[Devel] Re: IO scheduler based IO controller V10

Mon Oct 5 11:11:35 PDT 2009

On Mon, Oct 5, 2009 at 10:10 AM, Vivek Goyal <vgoyal at redhat.com> wrote:
> On Mon, Oct 05, 2009 at 11:55:35PM +0900, Ryo Tsuruta wrote:
>> Hi Vivek,
>>
>> Vivek Goyal <vgoyal at redhat.com> wrote:
>> > On Mon, Oct 05, 2009 at 07:38:08PM +0900, Ryo Tsuruta wrote:
>> > > Hi,
>> > >
>> > > Munehiro Ikeda <m-ikeda at ds.jp.nec.com> wrote:
>> > > > Vivek Goyal wrote, on 10/01/2009 10:57 PM:
>> > > > > Before finishing this mail, will throw a whacky idea in the ring. I was
>> > > > > going through the request based dm-multipath paper. Will it make sense
>> > > > > to implement request based dm-ioband? So basically we implement all the
>> > > > > group scheduling in CFQ and let dm-ioband implement a request function
>> > > > > to take the request and break it back into bios. This way we can keep
>> > > > > all the group control at one place and also meet most of the requirements.
>> > > > >
>> > > > > So request based dm-ioband will have a request in hand once that request
>> > > > > has passed group control and prio control. Because dm-ioband is a device
>> > > > > mapper target, one can put it on higher level devices (practically taking
>> > > > > CFQ at higher level device), and provide fairness there. One can also
>> > > > > put it on those SSDs which don't use IO scheduler (this is kind of forcing
>> > > > > them to use the IO scheduler.)
>> > > > >
>> > > > > I am sure that will be many issues but one big issue I could think of that
>> > > > > CFQ thinks that there is one device beneath it and dipsatches requests
>> > > > > from one queue (in case of idling) and that would kill parallelism at
>> > > > > higher layer and throughput will suffer on many of the dm/md configurations.
>> > > > >
>> > > > > Thanks
>> > > > > Vivek
>> > > >
>> > > > As long as using CFQ, your idea is reasonable for me.  But how about for
>> > > > other IO schedulers?  In my understanding, one of the keys to guarantee
>> > > > group isolation in your patch is to have per-group IO scheduler internal
>> > > > queue even with as, deadline, and noop scheduler.  I think this is
>> > > > great idea, and to implement generic code for all IO schedulers was
>> > > > concluded when we had so many IO scheduler specific proposals.
>> > > > If we will still need per-group IO scheduler internal queues with
>> > > > request-based dm-ioband, we have to modify elevator layer.  It seems
>> > > > out of scope of dm.
>> > > > I might miss something...
>> > >
>> > > IIUC, the request based device-mapper could not break back a request
>> > > into bio, so it could not work with block devices which don't use the
>> > > IO scheduler.
>> > >
>> >
>> > I think current request based multipath drvier does not do it but can't it
>> > be implemented that requests are broken back into bio?
>>
>> I guess it would be hard to implement it, and we need to hold requests
>> and throttle them at there and it would break the ordering by CFQ.
>>
>> > Anyway, I don't feel too strongly about this approach as it might
>> > introduce more serialization at higher layer.
>>
>> Yes, I know it.
>>
>> > > How about adding a callback function to the higher level controller?
>> > > CFQ calls it when the active queue runs out of time, then the higer
>> > > level controller use it as a trigger or a hint to move IO group, so
>> > > I think a time-based controller could be implemented at higher level.
>> > >
>> >
>> > Adding a call back should not be a big issue. But that means you are
>> > planning to run only one group at higher layer at one time and I think
>> > that's the problem because than we are introducing serialization at higher
>> > layer. So any higher level device mapper target which has multiple
>> > physical disks under it, we might be underutilizing these even more and
>> > take a big hit on overall throughput.
>> >
>> > The whole design of doing proportional weight at lower layer is optimial
>> > usage of system.
>>
>> But I think that the higher level approch makes easy to configure
>> against striped software raid devices.
>
> How does it make easier to configure in case of higher level controller?
>
> In case of lower level design, one just have to create cgroups and assign
> weights to cgroups. This mininum step will be required in higher level
> controller also. (Even if you get rid of dm-ioband device setup step).
>
>> If one would like to
>> combine some physical disks into one logical device like a dm-linear,
>> I think one should map the IO controller on each physical device and
>> combine them into one logical device.
>>
>
> In fact this sounds like a more complicated step where one has to setup
> one dm-ioband device on top of each physical device. But I am assuming
> that this will go away once you move to per reuqest queue like implementation.
>
> I think it should be same in principal as my initial implementation of IO
> controller on request queue and I stopped development on it because of FIFO
> dispatch.
>
> So you seem to be suggesting that you will move dm-ioband to request queue
> so that setting up additional device setup is gone. You will also enable
> it to do time based groups policy, so that we don't run into issues on
> seeky media. Will also enable dispatch from one group only at a time so
> that we don't run into isolation issues and can do time accounting
> accruately.

Will that approach solve the problem of doing bandwidth control on
logical devices? What would be the advantages compared to Vivek's
current patches?

>
> If yes, then that has the potential to solve the issue. At higher layer one
> can think of enabling size of IO/number of IO policy both for proportional
> BW and max BW type of control. At lower level one can enable pure time
> based control on seeky media.
>
> I think this will still left with the issue of prio with-in group as group
> control is separate and you will not be maintatinig separate queues for
> each process. Similarly you will also have isseus with read vs write
> ratios as IO schedulers underneath change.
>
> So I will be curious to see that implementation.
>
>> > > My requirements for IO controller are:
>> > > - Implement s a higher level controller, which is located at block
>> > >   layer and bio is grabbed in generic_make_request().
>> >
>> > How are you planning to handle the issue of buffered writes Andrew raised?
>>
>> I think that it would be better to use the higher-level controller
>> along with the memory controller and have limits memory usage for each
>> cgroup. And as Kamezawa-san said, having limits of dirty pages would
>> be better, too.
>>
>
> Ok. So if we plan to co-mount memory controller with per memory group
> dirty_ratio implemented, that can work with both higher level as well as
> low level controller. Not sure if we also require some kind of a per
> memory group flusher thread infrastructure also to make sure higher weight
> group gets more job done.
>
>> > > - Can work with any type of IO scheduler.
>> > > - Can work with any type of block devices.
>> > > - Support multiple policies, proportional wegiht, max rate, time
>> > >   based, ans so on.
>> > >
>> > > The IO controller mini-summit will be held in next week, and I'm
>> > > looking forard to meet you all and discuss about IO controller.
>> > > https://sourceforge.net/apps/trac/ioband/wiki/iosummit
>> >
>> > Is there a new version of dm-ioband now where you have solved the issue of
>> > sync/async dispatch with-in group? Before meeting at mini-summit, I am
>> > trying to run some tests and come up with numbers so that we have more
>> > clear picture of pros/cons.
>>
>> Yes, I've released new versions of dm-ioband and blkio-cgroup. The new
>> dm-ioband handles sync/async IO requests separately and
>> the write-starve-read issue you pointed out is fixed. I would
>> appreciate it if you would try them.
>> http://sourceforge.net/projects/ioband/files/
>
> Cool. Will get to testing it.
>
> Thanks
> Vivek
>
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