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Date: Thu, 11 Mar 2010 13:07:53 -0500
From: Vivek Goyal <vgoyal@redhat.com>
To: Andrea Righi <arighi@develer.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Balbir Singh <balbir@linux.vnet.ibm.com>,
       Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
       Peter Zijlstra <peterz@infradead.org>,
       Trond Myklebust <trond.myklebust@fys.uio.no>,
       Suleiman Souhlal <suleiman@google.com>,
       Greg Thelen <gthelen@google.com>,
       "Kirill A. Shutemov" <kirill@shutemov.name>,
       Andrew Morton <akpm@linux-foundation.org>,
       containers@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org
Subject: Re: [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v6)
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On Wed, Mar 10, 2010 at 12:00:31AM +0100, Andrea Righi wrote:
> Control the maximum amount of dirty pages a cgroup can have at any given time.
> 
> Per cgroup dirty limit is like fixing the max amount of dirty (hard to reclaim)
> page cache used by any cgroup. So, in case of multiple cgroup writers, they
> will not be able to consume more than their designated share of dirty pages and
> will be forced to perform write-out if they cross that limit.
> 
> The overall design is the following:
> 
>  - account dirty pages per cgroup
>  - limit the number of dirty pages via memory.dirty_ratio / memory.dirty_bytes
>    and memory.dirty_background_ratio / memory.dirty_background_bytes in
>    cgroupfs
>  - start to write-out (background or actively) when the cgroup limits are
>    exceeded
> 
> This feature is supposed to be strictly connected to any underlying IO
> controller implementation, so we can stop increasing dirty pages in VM layer
> and enforce a write-out before any cgroup will consume the global amount of
> dirty pages defined by the /proc/sys/vm/dirty_ratio|dirty_bytes and
> /proc/sys/vm/dirty_background_ratio|dirty_background_bytes limits.
> 

Hi Andrea,

I am doing a simple dd test of writting a 4G file. This machine has got
64G of memory and I have created one cgroup with 100M as limit_in_bytes.

I run following dd program both in root cgroup as well as test1/
cgroup(100M limit) one after the other.

In root cgroup
==============
dd if=/dev/zero of=/root/zerofile bs=4K count=1000000
1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 59.5571 s, 68.8 MB/s

In test1/ cgroup
===============
dd if=/dev/zero of=/root/zerofile bs=4K count=1000000
1000000+0 records in
1000000+0 records out
4096000000 bytes (4.1 GB) copied, 20.6683 s, 198 MB/s

It is strange that we are throttling process in root group much more than
process in test1/ cgroup?

Thanks
Vivek