linux-kernel - Re: CONFIG_HIGHMEM4G and 1GB RAM

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Message-Id: <20070430001755.e9f203c9.akpm@linux-foundation.org>
Date:	Mon, 30 Apr 2007 00:17:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Bradley Chapman" <kakadu@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: CONFIG_HIGHMEM4G and 1GB RAM

On Sat, 28 Apr 2007 21:26:42 +0100 "Bradley Chapman" <kakadu@...il.com> wrote:

> 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.
> 

I would expect the advantages of the additional 128MB to considerably
outweigh the cost of turning on CONFIG_HIGHMEM4G.

That cost will be a little extra CPU consumption inside the kernel, but the
great majority of CPU consumption usually happens in userspace anyway.

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