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Subject: Re: [PATCH 09/14] mm, compaction: Ignore the fragmentation avoidance
 boost for isolation and compaction
To:     Mel Gorman <mgorman@techsingularity.net>
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>
References: <20181214230310.572-1-mgorman@techsingularity.net>
 <20181214230310.572-10-mgorman@techsingularity.net>
 <f8aeec16-65de-b873-3362-3c7cb30c4ac6@suse.cz>
 <20181218135156.GK29005@techsingularity.net>
From:   Vlastimil Babka <vbabka@suse.cz>
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Date:   Tue, 18 Dec 2018 14:58:33 +0100
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On 12/18/18 2:51 PM, Mel Gorman wrote:
> 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,

Well, watermark boost is dynamic, but based on allocations stealing from
other migratetypes, not reflecting compaction chances of success.

> 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?

AFAIU the code in __isolate_free_page() reflects that if there's less
than 512 free pages gap, we might form a high-order page for THP but
won't be able to allocate it afterwards due to watermark.