to be perfectly - in sync with the actual time as defined by an atomic clock or - a better time source. A good installation using NTP, and that is - up to date before Erlang starts, will have properties that for - most users and programs will be near indistinguishable from the - perfect time. Note that any larger corrections to the time done - by hand, or after Erlang has started, will partly (or - temporarily) invalidate some of the properties, as the time is - no longer perfect.</item> - <tag>Less than one call per microsecond to erlang:now/0 is - done</tag> - <item>This means that at <em>any</em> microsecond interval in - time, there can be no more than one call to erlang:now/0 in the - system. However, for the system not to loose it's properties - completely, it's enough that it on average is no more than one - call per microsecond (in one Erlang node).</item> - </taglist> - <p>The uniqueness property of erlang:now/0 is the most limiting - property. It means that erlang:now() maintains a global state and - that there is a hard-to-check property of the system that needs to - be maintained. For most applications this is still not a problem, - but a future system might very well manage to violate the - frequency limit on the calls globally. The uniqueness property is - also quite useless, as there are globally unique references that - provide a much better unique value to programs. However the - property will need to be maintained unless a really subtle - backward compatibility issue is to be introduced.</p> + <title>Time Warp Modes</title> + + <p>Current <seealso marker="#Erlang_System_Time">Erlang system + time</seealso> is determined by adding current + <seealso marker="erlang#monotonic_time/0">Erlang monotonic time</seealso> + with current + <seealso marker="erlang#time_offset/0">time offset</seealso>. The + time offset is managed differently depending on which time + warp mode you use. The time warp mode is set by passing the + <seealso marker="erl#+C_"><c>+C + [no_time_warp|single_time_warp|multi_time_warp]</c></seealso> + command line argument to <c>erl</c>.</p> + + <marker id="No_Time_Warp_Mode"/> + <section> + <title>No Time Warp Mode</title> + <p>The time offset is determined at runtime system start + and will after this not change. This is the default behavior. + Not because it is the best mode (which it isn't). It is + default only because this is how the runtime system always + has behaved until ERTS version 7.0, and you have to ensure + that your Erlang code that may execute during a time warp is + <seealso marker="#Time_Warp_Safe_Code">time warp safe</seealso> + before you can enable other modes.</p> + + <p>Since the time offset is not allowed to change, time + correction needs to adjust the frequency of the Erlang + monotonic clock in order to smoothly align Erlang system + time with OS system time. A big downside of this approach + is that we on purpose will use a faulty frequency on the + Erlang monotonic clock if adjustments are needed. This + error may be as big as 1%. This error will show up in all + time measurements in the runtime system.</p> + + <p>If time correction is not enabled, the Erlang monotonic + time will freeze when the OS system time leap backwards. + The freeze of the monotonic time will continue until + OS system time catch up. The freeze may continue for + a very long time. When OS system time leaps forwards, + Erlang monotonic time will also leap forward.</p> + </section> + + <marker id="Single_Time_Warp_Mode"/> + <section> + <title>Single Time Warp Mode</title> + <p>This mode is more or less a backwards compatibility mode + as of its introduction.</p> + <p>On an embedded system it is not uncommon that the system + has no power supply at all, not even a battery, when it is + shut off. The system clock on such a system will typically + be way off when the system boots. If the + <seealso marker="#No_Time_Warp_Mode">no time warp mode</seealso> + is used, and the Erlang runtime system is started before + the OS system time has been corrected, the Erlang system + time may be wrong for a very long time, even centuries or + more.</p> + <p>If you for some reason need to use Erlang code that + is not + <seealso marker="#Time_Warp_Safe_Code">time warp safe</seealso>, + and you need to start the Erlang runtime system before the OS + system time has been corrected, you may want to use the single + time warp mode. Note that there are limitations to when you can + execute time warp unsafe code using this mode. If it is possible + to only utilize time warp safe code, it is much better to use + the <seealso marker="#Multi_Time_Warp_Mode">multi time warp + mode</seealso> instead. + </p> + + <p>Using the single time warp mode, the time offset is + handled in two phases:</p> + + <taglist> + <tag>Preliminary Phase</tag> + <item> + <p>The preliminary phase starts when the runtime + system starts. A preliminary time offset based on + current OS system time is determined. This offset will + from now on be fixed during the whole preliminary phase.