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Date: Mon, 24 Feb 2003 01:09:38 -0800
From: Andrew Morton <akpm@digeo.com>
To: Bill Huey (Hui) <billh@gnuppy.monkey.org>
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Subject: Re: Minutes from Feb 21 LSE Call
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Bill Huey (Hui) <billh@gnuppy.monkey.org> wrote:
>
> On Mon, Feb 24, 2003 at 12:40:05AM -0800, Andrew Morton wrote:
> > There is no evidence for any such thing.  Nor has any plausible
> > theory been put forward as to why such an improvement should occur.
> 
> I find what you're saying a rather unbelievable given some of the
> benchmarks I saw when the preempt patch started to floating around.
> 
> If you search linuxdevices.com for articles on preempt, you'll see a
> claim about IO performance improvements with the patch. If somethings
> changed then I'd like to know.
> 
> The numbers are here:
> 	http://kpreempt.sourceforge.net/
> 

That's a 5% difference across five dbench runs.  If it is even 
statistically significant, dbench is notoriously prone to chaotic
effects (less so in 2.5)  It is a long stretch to say that any
increase in dbench numbers can be generalised to "improved IO
performance" across the board.

The preempt stuff is all about *worst-case* latency.  I doubt if
it shifts the average latency (which is in the 50-100 microsecond
range) by more that 50 microseconds.

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