Re: mmap() is slower than read() on SCSI/IDE on 2.0 and 2.1

Rogier Wolff (R.E.Wolff@BitWizard.nl)
Sat, 19 Dec 1998 16:37:34 +0100 (MET)

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 **
*-- BitWizard writes Linux device drivers for any device you may have! --*
*   Never blow in a cat's ear because if you do, usually after three or  *
*   four times, they will bite your lips!  And they don't let go for at  *
*   least a minute. -- Lisa Coburn, age 9


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