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

[Andrea Righi]

On Tue, Apr 07, 2009 at 02:40:46AM -0400, Vivek Goyal wrote:
> On Sun, Apr 05, 2009 at 05:15:35PM +0200, Andrea Righi wrote:
> > On 2009-03-12 19:01, Vivek Goyal wrote:
> > > On Thu, Mar 12, 2009 at 12:11:46AM -0700, Andrew Morton wrote:
> > >> On Wed, 11 Mar 2009 21:56:46 -0400 Vivek Goyal <vgoyal at redhat.com> wrote:
> > [snip]
> > >> Also..  there are so many IO controller implementations that I've lost
> > >> track of who is doing what.  I do have one private report here that
> > >> Andreas's controller "is incredibly productive for us and has allowed
> > >> us to put twice as many users per server with faster times for all
> > >> users".  Which is pretty stunning, although it should be viewed as a
> > >> condemnation of the current code, I'm afraid.
> > >>
> > > 
> > > I had looked briefly at Andrea's implementation in the past. I will look
> > > again. I had thought that this approach did not get much traction.
> > 
> > Hi Vivek, sorry for my late reply. I periodically upload the latest
> > versions of io-throttle here if you're still interested:
> > http://download.systemimager.org/~arighi/linux/patches/io-throttle/
> > 
> > There's no consistent changes respect to the latest version I posted to
> > the LKML, just rebasing to the recent kernels.
> > 
> 
> Thanks Andrea. I will spend more time in looking through your patches
> and do a bit of testing.
> 
> > > 
> > > Some quick thoughts about this approach though.
> > > 
> > > - It is not a proportional weight controller. It is more of limiting
> > >   bandwidth in absolute numbers for each cgroup on each disk.
> > >  
> > >   So each cgroup will define a rule for each disk in the system mentioning
> > >   at what maximum rate that cgroup can issue IO to that disk and throttle
> > >   the IO from that cgroup if rate has excedded.
> > 
> > Correct. Add also the proportional weight control has been in the TODO
> > list since the early versions, but I never dedicated too much effort to
> > implement this feature, I can focus on this and try to write something
> > if we all think it is worth to be done.
> > 
> 
> Please do have a look at this patchset and would you do it differently
> to implement proportional weight control?
> 
> Few thoughts/queries.
> 
> - Max bandwidth control and Prportional weight control are two entirely
>   different ways of controlling the IO. Former one tries to put an upper
>   limit on the IO rate and later one kind of tries to  gurantee minmum
>   percentage share of disk.  

Agree.

> 
>   How does an determine what throughput rate you will get from a disk? That
>   is so much dependent on workload and miscalculations can lead to getting
>   lower BW for a particular cgroup?
> 
>   I am assuming that one can probably do some random read-write IO test
>   to try to get some idea of disk throughput. If that's the case, then
>   in proportional weight control also you should be able to predict the
>   minimum BW a cgroup will be getting? The only difference will be that
>   a cgroup can get higher BW also if there is no contention present and
>   I am wondring that how getting more BW than promised minumum is harmful?

IMHO we shouldn't care too much on how to extract the exact BW from a
disk. With proportional weights we can directly map different levels of
service to different weights.

With absolute limiting we can measure the consumed BW post facto and try
to do the best to satisfy the limits defined by the user (absolute max,
min or proportional). Predict a priori how much BW will consume a
particular application's workload is a very hard task (maybe even
impossible) and probably it doesn't give huge advantages respect to the
approach we're currently using. I think this is true for both solutions.

> 
> - I can think of atleast one usage of uppper limit controller where we
>   might have spare IO resources still we don't want to give it to a
>   cgroup because customer has not paid for that kind of service level. In
>   those cases we need to implement uppper limit also.
> 
>   May be prportional weight and max bw controller can co-exist depending
>   on what user's requirements are.
>  
>   If yes, then can't this control be done at the same layer/level where
>   proportional weight control is being done? IOW, this set of patches is
>   trying to do prportional weight control at IO scheduler level. I think
>   we should be able to store another max rate as another feature in 
>   cgroup (apart from weight) and not dispatch requests from the queue if
>   we have exceeded the max BW as specified by the user?

The more I think about a "perfect" solution (at least for my
requirements), the more I'm convinced that we need both functionalities.

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).

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.

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.

> - 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?

> 
> > > 
> > >   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.

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.

> 
> > So, my "ideal" IO controller should allow to define both rules:
> > absolute and proportional limits.
> > 
> > I still have to look closely at your patchset anyway. I will do and give
> > a feedback.
> 
> You feedback is always welcome.
> 
> Thanks
> Vivek

Thanks,
-Andrea
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