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



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.

> could the cgroup
> subsystem be a more flexible and customizable framework respect to the
> current ioprio model?
> 
> I'm not saying we have to ignore the problem, just trying to evaluate
> the impact and alternatives. And I'm still convinced that also providing
> per-cgroup ioprio would be an important feature.
> 
> > 
> > 
> > Without io-throttle patches
> > ----------------------------
> > - Two readers, first BE prio 7, second RT prio 0
> > 
> > 234179072 bytes (234 MB) copied, 2.81801 s, 83.1 MB/s
> > RT task finished
> > 234179072 bytes (234 MB) copied, 5.28238 s, 44.3 MB/s
> > 
> > Note: Because I can't limit the BW without io-throttle patches, so don't
> >       worry about increased BW. But the important point is that RT task
> >       gets much more BW than a BE prio 7 task.
> > 
> > Test2
> > ====
> > - Test 2 readers (One BE prio 0 and one BE prio 7) and see how BW is
> > distributed among these.
> > 
> > With io-throttle patches
> > ------------------------
> > - Two readers, first BE prio 7, second BE prio 0
> > 
> > 234179072 bytes (234 MB) copied, 55.8604 s, 4.2 MB/s
> > 234179072 bytes (234 MB) copied, 55.8918 s, 4.2 MB/s
> > High prio reader finished
> 
> Ditto.
> 
> > 
> > Without io-throttle patches
> > ---------------------------
> > - Two readers, first BE prio 7, second BE prio 0
> > 
> > 234179072 bytes (234 MB) copied, 4.12074 s, 56.8 MB/s
> > High prio reader finished
> > 234179072 bytes (234 MB) copied, 5.36023 s, 43.7 MB/s
> > 
> > Note: There is no service differentiation between prio 0 and prio 7 task
> >       with io-throttle patches.
> > 
> > Test 3
> > ======
> > - Run the one RT reader and one BE reader in root cgroup without any
> >   limitations. I guess this should mean unlimited BW and behavior should
> >   be same as with CFQ without io-throttling patches.
> > 
> > With io-throttle patches
> > =========================
> > Ran the test 4 times because I was getting different results in different
> > runs.
> > 
> > - Two readers, one RT prio 0  other BE prio 7
> > 
> > 234179072 bytes (234 MB) copied, 2.74604 s, 85.3 MB/s
> > 234179072 bytes (234 MB) copied, 5.20995 s, 44.9 MB/s
> > RT task finished
> > 
> > 234179072 bytes (234 MB) copied, 4.54417 s, 51.5 MB/s
> > RT task finished
> > 234179072 bytes (234 MB) copied, 5.23396 s, 44.7 MB/s
> > 
> > 234179072 bytes (234 MB) copied, 5.17727 s, 45.2 MB/s
> > RT task finished
> > 234179072 bytes (234 MB) copied, 5.25894 s, 44.5 MB/s
> > 
> > 234179072 bytes (234 MB) copied, 2.74141 s, 85.4 MB/s
> > 234179072 bytes (234 MB) copied, 5.20536 s, 45.0 MB/s
> > RT task finished
> > 
> > Note: Out of 4 runs, looks like twice it is complete priority inversion
> >       and RT task finished after BE task. Rest of the two times, the
> >       difference between BW of RT and BE task is much less as compared to
> >       without patches. In fact once it was almost same.
> 
> This is strange. If you don't set any limit there shouldn't be any
> difference respect to the other case (without io-throttle patches).
> 
> At worst a small overhead given by the task_to_iothrottle(), under
> rcu_read_lock(). I'll repeat this test ASAP and see if I'll be able to
> reproduce this strange behaviour.

Ya, I also found this strange. At least in root group there should not be
any behavior change (at max one might expect little drop in throughput
because of extra code).

Thanks
Vivek


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