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    SubjectRe: [PATCH 21/25] io-controller: Per cgroup request descriptor support
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    On Mon, Jul 20, 2009 at 10:37 PM, Gui
    Jianfeng<guijianfeng@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
    >>   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?

    Vivek,
    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.

    >
    > --
    > Regards
    > Gui Jianfeng
    >
    >
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