[Devel] Re: [patch 0/4] [RFC] Another proportional weight IO controller

[Vivek Goyal]

On Fri, Nov 14, 2008 at 02:44:22PM -0800, Nauman Rafique wrote:
> 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.

I have had a brief look at both Satoshi's patch and bfq. I kind of like
bfq's patches for keeping track of per cgroup, per queue data structures.
May be we can look there also.

> 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.

Does dm-iobnd support abosolute limit? I think till last version they did
not. I have not check the latest version though.

>  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
Vivek

> Thanks.
> Divyesh & Nauman
> 
> On Fri, Nov 14, 2008 at 8:05 AM, Vivek Goyal <vgoyal at redhat.com> 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
> >
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