Re: [patch, v2.6.22-rc6] sys_time() speedup

["Jesper Juhl" <jesper.juhl@gmail.com>]

On 26/06/07, Roman Zippel <zippel@linux-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@gmail.com>
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