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Date: Thu, 11 Sep 2003 20:18:06 +0200
From: Andi Kleen <ak@suse.de>
To: Jamie Lokier <jamie@shareable.org>
Cc: richard.brunner@amd.com, linux-kernel@vger.kernel.org, akpm@osdl.org,
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Subject: Re: [PATCH] 2.6 workaround for Athlon/Opteron prefetch errata
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On Thu, 11 Sep 2003 18:48:39 +0100
Jamie Lokier <jamie@shareable.org> wrote:

> Andi Kleen wrote:
> > signal exception path is thousands of cycles, we're talking about tens
> > of cycles here.
> 
> <hand-waving>
> 
> Tens vs thousands == percentage points.

It is more thousands than tens :-)

[just the page fault alone is quite costly]

> 
> Isn't it about 20 cycles per mispredicted branch on a P4?
> 
> Five of those and we're talking several percent slowdown, ridiculous
> as it seems.

There are a lot of conditional branches in the signal
path. If you don't believe me I can send you simics full instruction traces of it.
I'm not going to believe that 2-3 more make a significant difference.

-Andi
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