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Re: pthread_cond_timedwait with setclock(CLOCK_MONOTONIC) times out early




On Nov 26 10:47, James E. King III wrote:
> On Mon, Nov 26, 2018 at 10:35 AM Corinna Vinschen
> <corinna-cygwin@xxxxxxxxxx> wrote:
> >
> > 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:
> > [...]
> > 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.
> >
> > Thoughts?
> >
> >
> > Thanks,
> > Corinna
> >
> > (*) https://docs.microsoft.com/en-us/windows/desktop/Sync/wait-functions#wait-functions-and-time-out-intervals
> >
> > --
> > Corinna Vinschen
> > Cygwin Maintainer
> 
> Some thoughts:
> 
> https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/thread.cc;h=0bddaf345d255ae39187458dc6d02b1b4c8087c1;hb=HEAD#l2546
> 
> In pthread_convert_abstime, line 2564, care is taken to adjust for
> rounding errors.
> 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?

Au contraire:

- The end time you're waiting for is rounded *up*.
- The current time is rounded *down*
- So end time - current time is always bigger than required
  on the 100ns level.

Make sense?


Corinna

-- 
Corinna Vinschen
Cygwin Maintainer

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