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

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

On 04/26/2012 12:47 PM, Mel Gorman wrote:

> Instead of COMPACT_ASYNC_PARTIAL and COMPACT_ASYNC_FULL should we have
> COMPACT_ASYNC_MOVABLE and COMPACT_ASYNC_UNMOVABLE? The first pass from
> the page allocator (COMPACT_ASYNC_MOVABLE) would only consider MOVABLE
> blocks as migration targets. The second pass (COMPACT_ASYNC_UNMOVABLE)
> would examine UNMOVABLE blocks, rescue them and use what blocks it
> rescues as migration targets. The third pass (COMPACT_SYNC) would work
> as it does currently. kswapd would only ever use COMPACT_ASYNC_MOVABLE.
>
> That would avoid rescanning the movable blocks uselessly on the second
> pass but should still work for Bartlomiej's workload.
>
> What do you think?

This makes sense.

>> In other words, could it be better to always try to
>> rescue the unmovable blocks?
>
> I do not think we should always scan within unmovable blocks on the
> first pass. I strongly suspect it would lead to excessive amounts of CPU
> time spent in mm/compaction.c.

Maybe my systems are not typical.  I have not seen
more than about 10% of the memory blocks marked as
unmovable in my system.

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