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| -rw-r--r-- | erts/doc/src/time_correction.xml | 764 |
1 files changed, 393 insertions, 371 deletions
diff --git a/erts/doc/src/time_correction.xml b/erts/doc/src/time_correction.xml index aed38fbb92..aec9efa3d3 100644 --- a/erts/doc/src/time_correction.xml +++ b/erts/doc/src/time_correction.xml @@ -35,31 +35,35 @@ <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 + <note><p>As of OTP 18 (<c>ERTS</c> version 7.0) the time functionality of + Erlang has been extended. This includes a <seealso marker="#The_New_Time_API">new API</seealso> - for time, as well as + for time and <seealso marker="#Time_Warp_Modes">time warp - modes</seealso> which alters the behavior of the system when + modes</seealso> that alter the system behavior 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 + old API still works. Thus, 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 + <c>erlang:now/0</c> is deprecated, as it is and + will be a scalability bottleneck.</p> + + <p>By using the new API, you 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> + also enables you to use the + <seealso marker="#Multi_Time_Warp_Mode">multi-time warp mode</seealso> + that 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 + <title>Terminology</title> + <p>To make it easier to understand this section, some terms + are defined. This is a mix of our own terminology (Erlang/OS system time, Erlang/OS monotonic time, time warp) and globally accepted terminology.</p> @@ -67,7 +71,7 @@ <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 + that have a predecessor are either larger than or equal to its predecessor.</p> </section> @@ -82,19 +86,19 @@ <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> + <p>Universal Time. UT1 is based on the rotation of the earth + and conceptually means 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 + <p>Coordinated Universal Time. UTC almost aligns with + <seealso marker="#UT1">UT1</seealso>. However, UTC uses the + SI definition of a second, which has not exactly 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 + UT1. To keep UTC relatively in sync with UT1, leap seconds + are inserted, and potentially also deleted. That is, an UTC day can be 86400, 86401, or 86399 seconds long.</p> </section> @@ -104,14 +108,15 @@ <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. + 1970-01-01. <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 + Epoch is defined to be a time in UTC, and UTC has 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 + <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 + last second. If an UTC leap second would be deleted (which has not happened yet), POSIX time would make a one second leap forward.</p> </section> @@ -125,11 +130,11 @@ <marker id="Time_Precision"/> <section> <title>Time Precision</title> - <p>The shortest time interval that can be be distinguished + <p>The shortest time interval that can be distinguished repeatedly and reliably when reading time values. Precision is limited by the <seealso marker="#Time_Resolution">resolution</seealso>, but - resolution and precision might differ significantly.</p> + resolution and precision can differ significantly.</p> </section> <marker id="Time_Accuracy"/> @@ -143,21 +148,23 @@ <title>Time Warp</title> <p>A time warp is a leap forwards or backwards in time. That is, the difference of time values taken before and after the - time warp will not correspond to the actual elapsed time.</p> + time warp does not correspond to the actual elapsed time.</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="#POSIX_Time">POSIX time</seealso>. To + retrieve it, call <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, <seealso marker="#Time_Warp">time warps</seealso> - may be observed. You can get information about the Erlang runtime - system's source of OS system time by calling + may be observed.</p> + + <p>To get information about the Erlang runtime + system's source of OS system time, call <seealso marker="erlang#system_info_os_system_time_source"><c>erlang:system_info(os_system_time_source)</c></seealso>.</p> </section> @@ -165,15 +172,17 @@ <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 + system. This time does not leap and has 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 + uncommon that OS monotonic time stops if the system is + suspended. This time typically increases 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. You can get information about the Erlang - runtime system's source of OS monotonic time by calling + <seealso marker="#OS_System_Time">OS system time</seealso>. + This type of time is not necessarily provided by all + operating systems.</p> + + <p>To get information about the Erlang + runtime system's source of OS monotonic time, call <seealso marker="erlang#system_info_os_monotonic_time_source"><c>erlang:system_info(os_monotonic_time_source)</c></seealso>.</p> </section> @@ -181,14 +190,17 @@ <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 + <seealso marker="#POSIX_Time">POSIX time</seealso>. To + retrieve it, call + <seealso marker="erlang#system_time/0"><c>erlang:system_time()</c></seealso>.