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[dm-devel] Re: IO scheduler based IO Controller V2



On Wed, May 06, 2009 at 05:52:35PM -0400, Vivek Goyal wrote:
> On Wed, May 06, 2009 at 11:34:54PM +0200, Andrea Righi wrote:
> > On Wed, May 06, 2009 at 04:32:28PM -0400, Vivek Goyal wrote:
> > > Hi Andrea and others,
> > > 
> > > I always had this doubt in mind that any kind of 2nd level controller will
> > > have no idea about underlying IO scheduler queues/semantics. So while it
> > > can implement a particular cgroup policy (max bw like io-throttle or
> > > proportional bw like dm-ioband) but there are high chances that it will
> > > break IO scheduler's semantics in one way or other.
> > > 
> > > I had already sent out the results for dm-ioband in a separate thread.
> > > 
> > > http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-04/msg07258.html
> > > http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-04/msg07573.html
> > > http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-04/msg08177.html
> > > http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-04/msg08345.html
> > > http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-04/msg08355.html
> > > 
> > > Here are some basic results with io-throttle. Andrea, please let me know
> > > if you think this is procedural problem. Playing with io-throttle patches
> > > for the first time.
> > > 
> > > I took V16 of your patches and trying it out with 2.6.30-rc4 with CFQ
> > > scheduler.
> > > 
> > > I have got one SATA drive with one partition on it.
> > > 
> > > I am trying to create one cgroup and assignn 8MB/s limit to it and launch
> > > on RT prio 0 task and one BE prio 7 task and see how this 8MB/s is divided
> > > between these tasks. Following are the results.
> > > 
> > > Following is my test script.
> > > 
> > > *******************************************************************
> > > #!/bin/bash
> > > 
> > > mount /dev/sdb1 /mnt/sdb
> > > 
> > > mount -t cgroup -o blockio blockio /cgroup/iot/
> > > mkdir -p /cgroup/iot/test1 /cgroup/iot/test2
> > > 
> > > # Set bw limit of 8 MB/ps on sdb
> > > echo "/dev/sdb:$((8 * 1024 * 1024)):0:0" >
> > > /cgroup/iot/test1/blockio.bandwidth-max
> > > 
> > > sync
> > > echo 3 > /proc/sys/vm/drop_caches
> > > 
> > > echo $$ > /cgroup/iot/test1/tasks
> > > 
> > > # Launch a normal prio reader.
> > > ionice -c 2 -n 7 dd if=/mnt/sdb/zerofile1 of=/dev/zero &
> > > pid1=$!
> > > echo $pid1
> > > 
> > > # Launch an RT reader  
> > > ionice -c 1 -n 0 dd if=/mnt/sdb/zerofile2 of=/dev/zero &
> > > pid2=$!
> > > echo $pid2
> > > 
> > > wait $pid2
> > > echo "RT task finished"
> > > **********************************************************************
> > > 
> > > Test1
> > > =====
> > > Test two readers (one RT class and one BE class) and see how BW is
> > > allocated with-in cgroup
> > > 
> > > With io-throttle patches
> > > ------------------------
> > > - Two readers, first BE prio 7, second RT prio 0
> > > 
> > > 234179072 bytes (234 MB) copied, 55.8482 s, 4.2 MB/s
> > > 234179072 bytes (234 MB) copied, 55.8975 s, 4.2 MB/s
> > > RT task finished
> > > 
> > > Note: See, there is no difference in the performance of RT or BE task.
> > > Looks like these got throttled equally.
> > 
> > OK, this is coherent with the current io-throttle implementation. IO
> > requests are throttled without the concept of the ioprio model.
> > 
> > We could try to distribute the throttle using a function of each task's
> > ioprio, but ok, the obvious drawback is that it totally breaks the logic
> > used by the underlying layers.
> > 
> > BTW, I'm wondering, is it a very critical issue? I would say why not to
> > move the RT task to a different cgroup with unlimited BW? or limited BW
> > but with other tasks running at the same IO priority...
> 
> So one of hypothetical use case probably  could be following. Somebody
> is having a hosted server and customers are going to get there
> applications running in a particular cgroup with a limit on max bw.
> 
> 			root
> 		  /      |      \
> 	     cust1      cust2   cust3
> 	   (20 MB/s)  (40MB/s)  (30MB/s)
> 
> Now all three customers will run their own applications/virtual machines
> in their respective groups with upper limits. Will we say to these that
> all your tasks will be considered as same class and same prio level.
> 
> Assume cust1 is running a hypothetical application which creates multiple
> threads and assigns these threads different priorities based on its needs
> at run time. How would we handle this thing?
> 
> You can't collect all the RT tasks from all customers and move these to a
> single cgroup. Or ask customers to separate out their tasks based on
> priority level and give them multiple groups of different priority.

Clear.

Unfortunately, I think, with absolute BW limits at a certain point, if
we hit the limit, we need to block the IO request. That's the same
either, when we dispatch or submit the request. And the risk is to break
the logic of the IO priorities and fall in the classic priority
inversion problem.

The difference is that probably working at the CFQ level gives a better
control so we can handle these cases appropriately and avoid the
priority inversion problems.

Thanks,
-Andrea


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