[Devel] Re: [PATCH 01/10] Documentation

[Andrea Righi]

On Thu, Apr 16, 2009 at 02:37:53PM -0400, Vivek Goyal wrote:
> > I think it would be possible to implement both proportional and limiting
> > rules at the same level (e.g., the IO scheduler), but we need also to
> > address the memory consumption problem (I still need to review your
> > patchset in details and I'm going to test it soon :), so I don't know if
> > you already addressed this issue).
> > 
> 
> Can you please elaborate a bit on this? Are you concerned about that data
> structures created to solve the problem consume a lot of memory?

Sorry I was not very clear here. With memory consumption I mean wasting
the memory with hard/slow reclaimable dirty pages or pending IO
requests.

If there's only a global limit on dirty pages, any cgroup can exhaust
that limit and cause other cgroups/processes to block when they try to
write to disk.

But, ok, the IO controller is not probably the best place to implement
such functionality. I should rework on the per cgroup dirty_ratio:

https://lists.linux-foundation.org/pipermail/containers/2008-September/013140.html

Last time we focused too much on the best interfaces to define dirty
pages limit, and I never re-posted an updated version of this patchset.
Now I think we can simply provide the same dirty_ratio/dirty_bytes
interface that we provide globally, but per cgroup.

> 
> > IOW if we simply don't dispatch requests and we don't throttle the tasks
> > in the cgroup that exceeds its limit, how do we avoid the waste of
> > memory due to the succeeding IO requests and the increasingly dirty
> > pages in the page cache (that are also hard to reclaim)? I may be wrong,
> > but I think we talked about this problem in a previous email... sorry I
> > don't find the discussion in my mail archives.
> > 
> > IMHO a nice approach would be to measure IO consumption at the IO
> > scheduler level, and control IO applying proportional weights / absolute
> > limits _both_ at the IO scheduler / elevator level _and_ at the same
> > time block the tasks from dirtying memory that will generate additional
> > IO requests.
> > 
> > Anyway, there's no need to provide this with a single IO controller, we
> > could split the problem in two parts: 1) provide a proportional /
> > absolute IO controller in the IO schedulers and 2) allow to set, for
> > example, a maximum limit of dirty pages for each cgroup.
> > 
> 
> I think setting a maximum limit on dirty pages is an interesting thought.
> It sounds like as if memory controller can handle it?

Exactly, the same above.

> 
> I guess currently memory controller puts limit on total amount of memory
> consumed by cgroup and there are no knobs on type of memory consumed. So
> if one can limit amount of dirty page cache memory per cgroup, it
> automatically throttles the aysnc writes at the input itself.
>  
> So I agree that if we can limit the process from dirtying too much of
> memory than IO scheduler level controller should be able to do both
> proportional weight and max bw controller.
> 
> Currently doing proportional weight control for async writes is very
> tricky. I am not seeing constantly backlogged traffic at IO scheudler
> level and hence two different weight processes seem to be getting same
> BW.
> 
> I will dive deeper into the patches on dm-ioband to see how they have
> solved this issue. Looks like they are just waiting longer for slowest
> group to consume its tokens and that will keep the disk idle. Extended
> delays might now show up immediately as performance hog, because it might
> also promote increased merging but it should lead to increased latency of
> response. And proving latency issues is hard. :-)   
> 
> > Maybe I'm just repeating what we already said in a previous
> > discussion... in this case sorry for the duplicate thoughts. :)
> > 
> > > 
> > > - Have you thought of doing hierarchical control? 
> > > 
> > 
> > Providing hiearchies in cgroups is in general expensive, deeper
> > hierarchies imply checking all the way up to the root cgroup, so I think
> > we need to be very careful and be aware of the trade-offs before
> > providing such feature. For this particular case (IO controller)
> > wouldn't it be simpler and more efficient to just ignore hierarchies in
> > the kernel and opportunely handle them in userspace? for absolute
> > limiting rules this isn't difficult at all, just imagine a config file
> > and a script or a deamon that dynamically create the opportune cgroups
> > and configure them accordingly to what is defined in the configuration
> > file.
> > 
> > I think we can simply define hierarchical dependencies in the
> > configuration file, translate them in absolute values and use the
> > absolute values to configure the cgroups' properties.
> > 
> > For example, we can just check that the BW allocated for a particular
> > parent cgroup is not greater than the total BW allocated for the
> > children. And for each child just use the min(parent_BW, BW) or equally
> > divide the parent's BW among the children, etc.
> 
> IIUC, you are saying that allow hiearchy in user space and then flatten it
> out and pass it to kernel?
> 
> Hmm.., agree that handling hierarchies is hard and expensive. But at the
> same time rest of the controllers like cpu and memory are handling it in
> kernel so it probably makes sense to keep the IO controller also in line.
> 
> In practice I am not expecting deep hiearchices. May be 2- 3 levels would
> be good for most of the people.
> 
> > 
> > > - What happens to the notion of CFQ task classes and task priority. Looks
> > >   like max bw rule supercede everything. There is no way that an RT task
> > >   get unlimited amount of disk BW even if it wants to? (There is no notion
> > >   of RT cgroup etc)
> > 
> > What about moving all the RT tasks in a separate cgroup with unlimited
> > BW?
> 
> Hmm.., I think that should work. I have yet to look at your patches in
> detail but it looks like unlimited BW group will not be throttled at all
> hence RT tasks can just go right through without getting impacted.

Correct.