</p> + + <p>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 runtime system works towards aligning the two - system times. Depending on <seealso marker="#Time_Warp_Modes">time - warp mode</seealso> used, this may be achieved by letting the Erlang + system times. Depending on the + <seealso marker="#Time_Warp_Modes">time warp mode</seealso> used, + this can be achieved by letting Erlang system time perform a <seealso marker="#Time_Warp">time warp</seealso>.</p> </section> @@ -197,35 +209,43 @@ <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 + Erlang runtime system. Erlang monotonic time increases since + some unspecified point in time. To retrieve it, call <seealso marker="erlang#monotonic_time/0"><c>erlang:monotonic_time()</c></seealso>. - The - <seealso marker="#Time_Accuracy">accuracy</seealso>, and + </p> + + <p>The <seealso marker="#Time_Accuracy">accuracy</seealso> and <seealso marker="#Time_Precision">precision</seealso> 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 + monotonic time heavily depends on the following:</p> + + <list type="bulleted"> + <item>Accuracy and precision of + <seealso marker="#OS_Monotonic_Time">OS monotonic time</seealso> + </item> + <item>Accuracy and precision of + <seealso marker="#OS_System_Time">OS system time</seealso> + </item> + <item><seealso marker="#Time_Warp_Modes">time warp mode</seealso> used + </item> + </list> + + <p>On a system without OS monotonic time, Erlang monotonic + time guarantees monotonicity, but cannot give + other guarantees. The frequency adjustments made to + Erlang monotonic time depend on the time warp mode used.</p> + + <p>Internally in the runtime system, Erlang monotonic time is the "time engine" that is used for more or less - everything that has anything to do with time. All timers + 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 + 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 + offset, you get current Erlang system time.</p> + + <p>To retrieve current time offset, call <seealso marker="erlang#time_offset/0"><c>erlang:time_offset/0</c></seealso>. </p> </section> @@ -234,176 +254,169 @@ <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 - time in our programs, the Virtual Machine and the language has to be - very careful about what is considered a correct point in time and in + soft real-time properties and we can express + time in our programs, the Virtual Machine and the language must be + careful about what is considered a correct time and in how time functions behave.</p> - <p>In the beginning, Erlang was constructed assuming that the wall + <p>When Erlang was designed, it was assumed that the wall clock time in the system showed a monotonic time moving forward at - exactly the same pace as the definition of time. That more or less - meant that an atomic clock (or better) was expected to be attached + exactly the same pace as the definition of time. This more or less meant + that an atomic clock (or better time source) was expected to be attached to your hardware and that the hardware was then expected to be - locked away from any human (or unearthly) tinkering for all - eternity. While this might be a compelling thought, it's simply - never the case.</p> - - <p>A "normal" modern computer can not keep time. Not on itself and - not unless you actually have a chip level atomic clock wired to - it. Time, as perceived by your computer, will normally need to be - corrected. Hence the NTP protocol that together with the ntpd - process will do it's best to keep your computers time in sync with - the "real" time in the universe. Between NTP corrections, usually a + locked away from any human tinkering forever. While this can be a + compelling thought, it is simply never the case.</p> + + <p>A "normal" modern computer cannot keep time, not on itself and + not unless you have a chip-level atomic clock wired to it. Time, + as perceived by your computer, must normally be corrected. Hence + the Network Time Protocol (NTP) protocol, together with the <c>ntpd</c> + process, does its best to keep your computer time in sync with + the correct time. Between NTP corrections, usually a less potent time-keeper than an atomic clock is used.</p> - <p>But NTP is not fail safe. The NTP server can be unavailable, the - ntp.conf can be wrongly configured or your computer may from time to - time be disconnected from the internet. Furthermore you can have a - user (or even system administrator) on your system that thinks the - right way to handle daylight saving time is to adjust the clock one - hour two times a year (a tip, that is not the right way to do - it...). To further complicate things, this user fetched your - software from the internet and has never ever thought about what's - the correct time as perceived by a computer. The user simply does - not care about keeping the wall clock in sync with the rest of the - universe. The user expects your program to have omnipotent knowledge + <p>However, NTP is not fail-safe. The NTP server can be unavailable, + <c>ntp.conf</c> can be wrongly configured, or your computer may + sometimes be disconnected from Internet. Furthermore, you can have a + user (or even system administrator) who thinks the correct + way to handle Daylight Saving Time is to adjust the clock one + hour two times a year (which is the incorrect way to do it). + To complicate things further, this user fetched your + software from Internet and has not considered what + the correct time is as perceived by a computer. The user does + not care about keeping the wall clock in sync with the correct + time. The user expects your program to have unlimited knowledge about the time.