Thanks!
Hmm.. The timer waits 5 seconds before trying to prefetch, but then only stops
if it detects any activity elsewhere. It doesn't actually try to go idle in
between but it doesn't take much activity to put it back to sleep, hence
detecting yet another "not quite idle" period and then it goes to sleep
again. I guess the sleep interval can actually be changed as another tunable
from 5 seconds to whatever the user wanted.
Well the advantage is twofold potentially; 1. the pages that have been
prefecthed and become minor faults when they would have been major faults,
and 2. those that become minor faults (via 1) and then become major faults
again (since a copy is kept on backing store with swap prefetch). The
sp_tester only tests for 1, although it would be easy enough to simply do
another big malloc at the end and see how fast it swapped out again as a
marker of 2. As for an in-kernel option, it could get kind of expensive
tracking pages that have done one or both of these. I'll think about an
affordable way to do this, perhaps it could be just done as a
debugging/testing patch, but if would be nice to make it cheap enough to have
there permanently as well. The pages end up in swap cache (in the reverse
direction pages normally get to swap cache) so the accounting could be done
somewhere around there.
Thanks for comments!
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