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

[Nauman Rafique]

On Fri, Nov 7, 2008 at 6:19 AM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> On Thu, Nov 06, 2008 at 03:07:57PM -0800, Nauman Rafique wrote:
>> It seems that approaches with two level scheduling (DM-IOBand or this
>> patch set on top and another scheduler at elevator) will have the
>> possibility of undesirable interactions (see "issues" listed at the
>> end of the second patch). For example, a request submitted as RT might
>> get delayed at higher layers, even if cfq at elevator level is doing
>> the right thing.
>>
>
> Yep. Buffering of bios at higher layer can break underlying elevator's
> assumptions.
>
> What if we start keeping track of task priorities and RT tasks in higher
> level schedulers and dispatch the bios accordingly. Will it break the
> underlying noop, deadline or AS?

It will probably not. But then we have a cfq-like scheduler at higher
level and we can agree that the combinations "cfq(higher
level)-noop(lower level)", "cfq-deadline", "cfq-as" and "cfq-cfq"
would probably work.  But if we implement one high level cfq-like
scheduler at a higher level, we would not take care of somebody who
wants noop-noop or propotional-noop. The point I am trying to make is
that there is probably no single one-size-fits-all solution for a
higher level scheduler. And we should limit the arbitrary mixing and
matching of higher level schedulers and elevator schedulers. That
being said, the existence of a higher level scheduler is still a point
of debate I guess, see my comments below.


>
>> Moreover, if the requests in the higher level scheduler are dispatched
>> as soon as they come, there would be no queuing at the higher layers,
>> unless the request queue at the lower level fills up and causes a
>> backlog. And in the absence of queuing, any work-conserving scheduler
>> would behave as a no-op scheduler.
>>
>> These issues motivate to take a second look into two level scheduling.
>> The main motivations for two level scheduling seem to be:
>> (1) Support bandwidth division across multiple devices for RAID and LVMs.
>
> Nauman, can you give an example where we really need bandwidth division
> for higher level devices.
>
> I am beginning to think that real contention is at leaf level physical
> devices and not at higher level logical devices hence we should be doing
> any resource management only at leaf level and not worry about higher
> level logical devices.
>
> If this requirement goes away, then case of two level scheduler weakens
> and one needs to think about doing changes at leaf level IO schedulers.

I cannot agree with you more on this that there is only contention at
the leaf level physical devices and bandwidth should be managed only
there. But having seen earlier posts on this list, i feel some folks
might not agree with us. For example, if we have RAID-0 striping, we
might want to schedule requests based on accumulative bandwidth used
over all devices. Again, I myself don't agree with moving scheduling
at a higher level just to support that.

>
>> (2) Divide bandwidth between different cgroups without modifying each
>> of the existing schedulers (and without replicating the code).
>>
>> One possible approach to handle (1) is to keep track of bandwidth
>> utilized by each cgroup in a per cgroup data structure (instead of a
>> per cgroup per device data structure) and use that information to make
>> scheduling decisions within the elevator level schedulers. Such  a
>> patch can be made flag-disabled if co-ordination across different
>> device schedulers is not required.
>>
>
> Can you give more details about it. I am not sure I understand it. Exactly
> what information should be stored in each cgroup.
>
> I think per cgroup per device data structures are good so that an scheduer
> will not worry about other devices present in the system and will just try
> to arbitrate between various cgroup contending for that device. This goes
> back to same issue of getting rid of requirement (1) from io controller.

I was thinking that we can keep track of disk time used at each
device, and keep the cumulative number in a per cgroup data structure.
But that is only if we want to support bandwidth division across
devices. You and me both agree that we probably do not need to do
that.

>
>> And (2) can probably be handled by having one scheduler support
>> different modes. For example, one possible mode is "propotional
>> division between crgroups + no-op between threads of a cgroup" or "cfq
>> between cgroups + cfq between threads of a cgroup". That would also
>> help avoid combinations which might not work e.g RT request issue
>> mentioned earlier in this email. And this unified scheduler can re-use
>> code from all the existing patches.
>>
>
> IIUC, you are suggesting some kind of unification between four IO
> schedulers so that proportional weight code is not replicated and user can
> switch mode on the fly based on tunables?

Yes, that seems to be a solution to avoid replication of code. But we
should also look at any other solutions that avoid replication of
code, and also avoid scheduling in two different layers.
In my opinion, scheduling at two different layers is problematic because
(a) Any buffering done at a higher level will be artificial, unless
the queues at lower levels are completely full. And if there is no
buffering at a higher level, any scheduling scheme would be
ineffective.
(b) We cannot have an arbitrary mixing and matching of higher and
lower level schedulers.

(a) would exist in any solution in which requests are queued at
multiple levels. Can you please comment on this with respect to the
patch that you have posted?

