linux-kernel - Re: [RFC][PATCH] page reclaim throttle take2

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Message-Id: <20080227165139.18e5933e.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 27 Feb 2008 16:51:39 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Rik van Riel <riel@...hat.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [RFC][PATCH] page reclaim throttle take2

On Tue, 26 Feb 2008 23:19:08 -0800 (PST)
David Rientjes <rientjes@...gle.com> wrote:
> My suggestion is merely to make the number of concurrent page reclaim 
> threads be a function of how many online cpus there are.  Threads can 
> easily be added or removed for cpu hotplug events by callback functions.
> 
> That's different than allowing users to change the number of threads with 
> yet another sysctl.  Unless there are situations that can be presented 
> where tuning the number of threads is advantageous to reduce lock 
> contention, for example, and not simply working around other VM problems, 
> then I see no point for an additional sysctl.
> 
> So my suggestion is to implement this in terms of 
> CONFIG_NUM_RECLAIM_THREADS_PER_CPU and add callback functions for cpu 
> hotplug events that add or remove this number of threads.
> 

Hmm, but kswapd, which is main worker of page reclaiming, is per-node.
And reclaim is done based on zone.
per-zone/per-node throttling seems to make sense.

I know his environment has 4cpus per node but throttle to 3 was the best
number in his measurement. Then it seems num-per-cpu is excessive.
(At least, ratio(%) is better.)
When zone-reclaiming is improved to be scale well, we'll have to change
this throttle.

BTW, could someone try his patch on x86_64/ppc ? 
I'd like to see how contention is heavy on other machines.

Thanks,
-kame
 

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