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

On Thu, Nov 20, 2008 at 5:40 AM, Vivek Goyal [off-list ref] wrote:

On Thu, Nov 20, 2008 at 09:16:41AM +0100, Jens Axboe wrote:

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On Wed, Nov 19 2008, Divyesh Shah wrote:

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On Wed, Nov 19, 2008 at 6:24 AM, Jens Axboe [off-list ref] wrote:

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On Tue, Nov 18 2008, Nauman Rafique wrote:

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On Tue, Nov 18, 2008 at 11:12 AM, Jens Axboe [off-list ref] wrote:

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On Tue, Nov 18 2008, Fabio Checconi wrote:

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From: Vivek Goyal <vgoyal@redhat.com>
Date: Tue, Nov 18, 2008 09:07:51AM -0500

On Tue, Nov 18, 2008 at 01:05:08PM +0100, Fabio Checconi wrote:

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I have to think a little bit on how it would be possible to support
an option for time-only budgets, coexisting with the current behavior,
but I think it can be done.

IIUC, bfq and cfq are different in following manner.

a. BFQ employs WF2Q+ for fairness and CFQ employes weighted round robin.
b. BFQ uses the budget (sector count) as notion of service and CFQ uses
   time slices.
c. BFQ supports hierarchical fair queuing and CFQ does not.

We are looking forward for implementation of point C. Fabio seems to
thinking of supporting time slice as a service (B). It seems like
convergence of CFQ and BFQ except the point A (WF2Q+ vs weighted round
robin).

It looks like WF2Q+ provides tighter service bound and bfq guys mention
that they have been able to ensure throughput while ensuring tighter
bounds. If that's the case, does that mean BFQ is a replacement for CFQ
down the line?

BFQ started from CFQ, extending it in the way you correctly describe,
so it is indeed very similar.  There are also some minor changes to
locking, cic handling, hw_tag detection and to the CIC_SEEKY heuristic.

The two schedulers share similar goals, and in my opinion BFQ can be
considered, in the long term, a CFQ replacement; *but* before talking
about replacing CFQ we have to consider that:

  - it *needs* review and testing; we've done our best, but for sure
    it's not enough; review and testing are never enough;
  - the service domain fairness, which was one of our objectives, requires
    some extra complexity; the mechanisms we used and the design choices
    we've made may not fit all the needs, or may not be as generic as the
    simpler CFQ's ones;
  - CFQ has years of history behind and has been tuned for a wider
    variety of environments than the ones we've been able to test.

If time-based fairness is considered more robust and the loss of
service-domain fairness is not a problem, then the two schedulers can
be made even more similar.

My preferred approach here would be, in order or TODO:

- Create and test the smallish patches for seekiness, hw_tag checking,
 and so on for CFQ.
- Create and test a WF2Q+ service dispatching patch for CFQ.

and if there are leftovers after that, we could even conditionally
enable some of those if appropriate. I think the WF2Q+ is quite cool and
could be easily usable as the default, so it's definitely a viable
alternative.

1 Merge BFQ into CFQ (Jens and Fabio). I am assuming that this would
result in time slices being scheduled using WF2Q+

Yep, at least that is my preference.

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2 Do the following to support proportional division:
 a) Expose the per device weight interface to user, instead of calculating
 from priority.
 b) Add support for scheduling bandwidth among a hierarchy of cgroups
(besides threads)
3 Do the following to support the goals of 2 level schedulers:
 a) Limit the request descriptors allocated to each cgroup by adding
 functionality to elv_may_queue()
 b) Add support for putting an absolute limit on IO consumed by a
 cgroup. Such support is provided by Andrea
 Righi's patches too.
 c) Add support (configurable option) to keep track of total disk
time/sectors/count
 consumed at each device, and factor that into scheduling decision
 (more discussion needed here)
6 Incorporate an IO tracking approach which can allow tracking cgroups
for asynchronous reads/writes.
7 Start an offline email thread to keep track of progress on the above
goals.

Jens, what is your opinion everything beyond (1) in the above list?

