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SubjectRe: About highmem in 2.6
On Wed, 11 Feb 2004 20:13:10 +0100
Mark de Vries <m.devries@nl.tiscali.com> wrote:

> Dave McCracken wrote:
> > --On Wednesday, February 11, 2004 18:47:04 +0100 Luis Miguel García
> > <ktech@wanadoo.es> wrote:
> >
> >
> >>When I first installed 2.4, someone told me that if I had 1 gb ram it was
> >>better to not use highmem because those extra aditional mb was not worth
> >>the speed penalty of using the feature.
> >>
> >>Sorry for my ignorance (and my sucking english) but must I enable highmem
> >>now with 2.6? or have it any speed penalty althought?
> >
> >
> > I don't know if anyone has actually measured the relative performance, but
> > I'd expect the answer to be the same as 2.4. There is a small but
> > measurable performance penalty for enabling highmem which is higher than
> > the benefit of the extra 128 meg of memory you get when you have 1G. If
> > you have more than 1G it's better to enable highmem.
> >
>
> I've been using this patch for a while now on my box (with 1GB):
> http://www.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.23aa1/00_3.5G-address-space-5
> (kernel is 'vanilla' otherwise)
>
> This allows you to use your full 1GB w/out highmem support.... (2G/2G
> user/kernel addr space split, or something..)
>
> Anything (potentially) wrong/bad about this patch??
>
> Is there a simmilar patch for 2.6??
>

There is nothing wrong with that patch, the problem with Highmem support on x86 is that is uses an Intel hack to address the full 1Gb of memory, which make memory access a bit slower. The question is, does the 128Mb additional memory worth that penalty?
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