Re: swap cache

Rik van Riel (H.H.vanRiel@phys.uu.nl)
Mon, 21 Dec 1998 11:23:11 +0100 (CET)

On Mon, 21 Dec 1998, Ramakrishna K wrote:
> > ramak@wipinfo.soft.net (Ramakrishna K) said:
> >
> > > Also i was thinking about one of the other suggestions that was made
> > > regardin g the compression of a page(s) to a portion of the RAM, and
> > > uncompress it as part of the swap back in. Although the
> > > compression/un-compression has to be measured against the I/O cost.
> >
> > Not only. Your CPU can be happily computing away at process A while
> > process B pages in some. However, if this "pagein" is really "uncompress a
> > page from RAM", your process A looses the use of the CPU.
>
> All this * might * be a viable option only on the basis of the
> cost involved in eating up CPU cycles vis-a-vis large I/O due to
> swapping.

The cost of I/O is in seek time and not in disk read/througput
time. Compression will only reduce the already cheap part but
does nothing about the expensive seek times...

If you want cheaper swap I/O, use proper I/O clustering and
readahead. There is some (rather primitive) code for this in
the kernel and it is due for a revamp for 2.3...

cheers,

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