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Re: clock() time travel.


On Thursday 16 of January 2014 12:35:00 Carlos O'Donell wrote:
> On 01/16/2014 12:15 PM, PaweÅ Sikora wrote:
> > On Thursday 16 of January 2014 11:38:55 Carlos O'Donell wrote:
> >> On 01/16/2014 05:54 AM, PaweÅ Sikora wrote:> Hi,
> >> 
> >>> i've observed on my i3-540 cpu that subsequent clock() calls *sometimes*
> >>> give smaller number of ticks than previous one. is it a known issue?
> >>> 
> >>> BR,
> >>> PaweÅ.
> >>> 
> >>> % ./timing
> >>> t[current]: 10713902 < t[previous]: 10713903
> >>> zsh: abort (core dumped)  ./timing
> >> 
> >> This is either a compiler or kernel bug.
> >> 
> >> On glibc click() is just clock_gettime with
> >> CLOCK_PROCESS_CPUTIME_ID followed by the appropriate
> >> divisions to get the correctly rounded result.
> > 
> > hmm, there's interesting note in clock_gettime() manual not metioned
> > in clock() manual.
> > 
> >    "Note for SMP systems
> >    
> >        The  CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks
> >        are
> >        realized on many platforms using timers from the CPUs (TSC on i386,
> >        AR.ITC on Itanium). These registers may differ between CPUs and as
> >        a consequence these clocks may return bogus results if a process is
> >        migrated to another CPU. (....)"
> > 
> > i'm using an intel-i3 (1 processor, 4 cores), so probably subsequent
> > clock() snapshots in my testcase contain slightly different values from
> > different tsc registers.
> 
> IMO that's a kernel bug, but the kernel might disagree.
> 
> Either way you have no guarantee of monotonicity anyway.
> 
> You need to use clock_gettime and CLOCK_MONOTONIC.

i need to measure an overall cpu execution time with something like
a monotonic-process-cputime. an abstract monotonic timer which can
be speedup/slowdown via ntp updates is not what i really need.

maybe i should bind my process to a single cpu core with sched_setaffinity()
for reliable clock_gettime(CLOCK_PROCESS_CPUTIME_ID) results?

benchmarking is really paintful in these days :)


BR,
PaweÅ.


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