Message-ID: <4A655434.5060404@cn.fujitsu.com>
Date: Tue, 21 Jul 2009 13:37:56 +0800
From: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
User-Agent: Thunderbird 2.0.0.5 (Windows/20070716)
MIME-Version: 1.0
To: Vivek Goyal <vgoyal@redhat.com>
CC: linux-kernel@vger.kernel.org, containers@lists.linux-foundation.org,
       dm-devel@redhat.com, jens.axboe@oracle.com, nauman@google.com,
       dpshah@google.com, lizf@cn.fujitsu.com, mikew@google.com,
       fchecconi@gmail.com, paolo.valente@unimore.it, ryov@valinux.co.jp,
       fernando@oss.ntt.co.jp, s-uchida@ap.jp.nec.com, taka@valinux.co.jp,
       jmoyer@redhat.com, dhaval@linux.vnet.ibm.com, balbir@linux.vnet.ibm.com,
       righi.andrea@gmail.com, m-ikeda@ds.jp.nec.com, jbaron@redhat.com,
       agk@redhat.com, snitzer@redhat.com, akpm@linux-foundation.org,
       peterz@infradead.org
Subject: Re: [PATCH 21/25] io-controller: Per cgroup request descriptor support
References: <1246564917-19603-1-git-send-email-vgoyal@redhat.com> <1246564917-19603-22-git-send-email-vgoyal@redhat.com>
In-Reply-To: <1246564917-19603-22-git-send-email-vgoyal@redhat.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2544
Lines: 52

Vivek Goyal wrote:
> o Currently a request queue has got fixed number of request descriptors for
>   sync and async requests. Once the request descriptors are consumed, new
>   processes are put to sleep and they effectively become serialized. Because
>   sync and async queues are separate, async requests don't impact sync ones
>   but if one is looking for fairness between async requests, that is not
>   achievable if request queue descriptors become bottleneck.
> 
> o Make request descriptor's per io group so that if there is lots of IO
>   going on in one cgroup, it does not impact the IO of other group.
> 
> o This is just one relatively simple way of doing things. This patch will
>   probably change after the feedback. Folks have raised concerns that in
>   hierchical setup, child's request descriptors should be capped by parent's
>   request descriptors. May be we need to have per cgroup per device files
>   in cgroups where one can specify the upper limit of request descriptors
>   and whenever a cgroup is created one needs to assign request descritor
>   limit making sure total sum of child's request descriptor is not more than
>   of parent.
> 
>   I guess something like memory controller. Anyway, that would be the next
>   step. For the time being, we have implemented something simpler as follows.
> 
> o This patch implements the per cgroup request descriptors. request pool per
>   queue is still common but every group will have its own wait list and its
>   own count of request descriptors allocated to that group for sync and async
>   queues. So effectively request_list becomes per io group property and not a
>   global request queue feature.
> 
> o Currently one can define q->nr_requests to limit request descriptors
>   allocated for the queue. Now there is another tunable q->nr_group_requests
>   which controls the requests descriptr limit per group. q->nr_requests
>   supercedes q->nr_group_requests to make sure if there are lots of groups
>   present, we don't end up allocating too many request descriptors on the
>   queue.
> 

  Hi Vivek,

  In order to prevent q->nr_requests from becoming the bottle-neck of allocating
  requests, whether we can update nr_requests accordingly when allocating or removing
  a cgroup?

-- 
Regards
Gui Jianfeng

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/