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

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Message-ID: <51483D63.4070904@gmail.com>
Date:	Tue, 19 Mar 2013 18:26:43 +0800
From:	Simon Jeons <simon.jeons@...il.com>
To:	Mel Gorman <mgorman@...e.de>
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

Hi Mel,
On 03/19/2013 06:14 PM, Mel Gorman wrote:
> 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.
>

I mean since order && sc.reclaimed >= 2UL << order, it is reclaimed for 
high order allocation, if order == 0 is meet, should we do defrag for it?

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