> 
> > 
> > > 
> > > > > 
> > > > >   Above requirement can create configuration problems.
> > > > > 
> > > > > 	- If there are large number of disks in system, per cgroup one shall
> > > > > 	  have to create rules for each disk. Until and unless admin knows
> > > > > 	  what applications are in which cgroup and strictly what disk
> > > > > 	  these applications do IO to and create rules for only those
> > > > >  	  disks.
> > > > 
> > > > I don't think this is a huge problem anyway. IMHO a userspace tool, e.g.
> > > > a script, would be able to efficiently create/modify rules parsing user
> > > > defined rules in some human-readable form (config files, etc.), even in
> > > > presence of hundreds of disk. The same is valid for dm-ioband I think.
> > > > 
> > > > > 
> > > > > 	- I think problem gets compounded if there is a hierarchy of
> > > > > 	  logical devices. I think in that case one shall have to create
> > > > > 	  rules for logical devices and not actual physical devices.
> > > > 
> > > > With logical devices you mean device-mapper devices (i.e. LVM, software
> > > > RAID, etc.)? or do you mean that we need to introduce the concept of
> > > > "logical device" to easily (quickly) configure IO requirements and then
> > > > map those logical devices to the actual physical devices? In this case I
> > > > think this can be addressed in userspace. Or maybe I'm totally missing
> > > > the point here.
> > > 
> > > Yes, I meant LVM, Software RAID etc. So if I have got many disks in the system
> > > and I have created software raid on some of them, I need to create rules for
> > > lvm devices or physical devices behind those lvm devices? I am assuming
> > > that it will be logical devices.
> > > 
> > > So I need to know exactly to what all devices applications in a particular
> > > cgroup is going to do IO, and also know exactly how many cgroups are
> > > contending for that cgroup, and also know what worst case disk rate I can
> > > expect from that device and then I can do a good job of giving a
> > > reasonable value to the max rate of that cgroup on a particular device?
> > 
> > ok, I understand. For these cases dm-ioband perfectly addresses the
> > problem. For the general case, I think the only solution is to provide a
> > common interface that each dm subsystem must call to account IO and
> > apply limiting and proportional rules.
> > 
> > > 
> > > > 
> > > > > 
> > > > > - Because it is not proportional weight distribution, if some
> > > > >   cgroup is not using its planned BW, other group sharing the
> > > > >   disk can not make use of spare BW.  
> > > > > 	
> > > > 
> > > > Right.
> > > > 
> > > > > - I think one should know in advance the throughput rate of underlying media
> > > > >   and also know competing applications so that one can statically define
> > > > >   the BW assigned to each cgroup on each disk.
> > > > > 
> > > > >   This will be difficult. Effective BW extracted out of a rotational media
> > > > >   is dependent on the seek pattern so one shall have to either try to make
> > > > >   some conservative estimates and try to divide BW (we will not utilize disk
> > > > >   fully) or take some peak numbers and divide BW (cgroup might not get the
> > > > >   maximum rate configured).
> > > > 
> > > > Correct. I think the proportional weight approach is the only solution
> > > > to efficiently use the whole BW. OTOH absolute limiting rules offer a
> > > > better control over QoS, because you can totally remove performance
> > > > bursts/peaks that could break QoS requirements for short periods of
> > > > time.
> > > 
> > > Can you please give little more details here regarding how QoS requirements
> > > are not met with proportional weight?
> > 
> > With proportional weights the whole bandwidth is allocated if no one
> > else is using it. When IO is submitted other tasks with a higher weight
> > can be forced to sleep until the IO generated by the low weight tasks is
> > not completely dispatched. Or any extent of the priority inversion
> > problems.
> 
> Hmm..., I am not very sure here. When admin is allocating the weights, he
> has the whole picture. He knows how many groups are conteding for the disk
> and what could be the worst case scenario. So if I have got two groups
> with A and B with weight 1 and 2 and both are contending, then as an 
> admin one would expect to get 33% of BW for group A in worst case (if
> group B is continuously backlogged). If B is not contending than A can get
> 100% of BW. So while configuring the system, will one not plan for worst
> case (33% for A, and 66 % for B)?

OK, I'm quite convinced.. :)

To a large degree, if we want to provide a BW reservation strategy we
must provide an interface that allows cgroups to ask for time slices
such as max/min 5 IO requests every 50ms or something like that.
Probably the same functionality can be achieved translating time slices
from weights, percentages or absolute BW limits.

>   
> > 
> > Maybe it's not an issue at all for the most part of the cases, but using
> > a solution that is able to provide also a real partitioning of the
> > available resources can be profitely used by those who need to guarantee
> > _strict_ BW requirements (soft real-time, maximize the responsiveness of
> > certain services, etc.), because in this case we're sure that a certain
> > amount of "spare" BW will be always available when needed by some
> > "critical" services.
> > 
> 
> Will the same thing not happen in proportional weight? If it is an RT
> application, one can put it in RT groups to make sure it always gets
> the BW first even if there is contention. 
> 
> Even in regular group, the moment you issue the IO and IO scheduler sees
> it, you will start getting your reserved share according to your weight.
> 
> How it will be different in the case of io throttling? Even if I don't
> utilize the disk fully, cfq will still put the new guy in the queue and
> then try to give its share (based on prio).
> 
> Are you saying that by keeping disk relatively free, the latency of
> response for soft real time application will become better? In that
> case can't one simply underprovision the disk?
> 
> But having said that I am not disputing the need of max BW controller
> as some people have expressed the need of a constant BW view and don't
> want too big a fluctuations even if BW is available. Max BW controller
> can't gurantee the minumum BW hence can't avoid the fluctuations
> completely, but it can still help in smoothing the traffic because
> other competitiors will be stopped from doing too much of IO.

Agree.

-Andrea
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