[PATCH 21/25] io-controller: Per cgroup request descriptor support
nauman at google.com
Mon Jul 20 22:55:31 PDT 2009
On Mon, Jul 20, 2009 at 10:37 PM, Gui
Jianfeng<guijianfeng at cn.fujitsu.com> wrote:
> 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
> 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?
I agree with Gui here. In fact, it does not make much sense to keep
the nr_requests limit if we already have per cgroup limit in place.
This change also simplifies code quite a bit, as we can get rid of all
that sleep_on_global logic.
> Gui Jianfeng
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