Re: blk_congestion_wait racy?

From: Nick Piggin
Date: Thu Mar 11 2004 - 21:38:55 EST

Martin Schwidefsky wrote:

Yes, sorry, all the world's an x86 :( Could you please send me whatever
diffs were needed to get it all going?

I am just preparing that mail :-)

I thought you were running a 256MB machine? Two seconds for 400 megs of
swapout? What's up?

Roughly 400 MB of swapout. And two seconds isn't that bad ;-)

An ouch-per-second sounds reasonable. It could simply be that the CPUs
were off running other tasks - those timeout are less than scheduling

I don't understand why an ouch-per-second is reasonable. The mempig is
the only process that runs on the machine and the blk_congestion_wait
uses HZ/10 as timeout value. I'd expect about 100 ouches for the 10
seconds the test runs.

The 4x performance difference remains not understood.

It would still be blk_congestion_wait slowing things down, wouldn't
it? Performance was good when you took that out, wasn't it?

And it would not unusual for you to be waiting needlessly without
seeing the ouch.

I think I will try doing a non-racy blk_congestion_wait after Jens'
unplugging patch gets put into -mm. That should solve your problem.

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