Re: Over-eager swapping

[Christoph Lameter]

On Thu, 19 Aug 2010, Chris Webb wrote:

> 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?

If zone reclaim is off then pages from other nodes will be allocated if a
node is filled up with page cache.

zone reclaim typically only evicts clean page cache pages in order to keep
the additional overhead down. Enabling swapping allows a more aggressive
form of recovering memory in preference of going off line.

The VM should work fine even without zone reclaim.
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