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To: linux-kernel@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: highmem question
Date: 7 Dec 2001 16:01:33 -0800
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Copyright: Copyright 2001 H. Peter Anvin - All Rights Reserved
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Followup to:  <Pine.LNX.4.30.0112071404280.29154-100000@mustard.heime.net>
By author:    Roy Sigurd Karlsbakk <roy@karlsbakk.net>
In newsgroup: linux.dev.kernel
> 
> I heard that himem slows down systems.

It does, because it's a hack to extend 32-bit machines beyond their
architectural lifetime.

> - How much memory can Linux use without highmem enabled? (I've heard it's
>   1GB, but Linux found 1,2GB without ...)

On i386, it supports 896 MB without HIGHMEM.

> - How much is a system slowed down?

Depends completely on your application mix and amount of RAM -- and
whether or not you're using 4G or 64G HIGHMEM, the latter being more
severe across a whole bunch of axes.

> - How can this be fixed? I've heard it's a PCI issue (stuff being memory
>   mapped above the 2GB limit?)

Go to a 64-bit CPU architecture.

	-hpa

-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>
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