Re: hugepage compaction causes performance drop

[Vlastimil Babka <vbabka@suse.cz>]

On 11/24/2015 09:29 AM, Joonsoo Kim wrote:
> On Tue, Nov 24, 2015 at 03:27:43PM +0800, Aaron Lu wrote:
> 
> Thanks.
> 
> Okay. Output proves the theory. pagetypeinfo shows that there are
> too many unmovable pageblocks. isolate_freepages() should skip these
> so it's not easy to meet proper pageblock until need_resched(). Hence,
> updating cached pfn doesn't happen. (You can see unchanged free_pfn
> with 'grep compaction_begin tracepoint-output')

Hm to me it seems that the scanners meet a lot, so they restart at zone
boundaries and that's fine. There's nothing to cache.

> But, I don't think that updating cached pfn is enough to solve your problem.
> More complex change would be needed, I guess.

One factor is probably that THP only use async compaction and those don't result
in deferred compaction, which should help here. It also means that
pageblock_skip bits are not being reset except by kswapd...

Oh and pageblock_pfn_to_page is done before checking the pageblock skip bits, so
that's why it's prominent in the profiles. Although it was less prominent (9% vs
46% before) in the last data... was perf collected while tracing, thus
generating extra noise?

> Thanks.
> 
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