[RFC] pageout IO problem and possible solution

[RFC] pageout IO problem and possible solution

[Rik van Riel]

Something that I forgot during the VM summit yesterday.

One of the problems that network block devices and network swap can
have is that the VM can end up submitting too much I/O at once for
swapout.  This, in turn, could leave not enough free memory to finish
the IO, leaving the system stuck.

A related problem can happen with local disk subsystems, where we
submit a gazillion pages for writeout simultaneously and end up
causing horrible latency problems...

There are a number of solutions possible in the 2.6 kernel.

The first one would be to limit the amount of writes in progress
through the block layer ->congested test, but allowing less I/O
in flight if we are doing pageout and/or memory is low.

The second one would be to introduce the inactive_laundry list
from 2.4-rmap.  This eats a bit from the already scarce page
flags though.

Either of these approaches should work I suspect, as long as we
also throttle kswapd when there is too much I/O in flight.  I am
not sure the current kernel does this, or has an exception for
kswapd.

I am not sure what the threshold for I/O in flight should be
though.

Maybe let there be as many pages in flight as we have (free +
easily reclaimable free) pages around, to reduce the chance of
a deadlock to something really really low?

Maybe that is not high enough to achieve the best throughput
on some high-end I/O subsystems?

Any ideas?


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