linux-kernel - Re: [RFC][PATCH 0/2] memcg: replace memcg's per cpu status counter with array counter like vmstat

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Message-Id: <20091001094514.c9d2b3d9.nishimura@mxp.nes.nec.co.jp>
Date:	Thu, 1 Oct 2009 09:45:14 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	nishimura@....nes.nec.co.jp,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 0/2] memcg: replace memcg's per cpu status counter
 with array counter like vmstat

On Wed, 30 Sep 2009 19:04:17 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> Hi,
> 
> In current implementation, memcg uses its own percpu counters for counting
> evetns and # of RSS, CACHES. Now, counter is maintainer per cpu without
> any synchronization as vm_stat[] or percpu_counter. So, this is
>  update-is-fast-but-read-is-slow conter.
> 
> Because "read" for these counter was only done by memory.stat file, I thought
> read-side-slowness was acceptable. Amount of memory usage, which affects
> memory limit check, can be read by memory.usage_in_bytes. It's maintained
> by res_counter.
> 
> But in current -rc, root memcg's memory usage is calcualted by this per cpu
> counter and read side slowness may be trouble if it's frequently read.
> 
> And, in recent discusstion, I wonder we should maintain NR_DIRTY etc...
> in memcg. So, slow-read-counter will not match our requirements, I guess.
> I want some counter like vm_stat[] in memcg.
> 
I see your concern.

But IMHO, it would be better to explain why we need a new percpu array counter
instead of using array of percpu_counter(size or consolidation of related counters ?),
IOW, what the benefit of percpu array counter is.


Thanks,
Daisuke Nishimura.

> This 2 patches are for using counter like vm_stat[] in memcg.
> Just an idea level implementaion but I think this is not so bad.
> 
> I confirmed this patch works well. I'm now thinking how to test performance...
> 
> Any comments are welcome. 
> This patch is onto mmotm + some myown patches...so...this is just an RFC.
> 
> Regards,
> -Kame
> 
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