Re: [PATCH] 2.6 workaround for Athlon/Opteron prefetch errata

Andi Kleen
2003-09-11T18:18:11Z

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