Difference between CLOCK REALTIME and CLOCK MONOTONIC

Question

Could you explain the difference between CLOCK REALTIME and CLOCK MONOTONIC clocks returned by clock gettime   on Linux   Which is a better choice if I need to compute elapsed time between timestamps produced by an external source and the current time   Lastly  if I have an NTP daemon periodically adjusting system time  how do these adjustments interact with each of CLOCK REALTIME and CLOCK MONOTONIC

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Sorry  no reputation to add this as a comment   So it goes as an complementary answer  Depending on how often you will call clock gettime    you should keep in mind that only some of the  quot clocks quot  are provided by Linux in the VDSO  i e  do not require a syscall with all the overhead of one -- which only got worse when Linux added the defenses to protect against Spectre-like attacks   While clock gettime CLOCK MONOTONIC       clock gettime CLOCK REALTIME       and gettimeofday   are always going to be extremely fast  accelerated by the VDSO   this is not true for  e g  CLOCK MONOTONIC RAW or any of the other POSIX clocks  This can change with kernel version  and architecture  Although most programs don t need to pay attention to this  there can be latency spikes in clocks accelerated by the VDSO  if you hit them right when the kernel is updating the shared memory area with the clock counters  it has to wait for the kernel to finish  Here s the  quot proof quot   GitHub  to keep bots away from kernel org   https   github com torvalds linux commit 2aae950b21e4bc789d1fc6668faf67e8748300b7

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In addition to Ignacio s answer  CLOCK REALTIME can go up forward in leaps  and occasionally backwards  CLOCK MONOTONIC does neither  it just keeps going forwards  although it probably resets at reboot    A robust app needs to be able to tolerate CLOCK REALTIME leaping forwards occasionally  and perhaps backwards very slightly very occasionally  although that is more of an edge-case     Imagine what happens when you suspend your laptop - CLOCK REALTIME jumps forwards following the resume  CLOCK MONOTONIC does not  Try it on a VM

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Robert Love s book LINUX System Programming 2nd Edition  specifically addresses your question at the beginning of Chapter 11  pg 363      The important aspect of a monotonic time source is NOT the current   value  but the guarantee that the time source is strictly linearly   increasing  and thus useful for calculating the difference in time   between two samplings   That said  I believe he is assuming the processes are running on the same instance of an OS  so you might want to have a periodic calibration running to be able to estimate drift

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CLOCK REALTIME is affected by NTP  and can move forwards and backwards  CLOCK MONOTONIC is not  and advances at one tick per tick

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CLOCK REALTIME represents the machine s best-guess as to the current wall-clock  time-of-day time   As Ignacio and MarkR say  this means that CLOCK REALTIME can jump forwards and backwards as the system time-of-day clock is changed  including by NTP   CLOCK MONOTONIC represents the absolute elapsed wall-clock time since some arbitrary  fixed point in the past   It isn t affected by changes in the system time-of-day clock   If you want to compute the elapsed time between two events observed on the one machine without an intervening reboot  CLOCK MONOTONIC is the best option   Note that on Linux  CLOCK MONOTONIC does not measure time spent in suspend  although by the POSIX definition it should   You can use the Linux-specific CLOCK BOOTTIME for a monotonic clock that keeps running during suspend

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POSIX 7 quotes  POSIX 7 specifies both at http   pubs opengroup org onlinepubs 9699919799 functions clock getres html   CLOCK REALTIME      This clock represents the clock measuring real time for the system  For this clock  the values returned by clock gettime   and specified by clock settime   represent the amount of time  in seconds and nanoseconds  since the Epoch    CLOCK MONOTONIC  optional feature       For this clock  the value returned by clock gettime   represents the amount of time  in seconds and nanoseconds  since an unspecified point in the past  for example  system start-up time  or the Epoch   This point does not change after system start-up time  The value of the CLOCK MONOTONIC clock cannot be set via clock settime      clock settime   gives an important hint  POSIX systems are able to arbitrarily change CLOCK REALITME with it  so don t rely on it flowing neither continuously nor forward  NTP could be implemented using clock settime    and could only affect CLOCK REALITME   The Linux kernel implementation seems to take boot time as the epoch for CLOCK MONOTONIC  Starting point for CLOCK MONOTONIC

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There s one big difference between CLOCK REALTIME and MONOTONIC  CLOCK REALTIME can jump forward or backward according to NTP  By default  NTP allows the clock rate to be speeded up or slowed down by up to 0 05   but NTP cannot cause the monotonic clock to jump forward or backward

[linux] Difference between CLOCK_REALTIME and CLOCK_MONOTONIC?

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