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Re: clock() time travel.
- From: "Carlos O'Donell" <carlos at redhat dot com>
- To: PaweÅ Sikora <pluto at agmk dot net>
- Cc: libc-help at sourceware dot org
- Date: Thu, 16 Jan 2014 12:35:00 -0500
- Subject: Re: clock() time travel.
- Authentication-results: sourceware.org; auth=none
- References: <2a73b22e707bbe6dce3871aacf7d8a69 at agmk dot net> <52D80B1F dot 6070308 at redhat dot com> <6559318 dot s3zxh1qg9A at localhost dot localdomain>
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.
Cheers,
Carlos.