In an attempt to make sure that this discussion leads to something useful, we have summarized the points raised in this discussion and have come up with a strategy for future. The goal of this is to find common ground between all the approaches proposed on this mailing list. 1 Start with Satoshi's latest patches. 2 Do the following to support propotional division: a) Give time slices in proportion to weights (configurable option). We can support both priorities and weights by doing propotional division between requests with same priorities. 3 Schedule time slices using WF2Q+ instead of round robin. Test the performance impact (both throughput and jitter in latency). 4 Do the following to support the goals of 2 level schedulers: a) Limit the request descriptors allocated to each cgroup by adding functionality to elv_may_queue() b) Add support for putting an absolute limit on IO consumed by a cgroup. Such support exists in dm-ioband and is provided by Andrea Righi's patches too. c) Add support (configurable option) to keep track of total disk time/sectors/count consumed at each device, and factor that into scheduling decision (more discussion needed here) 5 Support multiple layers of cgroups to align IO controller behavior with CPU scheduling behavior (more discussion?) 6 Incorporate an IO tracking approach which re-uses memory resource controller code but is not dependent on it (may be biocgroup patches from dm-ioband can be used here directly) 7 Start an offline email thread to keep track of progress on the above goals. Please feel free to add/modify items to the list when you respond back. Any comments/suggestions are more than welcome. Thanks. Divyesh & Nauman On Fri, Nov 14, 2008 at 8:05 AM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > On Thu, Nov 13, 2008 at 02:57:29PM -0800, Divyesh Shah wrote: > > [..] >> > > > Ryo, do you still want to stick to two level scheduling? Given the problem >> > > > of it breaking down underlying scheduler's assumptions, probably it makes >> > > > more sense to the IO control at each individual IO scheduler. >> > > >> > > Vivek, >> > > I agree with you that 2 layer scheduler *might* invalidate some >> > > IO scheduler assumptions (though some testing might help here to >> > > confirm that). However, one big concern I have with proportional >> > > division at the IO scheduler level is that there is no means of doing >> > > admission control at the request queue for the device. What we need is >> > > request queue partitioning per cgroup. >> > > Consider that I want to divide my disk's bandwidth among 3 >> > > cgroups(A, B and C) equally. But say some tasks in the cgroup A flood >> > > the disk with IO requests and completely use up all of the requests in >> > > the rq resulting in the following IOs to be blocked on a slot getting >> > > empty in the rq thus affecting their overall latency. One might argue >> > > that over the long term though we'll get equal bandwidth division >> > > between these cgroups. But now consider that cgroup A has tasks that >> > > always storm the disk with large number of IOs which can be a problem >> > > for other cgroups. >> > > This actually becomes an even larger problem when we want to >> > > support high priority requests as they may get blocked behind other >> > > lower priority requests which have used up all the available requests >> > > in the rq. With request queue division we can achieve this easily by >> > > having tasks requiring high priority IO belong to a different cgroup. >> > > dm-ioband and any other 2-level scheduler can do this easily. >> > > >> > >> > Hi Divyesh, >> > >> > I understand that request descriptors can be a bottleneck here. But that >> > should be an issue even today with CFQ where a low priority process >> > consume lots of request descriptors and prevent higher priority process >> > from submitting the request. >> >> Yes that is true and that is one of the main reasons why I would lean >> towards 2-level scheduler coz you get request queue division as well. >> >> I think you already said it and I just >> > reiterated it. >> > >> > I think in that case we need to do something about request descriptor >> > allocation instead of relying on 2nd level of IO scheduler. >> > At this point I am not sure what to do. May be we can take feedback from the >> > respective queue (like cfqq) of submitting application and if it is already >> > backlogged beyond a certain limit, then we can put that application to sleep >> > and stop it from consuming excessive amount of request descriptors >> > (despite the fact that we have free request descriptors). >> >> This should be done per-cgroup rather than per-process. >> > > Yep, per cgroup limit will make more sense. get_request() already calls > elv_may_queue() to get a feedback from IO scheduler. May be here IO > scheduler can make a decision how many request descriptors are already > allocated to this cgroup. And if the queue is congested, then IO scheduler > can deny the fresh request allocation. > > Thanks > Vivek > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
