linux-kernel - Re: [patch, v2.6.22-rc6] sys

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Date:	Tue, 26 Jun 2007 00:20:27 +0200
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Roman Zippel" <zippel@...ux-m68k.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"John Stultz" <johnstul@...ibm.com>,
	"Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: [patch, v2.6.22-rc6] sys_time() speedup

On 26/06/07, Roman Zippel <zippel@...ux-m68k.org> wrote:
> Hi,
>
> On Mon, 25 Jun 2007, Jesper Juhl wrote:
>
> > > On Monday 25 June 2007, Ingo Molnar wrote:
> > >
> > > > the patch improves the sysbench OLTP macrobenchmark significantly:
> > >
> > > Has that any real practical relevance?
> > >
> > It seems to me that Ingo's patch offers slightly improved performance
> > for any program using the time() system call, with no real drawbacks,
> > so why wouldn't we want to use it?
>
> How do you come to the conclusion it has no real drawbacks?

His change to do_gettimeofday() will of course slow that path down a
tiny bit since he's adding an extra 'if', but since it's wrapped in
unlikely() and should hit at most one time pr second I would guess the
performance impact of that to be negligible.
The change to sys_time() does away with some local variables and
replaces the call to do_gettimeofday() with a memory barrier and a
simple read of xtime.tv_se. I find it hard to believe (although I have
not tested it) that that wouldn't be faster than the original.

So that's how I came to that conclusion; just reading the patch, going
over what it does in my head and thinking about it a bit.  Not the
most scientific of things I admit.

Even if it is not faster, what would make it slower?  Have you spotted
something I have not?

> Ingo provided no information about his test setup and his patch was a
> little strange, so I can't say that yet.
>
What did you find strange about it? I'm currious.
Sure it needs testing and of course it would be nice with some more
details on Ingo's test setup.

-- 
Jesper Juhl <jesper.juhl@...il.com>
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