linux-kernel - Re: [PATCH] mm:vmscan:replace zone_watermark_ok with zone_balanced for determining if kswapd will call compaction

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Date:	Mon, 23 Jun 2014 12:15:54 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Chen Yucong <slaoub@...il.com>
Cc:	hannes@...xchg.org, mhocko@...e.cz, riel@...hat.com,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm:vmscan:replace zone_watermark_ok with zone_balanced
 for determining if kswapd will call compaction

On Sun, Jun 22, 2014 at 04:51:00PM +0800, Chen Yucong wrote:
> According to the commit messages of "mm: vmscan: fix endless loop in kswapd balancing"
> and "mm: vmscan: decide whether to compact the pgdat based on reclaim progress", minor
> change is required to the following snippet.
> 
>         /*
>          * If any zone is currently balanced then kswapd will
>          * not call compaction as it is expected that the
>          * necessary pages are already available.
>          */
>         if (pgdat_needs_compaction &&
>                 zone_watermark_ok(zone, order,
>                                         low_wmark_pages(zone),
>                                         *classzone_idx, 0))
>                 pgdat_needs_compaction = false;
> 
> zone_watermark_ok() should be replaced by zone_balanced() in the above snippet. That's
> because zone_balanced() is more suitable for the context.
> 

What bug does this fix?

The intent here is to prevent kswapd compacting a node if an allocation
request within that node would succeed against the low watermark.
Your change alters that to check against hte high watermark + balance gap
without explaining why kswapd should compact until the high watermark is
reached.

-- 
Mel Gorman
SUSE Labs
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