Christoph Lameter <cl@linux-foundation.org> writes:

quoted text
> On Wed, 18 Aug 2010, Chris Webb wrote:
> 
> > > != 0.  And even then, zone reclaim should only reclaim file pages, not
> > > anon.  In theory...
> >
> > Hi. This is zero on all our machines:
> >
> > # sysctl vm.zone_reclaim_mode
> > vm.zone_reclaim_mode = 0
> 
> Set it to 1.

I tried this on a handful of the problem hosts before re-adding their swap.
One of them now runs without dipping into swap. The other three I tried had
the same behaviour of sitting at zero swap usage for a while, before
suddenly spiralling up with %wait going through the roof. I had to swapoff
on them to bring them back into a sane state. So it looks like it helps a
bit, but doesn't cure the problem.

I could definitely believe an explanation that we're swapping in preference
to allocating remote zone pages somehow, given the imbalance in free memory
between the nodes which we saw. However, I read the documentation for
vm.zone_reclaim_mode, which suggests to me that when it was set to zero,
pages from remote zones should be allocated automatically in preference to
swap given that zone_reclaim_mode & 4 == 0?

Cheers,

Chris.
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