[Devel] Re: [PATCH 0/5] blk-throttle: writeback and swap IO control

[Greg Thelen]

On Wed, Feb 23, 2011 at 4:40 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu at jp.fujitsu.com> wrote:
> On Wed, 23 Feb 2011 19:10:33 -0500
> Vivek Goyal <vgoyal at redhat.com> wrote:
>
>> On Thu, Feb 24, 2011 at 12:14:11AM +0100, Andrea Righi wrote:
>> > On Wed, Feb 23, 2011 at 10:23:54AM -0500, Vivek Goyal wrote:
>> > > > > Agreed. Granularity of per inode level might be accetable in many
>> > > > > cases. Again, I am worried faster group getting stuck behind slower
>> > > > > group.
>> > > > >
>> > > > > I am wondering if we are trying to solve the problem of ASYNC write throttling
>> > > > > at wrong layer. Should ASYNC IO be throttled before we allow task to write to
>> > > > > page cache. The way we throttle the process based on dirty ratio, can we
>> > > > > just check for throttle limits also there or something like that.(I think
>> > > > > that's what you had done in your initial throttling controller implementation?)
>> > > >
>> > > > Right. This is exactly the same approach I've used in my old throttling
>> > > > controller: throttle sync READs and WRITEs at the block layer and async
>> > > > WRITEs when the task is dirtying memory pages.
>> > > >
>> > > > This is probably the simplest way to resolve the problem of faster group
>> > > > getting blocked by slower group, but the controller will be a little bit
>> > > > more leaky, because the writeback IO will be never throttled and we'll
>> > > > see some limited IO spikes during the writeback.
>> > >
>> > > Yes writeback will not be throttled. Not sure how big a problem that is.
>> > >
>> > > - We have controlled the input rate. So that should help a bit.
>> > > - May be one can put some high limit on root cgroup to in blkio throttle
>> > >   controller to limit overall WRITE rate of the system.
>> > > - For SATA disks, try to use CFQ which can try to minimize the impact of
>> > >   WRITE.
>> > >
>> > > It will atleast provide consistent bandwindth experience to application.
>> >
>> > Right.
>> >
>> > >
>> > > >However, this is always
>> > > > a better solution IMHO respect to the current implementation that is
>> > > > affected by that kind of priority inversion problem.
>> > > >
>> > > > I can try to add this logic to the current blk-throttle controller if
>> > > > you think it is worth to test it.
>> > >
>> > > At this point of time I have few concerns with this approach.
>> > >
>> > > - Configuration issues. Asking user to plan for SYNC ans ASYNC IO
>> > >   separately is inconvenient. One has to know the nature of workload.
>> > >
>> > > - Most likely we will come up with global limits (atleast to begin with),
>> > >   and not per device limit. That can lead to contention on one single
>> > >   lock and scalability issues on big systems.
>> > >
>> > > Having said that, this approach should reduce the kernel complexity a lot.
>> > > So if we can do some intelligent locking to limit the overhead then it
>> > > will boil down to reduced complexity in kernel vs ease of use to user. I
>> > > guess at this point of time I am inclined towards keeping it simple in
>> > > kernel.
>> > >
>> >
>> > BTW, with this approach probably we can even get rid of the page
>> > tracking stuff for now.
>>
>> Agreed.
>>
>> > If we don't consider the swap IO, any other IO
>> > operation from our point of view will happen directly from process
>> > context (writes in memory + sync reads from the block device).
>>
>> Why do we need to account for swap IO? Application never asked for swap
>> IO. It is kernel's decision to move soem pages to swap to free up some
>> memory. What's the point in charging those pages to application group
>> and throttle accordingly?
>>
>
> I think swap I/O should be controlled by memcg's dirty_ratio.
> But, IIRC, NEC guy had a requirement for this...
>
> I think some enterprise cusotmer may want to throttle the whole speed of
> swapout I/O (not swapin)...so, they may be glad if they can limit throttle
> the I/O against a disk partition or all I/O tagged as 'swapio' rather than
> some cgroup name.
>
> But I'm afraid slow swapout may consume much dirty_ratio and make things
> worse ;)
>
>
>
>> >
>> > However, I'm sure we'll need the page tracking also for the blkio
>> > controller soon or later. This is an important information and also the
>> > proportional bandwidth controller can take advantage of it.
>>
>> Yes page tracking will be needed for CFQ proportional bandwidth ASYNC
>> write support. But until and unless we implement memory cgroup dirty
>> ratio and figure a way out to make writeback logic cgroup aware, till
>> then I think page tracking stuff is not really useful.
>>
>
> I think Greg Thelen is now preparing patches for dirty_ratio.
>
> Thanks,
> -Kame
>
>

Correct.  I am working on the memcg dirty_ratio patches with latest
mmotm memcg.  I am running some test cases which should be complete
tomorrow.  Once testing is complete, I will sent  the patches for
review.
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