linux-kernel - Re: General slowness with using 64Gb HIGHMEM option on 32-bit kernel

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Message-ID: <20070928140322.5fbbde5d@bree.surriel.com>
Date:	Fri, 28 Sep 2007 14:03:22 -0400
From:	Rik van Riel <riel@...hat.com>
To:	"Sergey Popov" <faijeya@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: General slowness with using 64Gb HIGHMEM option on 32-bit
 kernel

On Fri, 28 Sep 2007 21:57:55 +0400
"Sergey Popov" <faijeya@...il.com> wrote:

> Ok, I found a palliative.
> 
> Now I'm working with "mem=6770M" option, that gives me 6072MB RAM (of
> 6144 total).
> Interesting, that if you choose too much, like "mem=6800M", system
> behaviour is exactly the same as without any options.
> 
> Can it be that kernel incorrectly sets amount of memory to use so such
> a situation occurs?

Maybe the BIOS marks the top megabytes of memory as
uncacheable, which makes accesses ridiculously slow.

Cat /proc/mtrr to check.

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan
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