</p> + + <p>If time correction is enabled, the Erlang + monotonic clock will only use the OS monotonic time as + time source during this phase. That is, during the + preliminary phase changes in OS system time will have + no effect on Erlang system time and/or Erlang + monotonic time what so ever.</p> + + <p>If time correction is disabled, changes in OS system + time will effect the monotonic clock the same way as + when the <seealso marker="#No_Time_Warp_Mode">no time warp + mode</seealso> is used.</p> + </item> + + <tag>Final Phase</tag> + <item> + + <p>The final phase begin when the user finalize the time + offset by calling + <seealso marker="erlang#system_flag_time_offset"><c>erlang:system_flag(time_offset, finalize)</c></seealso>. + The finalization can only be performed once. + </p> + + <p>During finalization, the time offset is adjusted and + fixated so that current Erlang system time align with + current OS system time. Since the time offset + may be changed, the Erlang system time may do + a time warp at this point. The time offset will from + now on be fixed until the runtime system terminates. + If time correction has been enabled, the time correction + also begins when this phase begins. When the system is + in the final phase it behaves exactly as in the + <seealso marker="#No_Time_Warp_Mode">no time warp + mode</seealso>.</p> + + </item> + </taglist> + + <p>In order for this to work properly there are two + requirements that the user needs to ensure are + satisfied:</p> + + <taglist> + <tag>Forward Time Warp</tag> + <item><p>The time warp made when finalizing the time offset + can only be done forwards without encountering problems. + This implies that the user has to ensure that the OS + system time is set to a time earlier or equal to actual + POSIX time before starting the Erlang runtime system. If + you are not completely sure the OS system time is correct, + set it to a time that is guaranteed to be earlier than + actual POSIX time before starting the Erlang runtime + system just to be safe.</p></item> + + <tag>Finalize Correct OS System Time</tag> + <item><p>The OS system time needs to be correct when the + the user finalizes the time offset.</p></item> + </taglist> + + <p>If these requirements are not fulfilled, the system + may behave very bad. + </p> + + <p>Assuming that the requirements above are fulfilled, + time correction is enabled, and that the OS system time + is adjusted using some time adjustment protocol like NTP + or similar, only small adjustments of the Erlang monotonic + time should be needed in order to keep system times + aligned after finilization. As long as the system is not + suspended, the largest adjustments needed should be for + inserted (or deleted) leap seconds.</p> + + <warning><p>In order to be able to use this mode you have + to ensure that all Erlang code that will execute in + both phases are + <seealso marker="#Time_Warp_Safe_Code">time warp + safe</seealso>.</p> + <p>Code that only execute in the final phase does not have + to be able to cope with the time warp.</p></warning> + + </section> + + <marker id="Multi_Time_Warp_Mode"/> + <section> + <title>Multi Time Warp Mode</title> + + <p><em>Multi time warp mode in combination with time + correction is the preferred configuration</em>. This since, + on almost all platforms, the Erlang runtime system will have + better performance, will scale better, will behave better, + and since the accuracy, and precision of time measurements + will be better. Only Erlang runtime systems executing on + ancient platforms will benefit from another configuration.</p> + + <p>The time offset may change at any time without limitations. + That is, Erlang system time may perform time warps both + forwards and backwards at <em>any</em> time. Since we align + the Erlang system time with the OS system time by changing + the time offset, we can enable a time correction that tries + to adjust the frequency of the Erlang monotonic clock to be as + correct as possible. This will make time measurements using + the Erlang monotonic time more accurate and precise.</p> + + <p>If time correction is disabled, Erlang monotonic time + will leap forward if OS system time leaps forward. If the + OS system time leaps backwards, Erlang monotonic time will + stop briefly but it does not freeze for extended periods + of time. This since the time offset is changed in order to + align Erlang system time with OS system time.</p> + + <warning><p>In order to be able to use this mode you have + to ensure that all Erlang code that will execute on the + runtime system is + <seealso marker="#Time_Warp_Safe_Code">time warp + safe</seealso>.