Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for whenzone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

[Andrew Morton]

On Mon, 8 Jun 2009 16:11:51 +0100 Mel Gorman <mel@xxxxxxxxx> wrote:

> On Mon, Jun 08, 2009 at 10:55:55AM -0400, Christoph Lameter wrote:
> > On Mon, 8 Jun 2009, Mel Gorman wrote:
> > 
> > > > The tmpfs pages are unreclaimable and therefore should not be on the anon
> > > > lru.
> > > >
> > >
> > > tmpfs pages can be swap-backed so can be reclaimable. Regardless of what
> > > list they are on, we still need to know how many of them there are if
> > > this patch is to be avoided.
> > 
> > If they are reclaimable then why does it matter? They can be pushed out if
> > you configure zone reclaim to be that aggressive.
> > 
> 
> Because they are reclaimable by kswapd or normal direct reclaim but *not*
> reclaimable by zone_reclaim() if the zone_reclaim_mode is not configured
> appropriately.

Ah.  (zone_reclaim_mode & RECLAIM_SWAP) == 0.  That was important info.

Couldn't the lack of RECLAIM_WRITE cause a similar problem?

> I briefly considered setting zone_reclaim_mode to 7 instead of
> 1 by default for large NUMA distances but that has other serious consequences
> such as paging in preference to going off-node as a default out-of-box
> behaviour.

Maybe we should consider that a bit harder.  At what stage does
zone_reclaim decide to give up and try a different node?  Perhaps it's
presently too reluctant to do that?

> The point of the patch is that the heuristics that avoid the scan are not
> perfect. In the event they are wrong and a useless scan occurs, the response
> of the kernel after a useless scan should not be to uselessly scan a load
> more times around the LRU lists making no progress.

It would be sad to bring back a jiffies-based thing into page reclaim. 
Wall time has little correlation with the rate of page allocation and
reclaim activity.

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