linux-kernel - Re: [PATCH -mm] vmscan: bail out of page reclaim after swap_cluster

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Message-ID: <4920E869.9030501@linux.vnet.ibm.com>
Date:	Mon, 17 Nov 2008 09:13:37 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH -mm] vmscan: bail out of page reclaim after swap_cluster_max
 pages

KAMEZAWA Hiroyuki wrote:
> On Sun, 16 Nov 2008 16:38:56 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
>> One more point.
>>
>>> Sometimes the VM spends the first few priority rounds rotating back
>>> referenced pages and submitting IO.  Once we get to a lower priority,
>>> sometimes the VM ends up freeing way too many pages.
>>>
>>> The fix is relatively simple: in shrink_zone() we can check how many
>>> pages we have already freed and break out of the loop.
>>>
>>> However, in order to do this we do need to know how many pages we already
>>> freed, so move nr_reclaimed into scan_control.
>> IIRC, Balbir-san explained the implemetation of the memcgroup 
>> force cache dropping feature need non bail out at the past reclaim 
>> throttring discussion.
>>

Yes, for we used that for force_empty() in the past, but see below

>> I am not sure about this still right or not (iirc, memcgroup implemetation
>> was largely changed).
>>
>> Balbir-san, Could you comment to this patch?
>>
>>
> I'm not Balbir-san but there is no "force-cache-dropping" feature now.
> (I have no plan to do that.)
> 
> But, mem+swap controller will need to modify reclaim path to do "cache drop
> first" becasue the amount of "mem+swap" will not change when "mem+swap" hit
> limit. It's now set "sc.may_swap" to 0.
> 

Yes, there have been several changes to force_empty() and its meaning, including
movement of accounts. Since you've made most of the recent changes, your
comments are very relevant.

> Hmm, I hope memcg is a silver bullet to this kind of special? workload in
> long term.

:-) From my perspective, hierarchy, soft limits (sharing memory when there is no
contention), some form of over commit support and getting swappiness to work
correctly are very important for memcg.

-- 
	Balbir
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