linux-kernel - Re: [PATCH] i386-pda UP optimization

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Message-Id: <200611151846.31109.dada1@cosmosbay.com>
Date:	Wed, 15 Nov 2006 18:46:30 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Ingo Molnar <mingo@...e.hu>, akpm@...l.org,
	Arjan van de Ven <arjan@...radead.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386-pda UP optimization

On Wednesday 15 November 2006 18:24, Andi Kleen wrote:
> On Wednesday 15 November 2006 18:20, Ingo Molnar wrote:
> > * Andi Kleen <ak@...e.de> wrote:
> > > On Wednesday 15 November 2006 12:27, Eric Dumazet wrote:
> > > > Seeing %gs prefixes used now by i386 port, I recalled seeing strange
> > > > oprofile results on Opteron machines.
> > > >
> > > > I really think %gs prefixes can be expensive in some (most ?) cases,
> > > > even if the Intel/AMD docs say they are free.
> > >
> > > They aren't free, just very cheap.
> >
> > Eric's test shows a 5% slowdown. That's far from cheap.
>
> I have my doubts about the accuracy of his test results. That is why I
> asked him to double check.

Fair enough :)

I plan doing *lot* of tests as soon as possible (not possible during daytime 
unfortunately, I miss a dev machine)

By the way, I tried this patch to avoid reload %gs at syscall start. Since %gs 
is not anymore used inside kernel (after i386-pda UP optimization is 
applied) : We can let in %gs the User Program %gs value. (I still force a 
reload of %gs before syscall exit of course)

Machine boots but freeze when init starts. Any idea ?

Thank you
Eric

View attachment "entry.patch" of type "text/plain" (1399 bytes)