linux-kernel - Re: [PATCH v4 11/11] memcg: check memcg dirty limits in page writeback

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Date:	Mon, 1 Nov 2010 04:03:41 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"containers@...ts.osdl.org" <containers@...ts.osdl.org>,
	Andrea Righi <arighi@...eler.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Minchan Kim <minchan.kim@...il.com>,
	Ciju Rajan K <ciju@...ux.vnet.ibm.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v4 11/11] memcg: check memcg dirty limits in page
 writeback

On Sat, Oct 30, 2010 at 12:06:33AM +0800, Greg Thelen wrote:
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:
> 
> > On Fri, 29 Oct 2010 00:09:14 -0700
> > Greg Thelen <gthelen@...gle.com> wrote:
> >
> >> If the current process is in a non-root memcg, then
> >> balance_dirty_pages() will consider the memcg dirty limits
> >> as well as the system-wide limits.  This allows different
> >> cgroups to have distinct dirty limits which trigger direct
> >> and background writeback at different levels.
> >> 
> >> Signed-off-by: Andrea Righi <arighi@...eler.com>
> >> Signed-off-by: Greg Thelen <gthelen@...gle.com>
> >
> > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

The "check both memcg&global dirty limit" looks much more sane than
the V3 implementation. Although it still has misbehaviors in some
cases, it's generally a good new feature to have.

Acked-by: Wu Fengguang <fengguang.wu@...el.com>

> > Ideally, I think some comments in the code for "why we need double-check system's
> > dirty limit and memcg's dirty limit" will be appreciated.
> 
> I will add to the balance_dirty_pages() comment.  It will read:
> /*
>  * balance_dirty_pages() must be called by processes which are generating dirty
>  * data.  It looks at the number of dirty pages in the machine and will force
>  * the caller to perform writeback if the system is over `vm_dirty_ratio'.
                   ~~~~~~~~~~~~~~~~~                  ~~~~

To be exact, it tries to throttle the dirty speed so that
vm_dirty_ratio is not exceeded. In fact balance_dirty_pages() starts
throttling the dirtier slightly below vm_dirty_ratio.

>  * If we're over `background_thresh' then the writeback threads are woken to
>  * perform some writeout.  The current task may have per-memcg dirty
>  * limits, which are also checked.
>  */
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