linux-kernel - CONFIG_HIGHMEM4G and 1GB RAM

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Date:	Sat, 28 Apr 2007 21:26:42 +0100
From:	"Bradley Chapman" <kakadu@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: CONFIG_HIGHMEM4G and 1GB RAM

After installing a second 512MB SODIMM into my laptop, I noticed that
my 2.6.18.3 kernel only detected and used 896MB of the resulting 1GB,
and informed me that I needed to enable high memory in order to access
the remaining 127MB.

While doing so, I noticed that the help text stated that enabling high
memory (in my case, CONFIG_HIGHMEM4G) should only be done if the
system the kernel is running on has more than 1GB, but less than 4GB -
it said that systems with 1GB should leave it off, which resulted in
an eighth of the installed memory going missing. A bit more research
found a three year old thread on this list where it was stated that
enabling CONFIG_HIGHMEM4G sometimes slows the system down due to
sometimes undesirable changes to the way the kernel sets up the memory
map.

Basically, all I want to know is whether or not enabling
CONFIG_HIGHMEM4G for a laptop that has exactly 1GB of RAM will result
in any performance degradation.

TIA,

Brad

P.S: Direct replies please - I'm not subscribed to the list.
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