lkml.org 
[lkml]   [2010]   [Sep]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: slow nanosleep?
On Wed, 8 Sep 2010, Joakim Tjernlund wrote:

> Peter Zijlstra <peterz@infradead.org> wrote on 2010/09/08 15:00:18:
> >
> > On Wed, 2010-09-08 at 14:43 +0200, Thomas Gleixner wrote:
> > > > However nanosleep with 1 ns and prctl(PR_SET_TIMERSLACK, 1) takes
> > > > about 8 us on x86(Intel(R) Core(TM)2 Duo CPU E8500 @ 3.16GHz)
> > > > and 20 us on my slower ppc board. Is that system call overhead
> > > > or possibly some error?
> > >
> > > That's overhead I fear. We go way up to enqueue/arm the timer until we
> > > figure out that the timeout already happened.
> >
> > Well, there's also the fact that his ppc board is simply dead slow,
> > using the freq ratio: 3166/266 you'd expect (at a similar ins/clock
> > ratio) the ppc to take 95us.
> >
> > So in fact the pcc taking 20us is actually quite good.
>
> Actually, it takes 120 us. The 20 us was when I had Thomas
> timeout == 0 fast path patch applied(forgot to remove it).
> Without that patch it takes about 115 us. So it seems it takes
> 115-20=95 us to turn the timer wheel on my ppc.

You might fire up the tracer to look where it spends that time.

Thanks,

tglx


\
 
 \ /
  Last update: 2010-09-08 15:55    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site