</p> <p>Most programmers also expect time to be reliable, at least until - they realize that the wall clock time on their workstation is of by - a minute. Then they simply set it to the correct time, maybe or - maybe not in a smooth way. Most probably not in a smooth way.</p> + they realize that the wall clock time on their workstation is off by + a minute. Then they set it to the correct time, but most probably + not in a smooth way.</p> - <p>The amount of problems that arise when you expect the wall clock - time on the system to always be correct may be immense. Therefore Erlang + <p>The number of problems that arise when you always expect the wall clock + time on the system to be correct can be immense. Erlang therefore introduced the "corrected estimate of time", or the "time - correction" many years ago. The time correction relies on the fact + correction", many years ago. The time correction relies on the fact that most operating systems have some kind of monotonic clock, - either a real time extension or some built in "tick counter" that is - independent of the wall clock settings. This counter may have - microsecond resolution or much less, but generally it has a drift - that is not to be ignored.</p> - + either a real-time extension or some built-in "tick counter" that is + independent of the wall clock settings. This counter can have + microsecond resolution or much less, but it has a drift that cannot + be ignored.</p> </section> - <marker id="Time_Correction"/> <section> + <marker id="Time_Correction"/> <title>Time Correction</title> <p>If time correction is enabled, the Erlang runtime system - will make use of both + makes 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 + to adjust the frequency of the Erlang + monotonic clock. Time correction ensures 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 + does not warp and that the frequency is relatively accurate. + The type of frequency adjustments depends on the time warp mode used. + Section <seealso marker="#Time_Warp_Modes">Time Warp Modes</seealso> + provides more details.</p> + + <p>By default time correction is enabled if support for + it exists on the specific platform. Support for it includes + both OS monotonic time, provided by the OS, and an + implementation in the Erlang runtime system using + OS monotonic time. To check if your system has support + for OS monotonic time, call + <seealso marker="erlang#system_info_os_monotonic_time_source"><c>erlang:system_info(os_monotonic_time_source)</c></seealso>. + To check if time correction is enabled on your system, call <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 + <p>To enable or disable time correction, pass command-line argument <seealso marker="erl#+c"><c>+c [true|false]</c></seealso> - command line argument to <c>erl</c>.</p> + 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 + may warp forwards or stop, or even freeze for extended + periods of time. There are then 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, + Previously a performance penalty was associated with time + correction, but nowadays it is usually the other way around. + If time correction is disabled, you probably get bad scalability, bad performance, and bad time measurements.</p> </section> - - <marker id="Time_Warp_Safe_Code"/> <section> + <marker id="Time_Warp_Safe_Code"/> <title>Time Warp Safe Code</title> - <p>Time warp safe code is code that is able to handle + <p>Time warp safe code can handle a <seealso marker="#Time_Warp">time warp</seealso> of - <seealso marker="#Erlang_System_Time">Erlang system time</seealso>. - </p> + <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> + behaves bad when Erlang system time warps. When Erlang + system time does a time warp backwards, the values returned + from <c>erlang:now/0</c> freeze (if you disregard the + microsecond increments made because of the actual call) until + OS system time reaches the point of the last value returned by + <c>erlang:now/0</c>. This freeze can continue for a long time. It + can take years, decades, and even longer 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 + is 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> + the use of it with other functionality. For examples + of how to replace the use of <c>erlang:now/0</c>, see Section + <seealso marker="#Dos_and_Donts">How to Work with the New + API</seealso>.</p> </section> - <marker id="Time_Warp_Modes"/> <section> <title>Time Warp Modes</title> - + <marker id="Time_Warp_Modes"/> <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 + warp mode you use.