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

[Ryo Tsuruta]

Hi Vivek,

From: Vivek Goyal <vgoyal at redhat.com>
Subject: Re: [patch 0/4] [RFC] Another proportional weight IO controller
Date: Tue, 25 Nov 2008 11:27:20 -0500

> On Tue, Nov 25, 2008 at 11:33:59AM +0900, Ryo Tsuruta wrote:
> > Hi Vivek,
> > 
> > > > > 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.
> > > > 
> > > > I don't want to stick to it. I'm considering implementing dm-ioband's
> > > > algorithm into the block I/O layer experimentally.
> > > 
> > > Thanks Ryo. Implementing a control at block layer sounds like another
> > > 2 level scheduling. We will still have the issue of breaking underlying
> > > CFQ and other schedulers. How to plan to resolve that conflict.
> > 
> > I think there is no conflict against I/O schedulers.
> > Could you expain to me about the conflict?
> 
> Because we do the buffering at higher level scheduler and mostly release
> the buffered bios in the FIFO order, it might break the underlying IO
> schedulers. Generally it is the decision of IO scheduler to determine in
> what order to release buffered bios.
> 
> For example, If there is one task of io priority 0 in a cgroup and rest of
> the tasks are of io prio 7. All the tasks belong to best effort class. If
> tasks of lower priority (7) do lot of IO, then due to buffering there is
> a chance that IO from lower prio tasks is seen by CFQ first and io from
> higher prio task is not seen by cfq for quite some time hence that task
> not getting it fair share with in the cgroup. Similiar situations can
> arise with RT tasks also.

Thanks for your explanation. 
I think that the same thing occurs without the higher level scheduler,
because all the tasks issuing I/Os are blocked while the underlying
device's request queue is full before those I/Os are sent to the I/O
scheduler.

> > > What do you think about the solution at IO scheduler level (like BFQ) or
> > > may be little above that where one can try some code sharing among IO
> > > schedulers? 
> > 
> > I would like to support any type of block device even if I/Os issued
> > to the underlying device doesn't go through IO scheduler. Dm-ioband
> > can be made use of for the devices such as loop device.
> > 
> 
> What do you mean by that IO issued to underlying device does not go
> through IO scheduler? loop device will be associated with a file and
> IO will ultimately go to the IO scheduler which is serving those file
> blocks?

How about if the files is on an NFS-mounted file system?

> What's the use case scenario of doing IO control at loop device?
> Ultimately the resource contention will take place on actual underlying
> physical device where the file blocks are. Will doing the resource control
> there not solve the issue for you?

I don't come up with any use case, but I would like to make the
resource controller more flexible. Actually, a certain block device
that I'm using does not use the I/O scheduler.

Thanks,
Ryo Tsuruta
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