[Devel] Re: [PATCH 20/23] io-controller: Per cgroup request descriptor support

[Rik van Riel]

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 <nauman at google.com>
> Signed-off-by: Vivek Goyal <vgoyal at redhat.com>

Acked-by: Rik van Riel <riel at redhat.com>
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