Date: Wed, 16 Mar 2011 13:06:12 -0400
From: Vivek Goyal <vgoyal@redhat.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Greg Thelen <gthelen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        containers@lists.osdl.org, linux-fsdevel@vger.kernel.org,
        Andrea Righi <arighi@develer.com>,
        Balbir Singh <balbir@linux.vnet.ibm.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
        Minchan Kim <minchan.kim@gmail.com>,
        Ciju Rajan K <ciju@linux.vnet.ibm.com>,
        David Rientjes <rientjes@google.com>,
        Wu Fengguang <fengguang.wu@intel.com>,
        Chad Talbott <ctalbott@google.com>,
        Justin TerAvest <teravest@google.com>
Subject: Re: [PATCH v6 0/9] memcg: per cgroup dirty page accounting
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On Wed, Mar 16, 2011 at 05:35:10PM +0100, Johannes Weiner wrote:

[..]
> > IIUC, this sounds more like a solution to quickly come up with a list of
> > inodes one should be writting back. One could also come up with this kind of
> > list by going through memcg->lru list also (approximate). So this can be
> > an improvement over going through memcg->lru instead go through
> > memcg->mapping_list.
> 
> Well, if you operate on a large file it may make a difference between
> taking five inodes off the list and crawling through hundreds of
> thousands of pages to get to those same five inodes.
> 
> And having efficient inode lookup for a memcg makes targetted
> background writeback more feasible: pass the memcg in the background
> writeback work and have the flusher go through memcg->mappings,
> selecting those that match the bdi.
> 
> Am I missing something?  I feel like I missed your point.

No. Thanks for the explanation. I get it. Walking through the memcg->lru
list to figure out inodes memcg is writting to will be slow and can be
very painful for large files. Keeping a more direct mapping like
memcg_mapping list per memcg can simplify it a lot.

Thanks
Vivek 
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