</p></warning> + </section> </section> + + <marker id="The_New_Time_API"/> <section> - <title>Should I use erlang:now/0 or os:timestamp/0</title> - <p>The simple answer is to use erlang:now/0 for everything where - you want to keep real time characteristics, but use os:timestamp - for things like logs, user communication and debugging (typically - timer:ts uses os:timestamp, as it is a test tool, not a real world - application API). The benefit of using os:timestamp/0 is that it's - faster and does not involve any global state (unless the operating - system has one). The downside is that it will be vulnerable to wall - clock time changes.</p> + <title>The New Time API</title> + + <p>The old time API is based on + <seealso marker="erlang#now/0"><c>erlang:now/0</c></seealso>. + The major issue with <c>erlang:now/0</c> is that it was + intended to be used for so many unrelated things. This + tied these unrelated operations together and unnecessarily + caused performance, scalability as well as accuracy, and + precision issues for operations that do not need to have + such issues. The new API spreads different functionality + over multiple functions in order to improve on this.</p> + + <p>In order to be backwards compatible <c>erlang:now/0</c> will + remain as is, but <em>you are strongly discouraged from using + it</em>. A lot of uses of <c>erlang:now/0</c> will also + prevent you from using the new + <seealso marker="#Multi_Time_Warp_Mode">multi time warp + mode</seealso> which is an important part of this + new time functionality improvement.</p> + + <p>Some of the new BIFs on some systems, perhaps surprisingly, + return negative integer values on a newly started run time + system. This is not a bug, but a memory usage optimization.</p> + + <p>The new API consists of a number of new BIFs:</p> + <list> + <item><p><seealso marker="erlang#convert_time_unit/3"><c>erlang:convert_time_unit/3</c></seealso></p></item> + <item><p><seealso marker="erlang#monotonic_time/0"><c>erlang:monotonic_time/0</c></seealso></p></item> + <item><p><seealso marker="erlang#monotonic_time/1"><c>erlang:monotonic_time/1</c></seealso></p></item> + <item><p><seealso marker="erlang#system_time/0"><c>erlang:system_time/0</c></seealso></p></item> + <item><p><seealso marker="erlang#system_time/1"><c>erlang:system_time/1</c></seealso></p></item> + <item><p><seealso marker="erlang#time_offset/0"><c>erlang:time_offset/0</c></seealso></p></item> + <item><p><seealso marker="erlang#time_offset/1"><c>erlang:time_offset/1</c></seealso></p></item> + <item><p><seealso marker="erlang#timestamp/0"><c>erlang:timestamp/0</c></seealso></p></item> + <item><p><seealso marker="erlang#unique_integer/0"><c>erlang:unique_integer/0</c></seealso></p></item> + <item><p><seealso marker="erlang#unique_integer/1"><c>erlang:unique_integer/1</c></seealso></p></item> + <item><p><seealso marker="kernel:os#system_time/0"><c>os:system_time/0</c></seealso></p></item> + <item><p><seealso marker="kernel:os#system_time/1"><c>os:system_time/1</c></seealso></p></item> + </list> + <p>and a number of extensions of existing BIFs:</p> + <list> + <item><p><seealso marker="erlang#monitor/2"><c>erlang:monitor(time_offset, clock_service)</c></seealso></p></item> + <item><p><seealso marker="erlang#system_flag_time_offset"><c>erlang:system_flag(time_offset, finalize)</c></seealso></p></item> + <item><p><seealso marker="erlang#system_info_os_monotonic_time_source"><c>erlang:system_info(os_monotonic_time_source)</c></seealso></p></item> + <item><p><seealso marker="erlang#system_info_time_offset"><c>erlang:system_info(time_offset)</c></seealso></p></item> + <item><p><seealso marker="erlang#system_info_time_warp_mode"><c>erlang:system_info(time_warp_mode)</c></seealso></p></item> + <item><p><seealso marker="erlang#system_info_time_correction"><c>erlang:system_info(time_correction)</c></seealso></p></item> + <item><p><seealso marker="erlang#system_info_start_time"><c>erlang:system_info(start_time)</c></seealso></p></item> + </list> + + <marker id="The_New_Erlang_Monotonic_Time"/> + <section> + <title>The New Erlang Monotonic Time</title> + <p>The Erlang monotonic time as such is new as of ERTS + version 7.0. It has been introduced in order to be able + to detach time measurements such as elapsed time from + calender time. It is very common that one is interested + in measuring elapsed time or specifying a time relative + to another point in time without having any need to know + what the involved times are in UTC or any other + globally defined time scale. By introducing a time scale + that has a local definition of where it starts, it is + possible to manage time that do not concern calender + time on that time scale. Erlang monotonic time use + such a time scale with a locally defined start.