Re: [PATCH v3] mm: compaction: handle incorrect Unmovable type pageblocks

[Rik van Riel <riel@redhat.com>]

On 04/26/2012 10:36 AM, Mel Gorman wrote:

> Hmm, at what point does COMPACT_ASYNC_FULL get used? I see it gets
> used for the proc interface but it's not used via the page allocator at
> all.

He is using COMPACT_SYNC for the proc interface, and
COMPACT_ASYNC_FULL from kswapd.

> Minimally I was expecting to see if being used from the page allocator.

Makes sense, especially if we get the CPU overhead
saving stuff that we talked about at LSF to work :)

> A better option might be to track the number of MIGRATE_UNMOVABLE blocks that
> were skipped over during COMPACT_ASYNC_PARTIAL and if it was a high
> percentage and it looked like compaction failed then to retry with
> COMPACT_ASYNC_FULL. If you took this option, try_to_compact_pages()
> would still only take sync as a parameter and keep the decision within
> compaction.c

This I don't get.

If we have a small number of MIGRATE_UNMOVABLE blocks,
is it worth skipping over them?

If we have really large number of MIGRATE_UNMOVABLE blocks,
did we let things get out of hand?    By giving the page
allocator this many unmovable blocks to choose from, we
could have ended up with actually non-compactable memory.

If we have a medium number of MIGRATE_UNMOVABLE blocks,
is it worth doing a restart and scanning all the movable
blocks again?

In other words, could it be better to always try to
rescue the unmovable blocks?

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