It would be great if work on (1) and (2)-(7) can happen in parallel so
that we can see "proportional division of IO bandwidth to cgroups" in
tree sooner than later.

Sounds feasible, I'd like to see the cgroups approach get more traction.
My primary concern is just that I don't want to merge it into specific
IO schedulers.

Jens,
     So are you saying you don't prefer cgroups based proportional IO
division solutions in the IO scheduler but at a layer above so it can
be shared with all IO schedulers?

     If yes, then in that case, what do you think about Vivek Goyal's
patch or dm-ioband that achieve that. Of course, both solutions don't
meet all the requirements in the list above, but we can work on that
once we know which direction we should be heading in. In fact, it
would help if you could express the reservations (if you have any)
about these approaches. That would help in coming up with a plan that
everyone agrees on.

The dm approach has some merrits, the major one being that it'll fit
directly into existing setups that use dm and can be controlled with
familiar tools. That is a bonus. The draw back is partially the same -
it'll require dm. So it's still not a fit-all approach, unfortunately.

So I'd prefer an approach that doesn't force you to use dm.

Hi Jens,

My patches met the goal of not using the dm for every device one wants
to control.

Having said that, few things come to mind.

- In what cases do we need to control the higher level logical devices
 like dm. It looks like real contention for resources is at leaf nodes.
 Hence any kind of resource management/fair queueing should probably be
 done at leaf nodes and not at higher level logical nodes.

 If that makes sense, then probably we don't need to control dm device
 and we don't need such higher level solutions.


- Any kind of 2 level scheduler solution has the potential to break the
 underlying IO scheduler. Higher level solution requires buffering of
 bios and controlled release of bios to lower layers. This control breaks
 the assumptions of lower layer IO scheduler which knows in what order
 bios should be dispatched to device to meet the semantics exported by
 the IO scheduler.

- 2nd level scheduler does not keep track of tasks but task groups lets
 every group dispatch fair share. This has got little semantic problem in
 the sense that tasks and groups in root cgroup will not be considered at
 same level. "root" will be considered one group at same level with all
 child group hence competing with them for resources.

 This looks little odd. Considering tasks and groups same level kind of
 makes more sense. cpu scheduler also consideres tasks and groups at same
 level and deviation from that probably is not very good.

 Considering tasks and groups at same level will matter only if IO
 scheduler maintains separate queue for the task, like CFQ. Because
 in that case IO scheduler tries to provide fairness among various task
 queues. Some schedulers like noop don't have any notion of separate
 task queues and fairness among them. In that case probably we don't
 have a choice but to assume root group competing with child groups.

Keeping above points in mind, probably two level scheduling is not a
very good idea. If putting the code in a particular IO scheduler is a
concern we can probably explore ways regarding how we can maximize the
sharing of cgroup code among IO schedulers.

Thanks
Vivek

It seems that we have a solution if we can figure out a way to share
cgroup code between different schedulers. I am thinking how other
schedulers (AS, Deadline, No-op) would use cgroups. Will they have
proportional division between requests from different cgroups? And use
their own policy (e.g deadline scheduling) within a cgroup? How about
if we have both threads and cgroups at a particular level? I think
putting all threads in a default cgroup seems like a reasonable choice
in this case.

Here is a high level design that comes to mind.

Put proportional division code and state in common code. Each level of
the hierarchy which has more than one cgroup would have some state
maintained in common code. At leaf level of hiearchy, we can have a
cgroup specific scheduler (created when a cgroup is created). We can
choose a different scheduler for each cgroup (we can have a no-op for
one cgroup while cfq for another).

The scheduler gets a callback (just like it does right now from
driver) when the common code schedules a time slice (or budget) from
that cgroup. No buffering/queuing is done in the common code, so it
will still be a single level scheduler. When a request arrives, it is
routed to its scheduler's queues based on its cgroup.

The common code proportional scheduler uses specified weights to
schedule time slices. Please let me know if it makes any sense. And
then we can start talking about lower level details.