</p> + + <p>The introduction of Erlang monotonic time gives us + the possibility to adjust the two Erlang times (Erlang + monotonic time and Erlang system time) separately. By + doing this, accuracy of elapsed time does not have to + suffer just because the system time happened to be + wrong at some point in time. Separate adjustments + of the two times are only performed in the time warp + modes, and only fully separated in the + <seealso marker="#Multi_Time_Warp_Mode">multi + time warp mode</seealso>. All other modes than the + multi time warp mode are there for backwards + compatibility reasons, and when using these the + accuracy of Erlang monotonic time suffer since + the adjustments of Erlang monotonic time in these + modes are more or less tied to the Erlang system + time.</p> + + <p>The adjustment of system time could have been made + smother than using a time warp approach, but we think + that would be a bad choice. Since we are able to + express and measure time that aren't connected to + calender time by the use of Erlang monotonic time, it + is better to expose the change in Erlang system time + immediately. This since it makes it possible for the + Erlang applications executing on the system to react + on the change in system time as soon as possible. This + is also more or less exactly how most OSes handle this + (OS monotonic time and OS system time). By adjusting + system time smoothly we would just hide the fact that + system time changed and make it harder for the Erlang + applications to react to the change in a sensible way.</p> + + <p>In order to be able to react to a change in Erlang + system time you have to be able to detect that it + happened. The change in Erlang system time occurs when + current time offset is changed. We have therefore + introduced the possibility to monitor the time offset + using + <seealso marker="erlang#monitor/2"><c>erlang:monitor(time_offset, clock_service)</c></seealso>. A process monitoring the time + offset will be sent a message on the following format + when the time offset is changed:</p> + <code type="none">{'CHANGE', MonitorReference, time_offset, clock_service, NewTimeOffset}</code> + </section> + + <marker id="Unique_Values"/> + <section> + <title>Unique Values</title> + <p>Besides reporting time <c>erlang:now/0</c> also + produce unique and strictly monotonically increasing + values. In order to detach this functionality from + time measurements we have introduced + <seealso marker="erlang#unique_integer/1"><c>erlang:unique_integer()</c></seealso>. + </p> + </section> + + <marker id="Dos_and_Donts"/> + <section> + <title>Dos and Don'ts</title> + <p>Previously <c>erlang:now/0</c> was the only option for doing + quite a lot of things. We will look at a few different things + <c>erlang:now/0</c> could be used for, and how you want to do + this using the new API:</p> + + <marker id="Dos_and_Donts_Retrieve_Erlang_System_Time"/> + <section> + <title>Retrieve Erlang System Time</title> + <dont> + <p> + use <c>erlang:now/0</c> in order to retrieve current Erlang + system time. + </p> + </dont> + <do> + <p> + use + <seealso marker="erlang#system_time/1"><c>erlang:system_time/1</c></seealso> + in order to retrieve current Erlang system time on the + <seealso marker="erlang#type_time_unit">time unit</seealso> + of your choice.</p> + <p>If you want the same format as returned by <c>erlang:now/0</c>, use + <seealso marker="erlang#timestamp/0"><c>erlang:timestamp/0</c></seealso>. + </p> + </do> + </section> + + <marker id="Dos_and_Donts_Measure_Elapsed_Time"/> + <section> + <title>Measure Elapsed Time</title> + <dont> + <p> + take timestamps with <c>erlang:now/0</c> and calculate + the difference in time with + <seealso marker="stdlib:timer#now_diff/2"><c>timer:now_diff/2</c></seealso>. + </p> + </dont> + <do> + <p> + take timestamps with + <seealso marker="erlang#monotonic_time/0"><c>erlang:monotonic_time/0</c></seealso> + and calculate the time difference using ordinary subtraction. + The result will be in <c>native</c> + <seealso marker="erlang#type_time_unit">time unit</seealso>. + If you want to convert the + result to another time unit you can do this using + <seealso marker="erlang#convert_time_unit/3"><c>erlang:convert_time_unit/3</c></seealso>. + </p> + <p>Another easier way of doing this is to use + <seealso marker="erlang#monotonic_time/1"><c>erlang:monotonic_time/1</c></seealso> + with desired time unit. However, you may lose accuracy, + and precision this way. + </p> + </do> + </section> + + <marker id="Dos_and_Donts_Determine_Order_of_Events"/> + <section> + <title>Determine Order of Events</title> + <dont> + <p> + determine the order of events by saving a timestamp + with <c>erlang:now/0</c> when the event happens. + </p> + </dont> + <do> + <p> + determine the order of events by saving the integer + returned by + <seealso marker="erlang#unique_integer/1"><c>erlang:unique_integer([monotonic])</c></seealso> + when the event happens. These