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SubjectRe: excessive CPU utilization by isolate_freepages?
On Wed, 27 Jun 2012, Rik van Riel wrote:

> > I doubt compaction try to migrate continuously although we have no free
> > memory.
> > Could you apply this patch and retest?
> >
> > https://lkml.org/lkml/2012/6/21/30
>

Not sure if Jim is using memcg; if not, then this won't be helpful.

> Another possibility is that compaction is succeeding every time,
> but since we always start scanning all the way at the beginning
> and end of each zone, we waste a lot of CPU time rescanning the
> same pages (that we just filled up with moved pages) to see if
> any are free.
>
> In short, due to the way compaction behaves right now,
> compaction + isolate_freepages are essentially quadratic.
>
> What we need to do is remember where we left off after a
> successful compaction, so we can continue the search there
> at the next invocation.
>

So when the free and migration scanners meet and compact_finished() ==
COMPACT_CONTINUE, loop around to the start of the zone and continue until
you reach the pfn that it was started at? Seems appropriate.


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