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Date: Fri, 07 Dec 2001 17:52:48 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
Organization: Zytor Communications
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To: mjustice@austin.rr.com
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Subject: Re: highmem question
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Marvin Justice wrote:

>>The problem is that in the x86 architecture you don't have any reasonable
>>way of addressing the physical address space, so you need to map it into
>>the virtual address space.  You end up with a shortage of virtual address
>>space.
>>
> 
> Isn't this still just an artifact of the default 1:3 kernel/user virtual 
> address space split? I've never tried it myself but isn't there a 2:2 patch 
> available that has the effect of moving the highmem boundary up?
> 


You can tweak the split... both 2:2 and 0.5:3.5 splits have been used...
but it's not without side effects.  Cutting your user space breaks
applications which want large mmap() areas, for example.


> 
>>There is no way of fixing it.
>>
> 
> All I know is that a streaming io app I was playing with showed a drastic 
> performance hit when the kernel was compiled with CONFIG_HIGHMEM. On W2K we 
> saw no slowdown with 2 or even 4GB of RAM so I think solutions must exist.
> 


Of course you didn't.  Win2K runs with the equivalent of HIGHMEM all the
time.

	-hpa


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