From: Andrew Morton
Date: Mon Apr 30 2007 - 03:18:19 EST

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

> After installing a second 512MB SODIMM into my laptop, I noticed that
> my 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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