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SubjectRe: [PATCH 20/23] io-controller: Per cgroup request descriptor support
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 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.
> Signed-off-by: Nauman Rafique <>
> Signed-off-by: Vivek Goyal <>

Acked-by: Rik van Riel <>

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