swsusp performance problems in 2.6.15-rc3-mm1

[Andy Isaacson <adi@hexapodia.org>]

On recent kernels such as 2.6.14-rc2-mm1, a swsusp of my laptop (1.25
GB, P4M 1.4 GHz) was a pretty fast process; freeing memory took about 3
seconds or less, and writing out the swap image took less than 5
seconds, so within 15 seconds of running my suspend script power was
off.

The downside was that after suspend, *everything* needed to be paged
back in, so all my apps were *very* slow for the first few interactions.
It would take about 15 or 20 seconds for Firefox to repaint the first
time I switched to its virtual desktop, and it was perceptibly slower
than normal for the next 5 or 10 minutes of use.

Now that I'm running 2.6.15-rc3-mm1, the page-in problem seems to be
largely gone; I don't notice a significant lagginess after resuming from
swsusp.

But the suspend process is *slow*.  It takes a good 20 or 30 seconds to
write out the image, which makes the overall suspend process take close
to a minute; it's writing about 400 MB, and my disk seems to only be
good for about 18 MB/sec according to hdparm -t.

And, the resume is about the same amount slower, too.

Certainly there's a tradeoff to be made, and I'm glad to lose the slow
re-paging after resume, but I'm hoping that some kind of improvement can
be made in the suspend/resume time.

Could we perhaps throw away *half* the cached memory rather than all of
it?  Or keep a lazy list of pages that need re-reading and page them in
asynchronously after restarting userland?

-andy