Messages in this thread | | | Date | Wed, 15 Apr 2009 15:59:02 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 0/2] itimers: periodic timers fixes |
| |
* Stanislaw Gruszka <sgruszka@redhat.com> wrote:
> 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?
It's up to Thomas - but they certainly looked good to me.
Ingo
| |