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

Mon Apr 6 23:40:46 PDT 2009

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.  

  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?

- 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?

- Have you thought of doing hierarchical control? 

- 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)

> > 
> >   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?

> 
> > 
> > - 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?

> 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
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