Re: [patch v2 3/4] mm, compaction: add per-zone migration pfn cache for async compaction

David Rientjes
2014-05-06T00:29:35Z

On Mon, 5 May 2014, Vlastimil Babka wrote:

> I see, although I would still welcome some numbers to back such change.

It's pretty difficult to capture numbers for this in real-world scenarios
since it happens rarely (and when it happens, it's very significant
latency) and without an instrumented kernel that will determine how many
pageblocks have been skipped. I could create a synthetic example of it in
the kernel and get numbers for a worst-case scenario with a 64GB zone if
you'd like, I'm not sure how representative it will be.

> What I still don't like is the removal of the intent of commit 50b5b094e6. You
> now again call set_pageblock_skip() unconditionally, thus also on pageblocks
> that async compaction skipped due to being non-MOVABLE. The sync compaction
> will thus ignore them.
>

I'm not following you, with this patch there are two cached pfns for the
migration scanner: one is used for sync and one is used for async. When
cc->sync == true, both cached pfns are updated (async is not going to
succeed for a pageblock when sync failed for that pageblock); when
cc->sync == false, the async cached pfn is updated only and we pick up
again where we left off for subsequent async compactions. Sync compaction
will still begin where it last left off and consider these non-MOVABLE
pageblocks.