</p> + + <p>To set the time warp mode, pass command-line argument <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> + 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 + and does not change later. This is the default behavior, but + not because it is the best mode (which it is not). It is default <em>only</em> because this is how the runtime system - always has behaved up until ERTS version 7.0, and you have to - ensure that your Erlang code that may execute during a time + behaved until <c>ERTS</c> 7.0. + 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> + safe</seealso> before enabling 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 + <p>As the time offset is not allowed to change, time + correction must adjust the frequency of the Erlang + monotonic clock to align Erlang system time with OS + system time smoothly. A significant 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 + error can be as large 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> + <p>If time correction is not enabled, Erlang monotonic + time freezes when OS system time leaps backwards. + The freeze of monotonic time continues until + OS system time catches up. The freeze can continue for + a long time. When OS system time leaps forwards, + Erlang monotonic time also leaps forward.</p> </section> <marker id="Single_Time_Warp_Mode"/> @@ -411,26 +424,27 @@ <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 + has no power supply, not even a battery, when it is + shut off. The system clock on such a system is typically + way off when the system boots. If <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 + OS system time has been corrected, Erlang system time + can be wrong for a long time, centuries or even longer.</p> + + <p>If you 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 + and you need to start the Erlang runtime system before 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 + time warp mode.</p> + + <note><p>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 <em>much</em> better - to use the <seealso marker="#Multi_Time_Warp_Mode">multi time - warp mode</seealso> instead. - </p> + to use time warp safe code only, it is <em>much</em> better + to use the <seealso marker="#Multi_Time_Warp_Mode">multi-time + warp mode</seealso> instead.</p></note> <p>Using the single time warp mode, the time offset is handled in two phases:</p> @@ -438,158 +452,150 @@ <taglist> <tag>Preliminary Phase</tag> <item> - <p>The preliminary phase starts when the runtime + <p>This 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> + current OS system time is determined. This offset is from + now on to be fixed during the whole preliminary phase.</p> <p>If time correction is enabled, adjustments to the - Erlang monotonic clock will be made to keep its - frequency as correct as possible, but <em>no</em> - adjustments will be made trying to align Erlang system - time and OS system time. That is, during the preliminary - Erlang system time and OS system time might diverge - from each other, and no attempt to prevent this will - be made.</p> + Erlang monotonic clock are made to keep its + frequency as correct as possible. However, <em>no</em> + adjustments are made trying to align Erlang system + time and OS system time. That is, during the preliminary phase + Erlang system time and OS system time can diverge + from each other, and no attempt is made to prevent this.</p> <p>If time correction is disabled, changes in OS system - time will effect the monotonic clock the same way as + time affects 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 + <p>This phase begins when the user finalizes 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> + 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 - change during the finalization, 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. + fixated so that current Erlang system time aligns with + current OS system time. As the time offset can + change during the finalization, Erlang system time + can do a time warp at this point. The time offset is + from now on fixed until the runtime system terminates. If time correction has been enabled, the time - correction will from now on also make adjustments - in order to align Erlang system time with OS system - time. When the system is in the final phase it behaves + correction from now on also makes adjustments + to align Erlang system time with OS system + time. 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> + <p>In order for this to work properly, the user must ensure + that the following two requirements 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 + This implies that the user must ensure that 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, + POSIX time before starting the Erlang runtime system.</p> + + <p>If you are not sure that 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> + 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> + <item><p>OS system time must be correct when + 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 + may behave very bad.