Messages in this thread | | | Date | Fri, 8 May 2020 22:18:59 +0200 | From | Pali Rohár <> | Subject | Missing CLOCK_BOOTTIME_RAW? |
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Hello Thomas!
I'm looking at clocks which are provided by kernel for userspace applications via clock_gettime() syscall and I think that there is missing clock variant CLOCK_BOOTTIME_RAW.
If userspace application wants to measure time difference between two events then I think all of available clocks CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_MONOTONIC_RAW and CLOCK_BOOTTIME have some issues for such task.
CLOCK_REALTIME may be changed but userspace and therefore time difference between two events may be also negative.
CLOCK_MONOTONIC does not have to precise as there may be incremental jumps caused by NTP or measured time difference may be smaller as elapsed when system is suspended. During system suspend is clock CLOCK_MONOTONIC stopped.
CLOCK_MONOTONIC_RAW is not affected by NTP jumps but still is not suitable for measuring time differences when system is suspended.
CLOCK_BOOTTIME is affected by NTP jumps but provides correct time difference when system was suspended during measurement.
So for me it looks like that there is missing CLOCK_BOOTTIME_RAW clock which would not be affected by NTP jumps (like CLOCK_MONOTONIC_RAW) and also would not be affected by system suspend (like CLOCK_BOOTTIME).
Please correct me if I'm wrong in some of my above assumptions. It is how I understood documentation for clock_gettime() function from Linux manpage.
Is there any reason why kernel does not provide such CLOCK_BOOTTIME_RAW clock for userspace applications which would be interested in measuring time difference which occurred between two events?
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