linux-kernel - Re: [PATCH 03/10] mm: vmscan: Flatten kswapd priority loop

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Date:	Tue, 19 Mar 2013 10:14:28 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Simon Jeons <simon.jeons@...il.com>
Cc:	Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Rik van Riel <riel@...hat.com>,
	Zlatko Calusic <zcalusic@...sync.net>,
	Johannes Weiner <hannes@...xchg.org>,
	dormando <dormando@...ia.net>,
	Satoru Moriya <satoru.moriya@....com>,
	Michal Hocko <mhocko@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/10] mm: vmscan: Flatten kswapd priority loop

On Tue, Mar 19, 2013 at 11:08:23AM +0800, Simon Jeons wrote:
> Hi Mel,
> On 03/17/2013 09:04 PM, Mel Gorman wrote:
> >kswapd stops raising the scanning priority when at least SWAP_CLUSTER_MAX
> >pages have been reclaimed or the pgdat is considered balanced. It then
> >rechecks if it needs to restart at DEF_PRIORITY and whether high-order
> >reclaim needs to be reset. This is not wrong per-se but it is confusing
> 
> per-se is short for what?
> 

It means "in self" or "as such".

> >to follow and forcing kswapd to stay at DEF_PRIORITY may require several
> >restarts before it has scanned enough pages to meet the high watermark even
> >at 100% efficiency. This patch irons out the logic a bit by controlling
> >when priority is raised and removing the "goto loop_again".
> >
> >This patch has kswapd raise the scanning priority until it is scanningmm: vmscan: Flatten kswapd priority loop
> >enough pages that it could meet the high watermark in one shrink of the
> >LRU lists if it is able to reclaim at 100% efficiency. It will not raise
> 
> Which kind of reclaim can be treated as 100% efficiency?
> 

100% efficiency is where every page scanned can be reclaimed immediately.

> >  		/*
> >-		 * We do this so kswapd doesn't build up large priorities for
> >-		 * example when it is freeing in parallel with allocators. It
> >-		 * matches the direct reclaim path behaviour in terms of impact
> >-		 * on zone->*_priority.
> >+		 * Fragmentation may mean that the system cannot be rebalanced
> >+		 * for high-order allocations in all zones. If twice the
> >+		 * allocation size has been reclaimed and the zones are still
> >+		 * not balanced then recheck the watermarks at order-0 to
> >+		 * prevent kswapd reclaiming excessively. Assume that a
> >+		 * process requested a high-order can direct reclaim/compact.
> >  		 */
> >-		if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
> >-			break;
> >-	} while (--sc.priority >= 0);
> >+		if (order && sc.nr_reclaimed >= 2UL << order)
> >+			order = sc.order = 0;
> 
> If order == 0 is meet, should we do defrag for it?
> 

Compaction is unnecessary for order-0.

-- 
Mel Gorman
SUSE Labs
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