On 05/08/2014 07:28 AM, Joonsoo Kim wrote:
On Wed, May 07, 2014 at 02:09:10PM +0200, Vlastimil Babka wrote:
The compaction free scanner in isolate_freepages() currently remembers PFN of
the highest pageblock where it successfully isolates, to be used as the
starting pageblock for the next invocation. The rationale behind this is that
page migration might return free pages to the allocator when migration fails
and we don't want to skip them if the compaction continues.
Since migration now returns free pages back to compaction code where they can
be reused, this is no longer a concern. This patch changes isolate_freepages()
so that the PFN for restarting is updated with each pageblock where isolation
is attempted. Using stress-highalloc from mmtests, this resulted in 10%
reduction of the pages scanned by the free scanner.
Hello,
Although this patch could reduce page scanned, it is possible to skip
scanning fresh pageblock. If there is zone lock contention and we are on
asyn compaction, we stop scanning this pageblock immediately. And
then, we will continue to scan next pageblock. With this patch,
next_free_pfn is updated in this case, so we never come back again to this
pageblock. Possibly this makes compaction success rate low, doesn't
it?
Hm, you're right and thanks for catching that, but I think this is a
sign of a worse and older issue than skipping a pageblock?
When isolate_freepages_block() breaks loop due to lock contention, then
isolate_freepages() (which called it) should also immediately quit its
loop. Trying another pageblock in the same zone with the same zone->lock
makes no sense here? If this is fixed, then the issue you're pointing
out will also be fixed as next_free_pfn will still point to the
pageblock where the break occured.
Thanks.
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