</p> + + <p>Assuming that these requirements are fulfilled, + time correction is enabled, and that OS system time + is adjusted using a time adjustment protocol such as NTP, + only small adjustments of Erlang monotonic + time are needed to keep system times + aligned after finalization. As long as the system is not + suspended, the largest adjustments needed are 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 + <warning><p>To use this mode, 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 + <p>Code executing only 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> + <title>Multi-Time Warp Mode</title> + <p><em>Multi-time warp mode in combination with time + correction is the preferred configuration</em>. This as + the Erlang runtime system have better performance, scale + better, and behave better on almost all platforms. In + addition, the accuracy and precision of time measurements + are better. Only Erlang runtime systems executing on + ancient platforms 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 + forwards and backwards at <em>any</em> time. As we align + Erlang system time with 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> + correct as possible. This makes time measurements using + 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 + leaps forward if OS system time leaps forward. If + OS system time leaps backwards, Erlang monotonic time + stops briefly, but it does not freeze for extended periods + of time. This as the time offset is changed 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 + <warning><p>To use this mode, 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>The New Time API</title> - + <title>New Time API</title> + <marker id="The_New_Time_API"/> <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 + <c>erlang:now/0</c> was intended to be used for many unrelated + things. This tied these unrelated operations together and + caused issues with performance, scalability, accuracy, and + precision for operations that did not need to have + such issues. To improve this, the new API spreads different + functionality over multiple functions.</p> + + <p>To be backwards compatible, <c>erlang:now/0</c> + remains as is, but <em>you are strongly discouraged from using + it</em>. Much use of <c>erlang:now/0</c> + prevents 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> + return negative integer values on a newly started runtime + system. This is not a bug, but a memory use optimization.</p> + + <p>The new API consists of the following new BIFs:</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> @@ -604,7 +610,9 @@ <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> + + <p>The new API also consists of extensions of the following 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> @@ -619,102 +627,99 @@ <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 + <title>New Erlang Monotonic Time</title> + <p>Erlang monotonic time as such is new as of <c>ERTS</c> 7.0. + It is introduced to detach time measurements, such as elapsed + time from calendar time. Many programmers want to measure elapsed + time or specify a time relative to another point in time without + knowing the involved times in UTC or any other globally defined + time scale. By introducing a time scale + with a local definition of where it starts, time that do + not concern calendar time on that time scale can be managed. + Erlang monotonic time uses 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 + <p>The introduction of Erlang monotonic time allows + us 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 + doing this, the 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 + <seealso marker="#Multi_Time_Warp_Mode">multi-time + warp mode</seealso>. All other modes than the + multi-time warp mode are for backwards + compatibility reasons. When using these modes, the + accuracy of Erlang monotonic time suffer, as the adjustments of Erlang monotonic time in these - modes are more or less tied to the Erlang system - time.</p> + modes are more or less tied to 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 + that would be a bad choice. As we can + express and measure time that is not connected to + calendar 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 + immediately. This as the Erlang applications + executing on the system can react on the change in + system time as soon as possible. This is also more or + less exactly how most operating systems handle this (OS monotonic time and OS system time). By adjusting - system time smoothly we would just hide the fact that + 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 + <p>To be able to react to a change in Erlang + system time, you must 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 + 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 is 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 + <p>Besides reporting time, <c>erlang:now/0</c> also + produces unique and strictly monotonically increasing + values. 