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

On Fri, Nov 7, 2008 at 6:19 AM, Vivek Goyal [off-list ref] wrote:

On Thu, Nov 06, 2008 at 03:07:57PM -0800, Nauman Rafique wrote:

quoted

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.

quoted

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.

quoted

(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.

quoted

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

quoted

Thanks.
--
Nauman

On Thu, Nov 6, 2008 at 9:08 AM, Vivek Goyal [off-list ref] wrote:

quoted

On Thu, Nov 06, 2008 at 05:52:07PM +0100, Peter Zijlstra wrote:

quoted

On Thu, 2008-11-06 at 11:39 -0500, Vivek Goyal wrote:

quoted

On Thu, Nov 06, 2008 at 05:16:13PM +0100, Peter Zijlstra wrote:

quoted

On Thu, 2008-11-06 at 11:01 -0500, Vivek Goyal wrote:

quoted

quoted

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.

quoted

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.

quoted

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

quoted

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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