linux-kernel - Re: [PATCH 17/25] mm, compaction: Keep cached migration PFNs synced for unusable pageblocks

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Date:   Thu, 17 Jan 2019 18:17:28 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Linux-MM <linux-mm@...ck.org>
Cc:     David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>, ying.huang@...el.com,
        kirill@...temov.name, Andrew Morton <akpm@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 17/25] mm, compaction: Keep cached migration PFNs synced
 for unusable pageblocks

On 1/4/19 1:50 PM, Mel Gorman wrote:
> Migrate has separate cached PFNs for ASYNC and SYNC* migration on the
> basis that some migrations will fail in ASYNC mode. However, if the cached
> PFNs match at the start of scanning and pageblocks are skipped due to
> having no isolation candidates, then the sync state does not matter.
> This patch keeps matching cached PFNs in sync until a pageblock with
> isolation candidates is found.
> 
> The actual benefit is marginal given that the sync scanner following the
> async scanner will often skip a number of pageblocks but it's useless
> work. Any benefit depends heavily on whether the scanners restarted
> recently so overall the reduction in scan rates is a mere 2.8% which
> is borderline noise.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

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

My easlier suggestion to check more thoroughly if pages can be migrated (which
depends on the mode) before isolating them wouldn't play nice with this :)