Message-Id: <199812191537.QAA03388@cave.bitwizard.nl>
Subject: Re: mmap() is slower than read() on SCSI/IDE on 2.0 and 2.1
In-Reply-To: <19981218010838.D28066@cerebro.laendle> from Marc Lehmann at "Dec 18, 98 01:08:38 am"
To: pcg@goof.com (Marc Lehmann)
Date: Sat, 19 Dec 1998 16:37:34 +0100 (MET)
Cc: linux-kernel@vger.rutgers.edu
From: R.E.Wolff@BitWizard.nl (Rogier Wolff)
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Marc Lehmann wrote:
> On Thu, Dec 17, 1998 at 06:52:50AM +0000, Linus Torvalds wrote:
> > Umm, the easiest hint is probably to just look at the faulting address. 
> > We have it available, after all. 
> > 
> > I suspect that such a simple heuristic would be fairly accurate, and it
> > can be coupled with other heuristics to further increase the accuracy. 
> 
> file copy, yes. But grep (and probably lots of others) don't access memory
> sequentially (as faster search algorithms exist)

A fast search algorithm, touches memory every n bytes where n is the
size of the largest constant string that you're searching for. 

That way "n" is typically small, so that you end up hitting the first
n bytes pretty often.

If you're acessing 

	struct blabla {
        int ...
	float ...
	char [];
	}

mmapped from a file, you might be looking for a certain float. As long
as you're doing a linear search, you'll again hit the first few bytes
of a page pretty often.


				Roger.

-- 
** R.E.Wolff@BitWizard.nl ** http://www.BitWizard.nl/ ** +31-15-2137555 **
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