Re: [PATCH 0/2] itimers: periodic timers fixes

Stanislaw Gruszka
2009-04-15T12:29:39Z

Hi Ingo.

On Thu, 2 Apr 2009 18:57:53 +0200
Ingo Molnar <mingo@elte.hu> wrote:
> > We found the periodic timers ITIMER_PROF and ITIMER_VIRT are
> > unreliable, they have systematic timing error. For example period
> > of 10000 us will not be represented by the kernel as 10 ticks, but
> > 11 (for HZ=1000). The reason is that the frequency of the hardware
> > timer can only be chosen in discrete steps and the actual
> > frequency is about 1000.152 Hz. So 10 ticks would take only about
> > 9.9985 ms, the kernel decides it must never return earlier than
> > requested, so it rounds the period up to 11 ticks. This results in
> > a systematic multiplicative timing error of -10 %. The situation
> > is even worse where application try to request with 1 thick
> > period. It will get the signal once per two kernel ticks, not on
> > every tick. The systematic multiplicative timing error is -50 %.
> > He have program [1] that shows itimers systematic error, results
> > are below [2].
> >
> > To fix situation we wrote two patches. First one just simplify
> > code related with itimers. Second is fix, it change intervals
> > measurement resolutions and correct times when signal is
> > generated. However this add some drawback, that I'm not sure if
> > are acceptable:
> >
> > - the time between two consecutive tics can be smaller than
> > requested interval
> >
> > - intervals values which are returned to user by getitimer() are
> > not rounded up
> >
> > Second drawback mean that applications which first call
> > setitimer() then call getitimer() to see if interval was round up
> > and to correct timings, will potentially stop works. However this
> > can be only problem with requested interval smaller than 1/HZ, as
> > for intervals > 1/Hz we can generate signals with proper
> > resolution.
>
> Converting those to GTOD sampling instead of jiffies sampling is a
> worthwile change IMO and a good concept.
>
> The unificaton of ITIMER_PROF and ITIMER_VIRT is a nice observation
> and a good patch.
>
> The second one, changing all the sampling from cputime to ktime_t is
> nicely done too:
>
> We could do more though, there's still a bit of cputime legacies
> around:
>
> + cputime_t cval, nval;
>
> Couldnt all of that go over into the ktime_t space as well, phasing
> out cputime logic from the itimer code?
>
> The user ABI is struct timeval based, so there's no need to have
> cputime anywhere. The scheduler does nanoseconds accurate stats so
> it can be connected up there too.

Could the patches be merged and possible other work done in later time?
Or perhaps I should rework on them?

Regards
Stanislaw