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Subject: Re: [PATCH 21/25] mm, compaction: Round-robin the order while
 searching the free lists for a target
References: <20190104125011.16071-1-mgorman@techsingularity.net>
 <20190104125011.16071-22-mgorman@techsingularity.net>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <71ad4f46-ca81-9a70-0b98-d1a1c46df47a@suse.cz>
Date: Fri, 18 Jan 2019 10:17:58 +0100
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In-Reply-To: <20190104125011.16071-22-mgorman@techsingularity.net>
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To: Mel Gorman <mgorman@techsingularity.net>, Linux-MM <linux-mm@kvack.org>
Cc: David Rientjes <rientjes@google.com>, Andrea Arcangeli <aarcange@redhat.com>, ying.huang@intel.com, kirill@shutemov.name, Andrew Morton <akpm@linux-foundation.org>, Linux List Kernel Mailing <linux-kernel@vger.kernel.org>

On 1/4/19 1:50 PM, Mel Gorman wrote:
> As compaction proceeds and creates high-order blocks, the free list
> search gets less efficient as the larger blocks are used as compaction
> targets. Eventually, the larger blocks will be behind the migration
> scanner for partially migrated pageblocks and the search fails. This
> patch round-robins what orders are searched so that larger blocks can be
> ignored and find smaller blocks that can be used as migration targets.
> 
> The overall impact was small on 1-socket but it avoids corner cases where
> the migration/free scanners meet prematurely or situations where many of
> the pageblocks encountered by the free scanner are almost full instead of
> being properly packed. Previous testing had indicated that without this
> patch there were occasional large spikes in the free scanner without this
> patch. By co-incidence, the 2-socket results showed a 54% reduction in
> the free scanner but will not be universally true.
> 
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>