Re: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression

[David Rientjes <rientjes@google.com>]

On Wed, 5 Dec 2018, Andrea Arcangeli wrote:

> __GFP_COMPACT_ONLY gave an hope it could give some middle ground but
> it shows awful compaction results, it basically destroys compaction
> effectiveness and we know why (COMPACT_SKIPPED must call reclaim or
> compaction can't succeed because there's not enough free memory in the
> node). If somebody used MADV_HUGEPAGE compaction should still work and
> not fail like that. Compaction would fail to be effective even in the
> local node where __GFP_THISNODE didn't fail. Worst of all it'd fail
> even on non-NUMA systems (that would be easy to fix though by making
> the HPAGE_PMD_ORDER check conditional to NUMA being enabled at
> runtime).
> 

Note that in addition to COMPACT_SKIPPED that you mention, compaction can 
fail with COMPACT_COMPLETE, meaning the full scan has finished without 
freeing a hugepage, or COMPACT_DEFERRED, meaning that doing another scan 
is unlikely to produce a different result.  COMPACT_SKIPPED makes sense to 
do reclaim if it can become accessible to isolate_freepages() and 
hopefully another allocator does not allocate from these newly freed pages 
before compaction can scan the zone again.  For COMPACT_COMPLETE and 
COMPACT_DEFERRED, reclaim is unlikely to ever help.