integers will be strictly + monotonically ordered on current runtime system instance + corresponding to creation time. + </p> + </do> + </section> + + <marker id="Dos_and_Donts_Determine_Order_of_Events_With_Time_of_the_Event"/> + <section> + <title>Determine Order of Events With Time of the Event</title> + <dont> + <p> + determine the order of events by saving a timestamp + with <c>erlang:now/0</c> when the event happens. + </p> + </dont> + <do> + <p> + determine the order of events by saving a tuple + containing + <seealso marker="erlang#monotonic_time/0">monotonic time</seealso> + and a <seealso marker="erlang#unique_integer/1">strictly + monotonically increasing integer</seealso> like this:</p> + <code type="none"> +Time = erlang:monotonic_time(), +UMI = erlang:unique_integer([monotonic]), +EventTag = {Time, UMI}</code> + <p>These tuples will be strictly monotonically ordered + on the current runtime system instance according to + creation time. Note that it is important that the + monotonic time is in the first element (the most + significant element when comparing 2-tuples). Using + the monotonic time in the tuples, you can calculate time + between events.</p> + <p>If you are interested in the Erlang system time at the + time when the event occurred you can also save the time + offset before or after saving the events using + <seealso marker="erlang#time_offset/0"><c>erlang:time_offset/0</c></seealso>. + Erlang monotonic time added with the time + offset corresponds to Erlang system time.</p> + <p>If you are executing in a mode where time offset + may change and you want to be able to get the actual + Erlang system time when the event occurred you can + save the time offset as a third element in the tuple + (the least significant element when comparing 3-tuples).</p> + </do> + </section> + + <marker id="Dos_and_Donts_Create_a_Unique_Name"/> + <section> + <title>Create a Unique Name</title> + <dont> + <p> + use the values returned from <c>erlang:now/0</c> + in order to create a name unique on the current + runtime system instance. + </p> + </dont> + <do> + <p> + use the value returned from + <seealso marker="erlang#unique_integer/0"><c>erlang:unique_integer/0</c></seealso> + in order to create a name unique on the current runtime system + instance. If you only want positive integers, you can use + <seealso marker="erlang#unique_integer/1"><c>erlang:unique_integer([positive])</c></seealso>. + </p> + </do> + </section> + + <marker id="Dos_and_Donts_Seed_Random_Number_Generation_With_a_Unique_Value"/> + <section> + <title>Seed Random Number Generation With a Unique Value</title> + <dont> + <p> + seed random number generation using <c>erlang:now()</c>. + </p> + </dont> + <do> + <p> + seed random number generation using a combination of + <seealso marker="erlang#monotonic_time/0"><c>erlang:monotonic_time()</c></seealso>, + <seealso marker="erlang#time_offset/0"><c>erlang:time_offset()</c></seealso>, + <seealso marker="erlang#unique_integer/0"><c>erlang:unique_integer()</c></seealso>, and other functionality. + </p> + </do> + </section> + + <p>To sum this section up: <em>Don't use <c>erlang:now/0</c>!</em></p> + </section> </section> + + <marker id="Supporting_Both_New_and_Old_OTP_Releases"/> <section> - <title>Turning off time correction</title> - <p>If, for some reason, time correction causes trouble and you are - absolutely confident that the wall clock on the system is nearly - perfect, you can turn off time correction completely by giving the - <c>+c</c> option to <c>erl</c>. The probability for this being a - good idea, is very low.</p> + <title>Supporting Both New and Old OTP Releases</title> + <p>Your code may be required to be able to run on a variety + of OTP installations of different OTP releases. If so, you + can not just use the new API out of the box, since it will + not be available on old pre OTP 18 releases. The solution + is <em>not</em> to avoid using the new API, since your + code then won't be able to benefit from the scalability + and accuracy improvements made. Instead you want to use the + new API when available, and fall back on <c>erlang:now/0</c> + when it is not available. Fortunately almost all of the new + API can easily be implemented using existing primitives + (except for + <seealso marker="erlang#system_info_start_time"><c>erlang:system_info(start_time)</c></seealso>, and + <seealso marker="erlang#system_info_os_monotonic_time_source"><c>erlang:system_info(os_monotonic_time_source)</c></seealso>). + By wrapping the API with functions that fall back on + <c>erlang:now/0</c> when the new API is not available, + and using these wrappers instead of using the API directly + the problem is solved. These wrappers can for example + be implemented as in + <url href="time_compat.erl"><c>$ERL_TOP/erts/example/time_compat.erl</c></url>.</p> </section> </chapter> - |