On Mon, Jul 20, 2009 at 10:37 PM, Gui Jianfeng<guijianfeng@xxxxxxxxxxxxxx> 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 > > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
