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Subject: Re: [PATCH 21/22] mm, compaction: Be selective about what pageblocks
 to clear skip hints
To:     Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     David Rientjes <rientjes@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
References: <20190118175136.31341-1-mgorman@techsingularity.net>
 <20190118175136.31341-22-mgorman@techsingularity.net>
From:   Vlastimil Babka <vbabka@suse.cz>
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Date:   Thu, 31 Jan 2019 16:45:16 +0100
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On 1/18/19 6:51 PM, Mel Gorman wrote:
> Pageblock hints are cleared when compaction restarts or kswapd makes enough
> progress that it can sleep but it's over-eager in that the bit is cleared
> for migration sources with no LRU pages and migration targets with no free
> pages. As pageblock skip hint flushes are relatively rare and out-of-band
> with respect to kswapd, this patch makes a few more expensive checks to
> see if it's appropriate to even clear the bit. Every pageblock that is
> not cleared will avoid 512 pages being scanned unnecessarily on x86-64.
> 
> The impact is variable with different workloads showing small differences
> in latency, success rates and scan rates. This is expected as clearing
> the hints is not that common but doing a small amount of work out-of-band
> to avoid a large amount of work in-band later is generally a good thing.
> 
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

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