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Date: Mon, 30 Apr 2007 23:18:28 +0100
From: Matt Keenan <tank.en.mate@gmail.com>
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To: William Lee Irwin III <wli@holomorphy.com>
CC: Andrew Morton <akpm@linux-foundation.org>,
       Bradley Chapman <kakadu@gmail.com>, linux-kernel@vger.kernel.org
Subject: Re: CONFIG_HIGHMEM4G and 1GB RAM
References: <e294b46e0704281326w60f9cecfuace3442791f147b3@mail.gmail.com> <20070430001755.e9f203c9.akpm@linux-foundation.org> <20070430074044.GB19966@holomorphy.com>
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William Lee Irwin III wrote:
> On Sat, 28 Apr 2007 21:26:42 +0100 "Bradley Chapman" <kakadu@gmail.com> wrote:
>   
>>> 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.
>>>       
>
> On Mon, Apr 30, 2007 at 12:17:55AM -0700, Andrew Morton wrote:
>   
>> 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.
>>     
>
> The CONFIG_VMSPLIT config options were merged for such cases.
>
> It should be able to split on any 4MB-aligned boundary in
> CONFIG_HIGHMEM4G. CONFIG_VMSPLIT_3G_OPT appears to do something of
> this sort to use an entire 1GB RAM with minimal user address space
> reduction.
>
> This is an ELF ABI violation but the number of major applications
> that break is apparently low.
>
>   
wine and some java implementations being two of the big caveats.

Matt