linux-kernel - Re: x86_64 ten times slower than i386

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Message-ID: <20071106194009.GC1045@1wt.eu>
Date:	Tue, 6 Nov 2007 20:40:09 +0100
From:	Willy Tarreau <w@....eu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ray Lee <ray-lk@...rabbit.org>,
	Bo Brantén <bosse@....umu.se>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jesse Barnes <jesse.barnes@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: x86_64 ten times slower than i386

On Mon, Nov 05, 2007 at 05:19:44PM -0800, H. Peter Anvin wrote:
> Andi Kleen wrote:
> >>Jesse Barnes (cc:d) wrote a patch to address this, I think (x86: trim
> >>memory not covered by WB MTRRs), but as far as I can tell it hasn't
> >>been merged yet. System is Intel, 4gb of RAM.
> >
> >It wasn't merged because it broke booting on some systems.
> >Besides the memory would be still lost -- all it did was to automate
> >the "mem=XXXX" line.
> 
> There really are only two ways to deal with this -- drop the memory 
> (which should be automated, and a warning printed) or adjust the MTRRs. 
>  The problem is that at some point we run out of MTRRs, partially 
> because they're masks instead of base/limit.

Just out of curiosity, what would be the problem if the MTRRs covered more
than the memory size ? For instance, instead of having 512 MB at 4G, why
not have 1G at 4G ?

regards,
Willy

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