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Date: Fri, 07 Dec 2001 17:58:16 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
Organization: Zytor Communications
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To: Jens Axboe <axboe@suse.de>
CC: Marvin Justice <mjustice@austin.rr.com>, linux-kernel@vger.kernel.org
Subject: Re: highmem question
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Jens Axboe wrote:

> On Fri, Dec 07 2001, Marvin Justice wrote:
> 
>>>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.
>>
> 
> That's because of highmem page bouncing when doing I/O. There is indeed
> a solution for this -- 2.5 or 2.4 + block-highmem-all patches will
> happily do I/O directly to any page in your system as long as your
> hardware supports it. I'm sure we're beating w2k with that enabled :-)
> 


I didn't realize we were doing page bouncing for I/O in the 1-4 GB range.
 Yes, this would be an issue.

	-hpa




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