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diff --git a/erts/doc/src/time_correction.xml b/erts/doc/src/time_correction.xml index 7f7c28fc30..8af98acc19 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 +31,209 @@ <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="Time_Resolution"/> + <section> + <title>Time Resolution</title> + <p>The shortest time interval that can be distinguished when + reading time values.</p> + </section> + + <marker id="Time_Precision"/> + <section> + <title>Time Precision</title> + <p>The shortest time interval that can be 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> + </section> + + <marker id="Time_Accuracy"/> + <section> + <title>Time Accuracy</title> + <p>The correctness of time values.</p> + </section> + + <marker id="Time_Warp"/> + <section> + <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> + </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, <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 + <seealso marker="erlang#system_info_os_system_time_source"><c>erlang:system_info(os_system_time_source)</c></seealso>.</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. You can get information about the Erlang + runtime system's source of OS monotonic time by calling + <seealso marker="erlang#system_info_os_monotonic_time_source"><c>erlang:system_info(os_monotonic_time_source)</c></seealso>.</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 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 time perform a <seealso marker="#Time_Warp">time + warp</seealso>.</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 + <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 + 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> + + </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 +286,592 @@ 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 <seealso marker="#Time_Warp">time warp</seealso> 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 <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 + 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 <em>much</em> 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, 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> + + <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 + 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. + 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 + 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_os_system_time_source"><c>erlang:system_info(os_system_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>, + <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 + be implemented as in + <url href="time_compat.erl"><c>$ERL_TOP/erts/example/time_compat.erl</c></url>.</p> </section> </chapter> - |