linux-kernel - Re: slow nanosleep?

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Date:	Wed, 8 Sep 2010 11:51:13 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
cc:	Eric Dumazet <eric.dumazet@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: slow nanosleep?

On Wed, 8 Sep 2010, Joakim Tjernlund wrote:

> Eric Dumazet <eric.dumazet@...il.com> wrote on 2010/09/08 10:24:49:
> >
> > Le mercredi 08 septembre 2010 à 10:04 +0200, Joakim Tjernlund a écrit :
> >
> > > That makes litte difference:
> > > root@...alhost ~ # ./nanosleep
> > > nanosleep
> > > req:0 :0
> > > tv_res:0 :112
> > >
> >
> > Here, result is 30 (with prctl())
> > instead of 95 (without)
> 
> On x86 I notice a difference:
>   7 vs 57.
> however select is still faster: 2
> The system call OH seems to be bigger for nanosleep than
> for select and on my ppc is is about 112 us. Can anything
> be done about that?

Hmm, the only reason I can see is that select calls finally
schedule_hrtimeout_range_clock() which optimizes the expiry = 0 case
while nanosleep does not. So the difference is only visiable when the
relative timeout is 0. For timeouts > 0 nanosleep and select should
behave the same way.

Thanks,

	tglx
---
 kernel/hrtimer.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux-2.6/kernel/hrtimer.c
===================================================================
--- linux-2.6.orig/kernel/hrtimer.c
+++ linux-2.6/kernel/hrtimer.c
@@ -1609,6 +1609,9 @@ SYSCALL_DEFINE2(nanosleep, struct timesp
 	if (!timespec_valid(&tu))
 		return -EINVAL;

+	if (!tu.tv_sec && !tu.tv_usec)
+		return 0;
+
 	return hrtimer_nanosleep(&tu, rmtp, HRTIMER_MODE_REL, CLOCK_MONOTONIC);
 }