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

Rik van Riel (H.H.vanRiel@phys.uu.nl)
Thu, 17 Dec 1998 19:37:05 +0100 (CET)

On Thu, 17 Dec 1998, Stephen C. Tweedie wrote:
> On Thu, 17 Dec 1998 01:27:05 +0100 (CET), Rik van Riel
> <H.H.vanRiel@phys.uu.nl> said:
>
> > Once again, you seem to be forgetting about read-behind.
>
> No, I'm ignoring read-behind. If people access data backwards
> then tough. Real disks don't work that way.

This is exactly why we want to do proper read-behind...

There are quite a lot of programs that do tiled backwards
reading of data, just think about large matrix stuff or
some image viewing or manipulation stuff.

It is relatively cheap to do read-behind, so I don't see
any reason to let the backwards-reading programs slow
down the rest of the system by causing 'improper' disk
access patterns.

We can simply implement the read-behind in the same way
we do read-ahead, but we start at address-16 instead of
address itself.

cheers,

Rik -- the flu hits, the flu hits, the flu hits -- MORE
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