On 06/28/2012 01:16 PM, Jim Schutt wrote:

On 06/27/2012 09:37 PM, Rik van Riel wrote:

quoted

Order> 0 compaction stops when enough free pages of the correct
page order have been coalesced. When doing subsequent higher order
allocations, it is possible for compaction to be invoked many times.

However, the compaction code always starts out looking for things to
compact at the start of the zone, and for free pages to compact things
to at the end of the zone.

This can cause quadratic behaviour, with isolate_freepages starting
at the end of the zone each time, even though previous invocations
of the compaction code already filled up all free memory on that end
of the zone.

This can cause isolate_freepages to take enormous amounts of CPU
with certain workloads on larger memory systems.

The obvious solution is to have isolate_freepages remember where
it left off last time, and continue at that point the next time
it gets invoked for an order> 0 compaction. This could cause
compaction to fail if cc->free_pfn and cc->migrate_pfn are close
together initially, in that case we restart from the end of the
zone and try once more.

Forced full (order == -1) compactions are left alone.

Reported-by: Jim Schutt<redacted>
Signed-off-by: Rik van Riel<redacted>

Tested-by: Jim Schutt<redacted>

Please let me know if you further refine this patch
and would like me to test it with my workload.

Mel pointed out a serious problem with the way wrapping
cc->free_pfn back to the top of the zone is handled.

I will send you a new patch once I have a fix for that.

So far I've run a total of ~20 TB of data over fifty minutes
or so through 12 machines running this patch; no hint of
trouble, great performance.

Without this patch I would typically start having trouble
after just a few minutes of this load.

Good to hear that!

Thank you for testing last night's version.

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