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

[Wu Fengguang <fengguang.wu@intel.com>]

On Tue, Jun 09, 2009 at 11:06:19PM +0800, Mel Gorman wrote:
> On Tue, Jun 09, 2009 at 09:38:04PM +0800, Wu Fengguang wrote:
> > On Tue, Jun 09, 2009 at 05:40:50PM +0800, Mel Gorman wrote:
> > > 
> > > Conceivably though, zone_reclaim_interval could automatically tune
> > > itself based on a heuristic like this if the administrator does not give
> > > a specific value. I think that would be an interesting follow on once
> > > we've brought back zone_reclaim_interval and get a feeling for how often
> > > it is actually used.
> > 
> > Well I don't think that's good practice. There are heuristic
> > calculations all over the kernel. Shall we exporting parameters to
> > user space just because we are not absolutely sure? Or shall we ship
> > the heuristics and do adjustments based on feedbacks and only export
> > parameters when we find _known cases_ that cannot be covered by pure
> > heuristics?
> > 
> 
> Good question - I don't have a satisfactory answer but I intuitively find
> the zone_reclaim_interval easier to deal with than the heuristic.  That said,
> I would prefer if neither was required.

Yes - can we rely on the (improved) accounting to make our "failure feedback"
patches unnecessary? :)

Thanks,
Fengguang

> In the patchset, I've added a counter for the number of times that the
> scan-avoidance heuristic fails. If the tmpfs problem has been resolved
> (patch with bug reporter, am awaiting test), I'll drop zone_reclaim_interval
> altogether and we'll use the counter to detect if/when this situation
> occurs again.

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