linux-kernel - Re: [PATCH v2 02/10] mm, compaction: remove redundant watermark check in compact

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Message-ID: <20170214163453.GC2450@cmpxchg.org>
Date:   Tue, 14 Feb 2017 11:34:53 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v2 02/10] mm, compaction: remove redundant watermark
 check in compact_finished()

On Fri, Feb 10, 2017 at 06:23:35PM +0100, Vlastimil Babka wrote:
> When detecting whether compaction has succeeded in forming a high-order page,
> __compact_finished() employs a watermark check, followed by an own search for
> a suitable page in the freelists. This is not ideal for two reasons:
> 
> - The watermark check also searches high-order freelists, but has a less strict
>   criteria wrt fallback. It's therefore redundant and waste of cycles. This was
>   different in the past when high-order watermark check attempted to apply
>   reserves to high-order pages.
> 
> - The watermark check might actually fail due to lack of order-0 pages.
>   Compaction can't help with that, so there's no point in continuing because of
>   that. It's possible that high-order page still exists and it terminates.
> 
> This patch therefore removes the watermark check. This should save some cycles
> and terminate compaction sooner in some cases.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Johannes Weiner <hannes@...xchg.org>