linux-kernel - Re: More breakage in native_rdtsc out of line in git-x86

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Message-ID: <20080109204028.GA15612@one.firstfloor.org>
Date:	Wed, 9 Jan 2008 21:40:28 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: More breakage in native_rdtsc out of line in git-x86

On Wed, Jan 09, 2008 at 12:25:59PM -0800, Arjan van de Ven wrote:
> On Wed, 9 Jan 2008 18:48:00 +0100
> > And after all that's still by far the most common system call
> > (not only for databases; i profiled this using systemtap in some
> > loads some time ago and it usually came up with >50%) 
> > and quite important for many workloads.
> > 
> 
> btw be careful with this; the X server uses gettimeofday in it's equivalent of udelay()...
> (and we all know how useful it is to make the delay loops faster ;-)

People tend to make jokes about optimizing the idle loop too, but they're
actually wrong. Exit latency for the idle loop is important -- it 
decides how quickly you can react to load changes on idle CPUs.

For short udelays I suspect shorter exit latency is also moderately useful.

But anyways there are plenty of gtod users outside the X server.

e.g. common not user space case is packet timestamps.

-Andi
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