linux-kernel - Re: slow nanosleep?

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Message-ID: <OFA5A9E9D0.0B55FA15-ONC1257798.004A7BB3-C1257798.004B86F2@transmode.se>
Date:	Wed, 8 Sep 2010 15:44:57 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: slow nanosleep?

Peter Zijlstra <peterz@...radead.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.

 Jocke

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