Thanks.
--
Nauman

>
> Thanks
> Vivek
>
>> Thanks.
>> --
>> Nauman
>>
>> On Thu, Nov 6, 2008 at 9:08 AM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>> > On Thu, Nov 06, 2008 at 05:52:07PM +0100, Peter Zijlstra wrote:
>> >> On Thu, 2008-11-06 at 11:39 -0500, Vivek Goyal wrote:
>> >> > On Thu, Nov 06, 2008 at 05:16:13PM +0100, Peter Zijlstra wrote:
>> >> > > On Thu, 2008-11-06 at 11:01 -0500, Vivek Goyal wrote:
>> >> > >
>> >> > > > > Does this still require I use dm, or does it also work on regular block
>> >> > > > > devices? Patch 4/4 isn't quite clear on this.
>> >> > > >
>> >> > > > No. You don't have to use dm. It will simply work on regular devices. We
>> >> > > > shall have to put few lines of code for it to work on devices which don't
>> >> > > > make use of standard __make_request() function and provide their own
>> >> > > > make_request function.
>> >> > > >
>> >> > > > Hence for example, I have put that few lines of code so that it can work
>> >> > > > with dm device. I shall have to do something similar for md too.
>> >> > > >
>> >> > > > Though, I am not very sure why do I need to do IO control on higher level
>> >> > > > devices. Will it be sufficient if we just control only bottom most
>> >> > > > physical block devices?
>> >> > > >
>> >> > > > Anyway, this approach should work at any level.
>> >> > >
>> >> > > Nice, although I would think only doing the higher level devices makes
>> >> > > more sense than only doing the leafs.
>> >> > >
>> >> >
>> >> > I thought that we should be doing any kind of resource management only at
>> >> > the level where there is actual contention for the resources.So in this case
>> >> > looks like only bottom most devices are slow and don't have infinite bandwidth
>> >> > hence the contention.(I am not taking into account the contention at
>> >> > bus level or contention at interconnect level for external storage,
>> >> > assuming interconnect is not the bottleneck).
>> >> >
>> >> > For example, lets say there is one linear device mapper device dm-0 on
>> >> > top of physical devices sda and sdb. Assuming two tasks in two different
>> >> > cgroups are reading two different files from deivce dm-0. Now if these
>> >> > files both fall on same physical device (either sda or sdb), then they
>> >> > will be contending for resources. But if files being read are on different
>> >> > physical deivces then practically there is no device contention (Even on
>> >> > the surface it might look like that dm-0 is being contended for). So if
>> >> > files are on different physical devices, IO controller will not know it.
>> >> > He will simply dispatch one group at a time and other device might remain
>> >> > idle.
>> >> >
>> >> > Keeping that in mind I thought we will be able to make use of full
>> >> > available bandwidth if we do IO control only at bottom most device. Doing
>> >> > it at higher layer has potential of not making use of full available bandwidth.
>> >> >
>> >> > > Is there any reason we cannot merge this with the regular io-scheduler
>> >> > > interface? afaik the only problem with doing group scheduling in the
>> >> > > io-schedulers is the stacked devices issue.
>> >> >
>> >> > I think we should be able to merge it with regular io schedulers. Apart
>> >> > from stacked device issue, people also mentioned that it is so closely
>> >> > tied to IO schedulers that we will end up doing four implementations for
>> >> > four schedulers and that is not very good from maintenance perspective.
>> >> >
>> >> > But I will spend more time in finding out if there is a common ground
>> >> > between schedulers so that a lot of common IO control code can be used
>> >> > in all the schedulers.
>> >> >
>> >> > >
>> >> > > Could we make the io-schedulers aware of this hierarchy?
>> >> >
>> >> > You mean IO schedulers knowing that there is somebody above them doing
>> >> > proportional weight dispatching of bios? If yes, how would that help?
>> >>
>> >> Well, take the slightly more elaborate example or a raid[56] setup. This
>> >> will need to sometimes issue multiple leaf level ios to satisfy one top
>> >> level io.
>> >>
>> >> How are you going to attribute this fairly?
>> >>
>> >
>> > I think in this case, definition of fair allocation will be little
>> > different. We will do fair allocation only at the leaf nodes where
>> > there is actual contention, irrespective of higher level setup.
>> >
>> > So if higher level block device issues multiple ios to satisfy one top
>> > level io, we will actually do the bandwidth allocation only on
>> > those multiple ios because that's the real IO contending for disk
>> > bandwidth.  And if these multiple ios are going to different physical
>> > devices, then contention management will take place on those devices.
>> >
>> > IOW, we will not worry about providing fairness at bios submitted to
>> > higher level devices. We will just pitch in for contention management
>> > only when request from various cgroups are contending for physical
>> > device at bottom most layers. Isn't if fair?
>> >
>> > Thanks
>> > Vivek
>> >
>> >> I don't think the issue of bandwidth availability like above will really
>> >> be an issue, if your stripe is set up symmetrically, the contention
>> >> should average out to both (all) disks in equal measures.
>> >>
>> >> The only real issue I can see is with linear volumes, but those are
>> >> stupid anyway - non of the gains but all the risks.
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