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Date:   Tue, 18 Dec 2018 13:51:56 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Linux-MM <linux-mm@kvack.org>,
        David Rientjes <rientjes@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>, ying.huang@intel.com,
        kirill@shutemov.name, Andrew Morton <akpm@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 09/14] mm, compaction: Ignore the fragmentation avoidance
 boost for isolation and compaction
Message-ID: <20181218135156.GK29005@techsingularity.net>
References: <20181214230310.572-1-mgorman@techsingularity.net>
 <20181214230310.572-10-mgorman@techsingularity.net>
 <f8aeec16-65de-b873-3362-3c7cb30c4ac6@suse.cz>
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On Tue, Dec 18, 2018 at 01:36:42PM +0100, Vlastimil Babka wrote:
> On 12/15/18 12:03 AM, Mel Gorman wrote:
> > When pageblocks get fragmented, watermarks are artifically boosted to pages
> > are reclaimed to avoid further fragmentation events. However, compaction
> > is often either fragmentation-neutral or moving movable pages away from
> > unmovable/reclaimable pages. As the actual watermarks are preserved,
> > allow compaction to ignore the boost factor.
> 
> Right, I should have realized that when reviewing the boost patch. I
> think it would be useful to do the same change in
> __compaction_suitable() as well. Compaction has its own "gap".
> 

That gap is somewhat static though so I'm a bit more wary of it.  However,
the check in __isolate_free_page looks too agressive. We isolate in
units of COMPACT_CLUSTER_MAX yet the watermark check there is based on
the allocation request. That means for THP that we check if 512 pages
can be allocated when only somewhere between 1 and 32 is needed for that
compaction cycle to complete. Adjusting that might be more appropriate?

-- 
Mel Gorman
SUSE Labs