Re: pthread_cond_timedwait with setclock(CLOCK_MONOTONIC) times out early
- Date: Mon, 26 Nov 2018 10:47:38 -0500
- From: "James E. King III" <jking@xxxxxxxxxx>
- Subject: Re: pthread_cond_timedwait with setclock(CLOCK_MONOTONIC) times out early
On Mon, Nov 26, 2018 at 10:35 AM Corinna Vinschen
> On Nov 25 09:01, James E. King III wrote:
> > I have isolated a problem in pthread_cond_timedwait when the condattr
> > is used to set the clock type to CLOCK_MONOTONIC. In this case even
> > though a target time point in the future is specified, the call
> > returns ETIMEDOUT but a subsequent call to
> > clock_gettime(CLOCK_MONOTONIC) shows the desired time point was not
> > reached.
> > $ gcc timed_wait_short.c -o timed_wait_short
> > $ ./timed_wait_short.exe
> > [...]
> > begin: 521056s 671907500n
> > target: 521056s 721907500n
> > end: 521056s 721578000n
> > ok: false
> > I have attached the source code.
> Thanks for the testcase. The problem is this:
> The underlying implementation uses a Windows waitable time set to
> implement the timeout. In case of a CLOCK_REALTIME timer, we can use
> the given absolut timestamp in 100ns resolution for the timer.
> On the other hand, the CLOCK_MONOTONIC timer is not running in absolut
> time but uses the hi-res timestamps generated by QueryPerformanceCounter.
> The perf counter uses an arbitrary "ticks per second" unit which is
> converted to nsecs on the fly on the POSIX API level. However, perf
> counters are not waitable objects, only waitable timers are, so we have
> to use the perf timer values to prime a waitable timer evetually.
> The side effect is that we have to use relative offset under the hood as
> soon as the base timer is CLOCK_MONOTONIC, since there's no direct
> relation to the absolute system time as used by the waitable timer in
> absolute mode.
> Combine this with the inaccuracy of the Windows waitable timer and wait
> functions in general(*) and you know what uphill battle accuracy is in
> this scenario.
> Having said that, I don't have a *good*, reliable solution to this
> At the moment I only have an *ugly* idea: We can always add the
> coarsest resolution of the wait functions (typically 15.625 ms) to the
> relative timeout value computed from the absolute timeout given to
> pthread_cond_timedwait. In my testing this is sufficient since the
> difference between target and actual end time is always < 15ms, in
> thousands of runs.
> (*) https://docs.microsoft.com/en-us/windows/desktop/Sync/wait-functions#wait-functions-and-time-out-intervals
> Corinna Vinschen
> Cygwin Maintainer
In pthread_convert_abstime, line 2564, care is taken to adjust for
At line 2574, the rounding is not accounted for when adjusting for a
relative wait because it is a monotonic clock.
Wouldn't that rounding error cause it to wait less time?
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