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> + <title>How to Work with the New API</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> + many things. This section deals with some things that + <c>erlang:now/0</c> can be used for, and how you are to + these 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. + Use <c>erlang:now/0</c> to retrieve current Erlang system time. </p> </dont> <do> <p> - use + Use <seealso marker="erlang#system_time/1"><c>erlang:system_time/1</c></seealso> - in order to retrieve current Erlang system time on the + 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> + </p> </do> </section> @@ -723,26 +728,27 @@ <title>Measure Elapsed Time</title> <dont> <p> - take timestamps with <c>erlang:now/0</c> and calculate + 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 + 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 + result to another time unit, you can use <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 + + <p>An easier way to do 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. + with the desired time unit. However, you can then lose accuracy + and precision. </p> </do> </section> @@ -752,16 +758,16 @@ <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. + Determine the order of events by saving a timestamp + with <c>erlang:now/0</c> when the event occurs. </p> </dont> <do> <p> - determine the order of events by saving the integer + 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 + when the event occurs. These integers will be strictly monotonically ordered on current runtime system instance corresponding to creation time. </p> @@ -770,40 +776,43 @@ <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> + <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. + Determine the order of events by saving a timestamp + with <c>erlang:now/0</c> when the event occurs. </p> </dont> <do> <p> - determine the order of events by saving a tuple - containing + 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> + monotonically increasing integer</seealso> as follows:</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 + on current runtime system instance according to + creation time. 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 + + <p>If you are interested in 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 + can change, and you want 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> @@ -814,16 +823,15 @@ EventTag = {Time, UMI}</code> <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. + Use the values returned from <c>erlang:now/0</c> + to create a name unique on the current runtime system instance. </p> </dont> <do> <p> - use the value returned from + 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 + 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> @@ -832,48 +840,62 @@ EventTag = {Time, UMI}</code> <marker id="Dos_and_Donts_Seed_Random_Number_Generation_With_a_Unique_Value"/> <section> - <title>Seed Random Number Generation With a Unique Value</title> + <title>Seed Random Number Generation with a Unique Value</title> <dont> <p> - seed random number generation using <c>erlang:now()</c>. + Seed random number generation using <c>erlang:now()</c>. </p> </dont> <do> <p> - seed random number generation using a combination of + 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. + <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> + <p>To sum up this section: <em>Do not use <c>erlang:now/0</c>.</em></p> </section> </section> - <marker id="Supporting_Both_New_and_Old_OTP_Releases"/> <section> - <title>Supporting Both New and Old OTP Releases</title> - <p>Your code may be required to be able to run on a variety + <marker id="Supporting_Both_New_and_Old_OTP_Releases"/> + <title>Support of Both New and Old OTP Releases</title> + <p>It can be required that your code must 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 + cannot use the new API out of the box, as 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 + is <em>not</em> to avoid using the new API, as your + code then would not benefit from the scalability + and accuracy improvements made. Instead, 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>, - <seealso marker="erlang#system_info_end_time"><c>erlang:system_info(end_time)</c></seealso>, - <seealso marker="erlang#system_info_os_monotonic_time_source"><c>erlang:system_info(os_monotonic_time_source)</c></seealso>, and - <seealso marker="erlang#system_info_os_system_time_source"><c>erlang:system_info(os_system_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 + when the new API is unavailable.</p> + + <p>Fortunately most of the new API can easily be + implemented using existing primitives, except for:</p> + + <list type="bulleted"> + <item> + <seealso marker="erlang#system_info_start_time"><c>erlang:system_info(start_time)</c></seealso> + </item> + <item> + <seealso marker="erlang#system_info_end_time"><c>erlang:system_info(end_time)</c></seealso> + </item> + <item> + <seealso marker="erlang#system_info_os_monotonic_time_source"><c>erlang:system_info(os_monotonic_time_source)</c></seealso> + </item> + <item> + <seealso marker="erlang#system_info_os_system_time_source"><c>erlang:system_info(os_system_time_source)</c></seealso>) + </item> + </list> + + <p>By wrapping the API with functions that fall back on + <c>erlang:now/0</c> when the new API is unavailable, + 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> |