linux-kernel - Re: [PATCH V3 00/11] block-throttle: add .high limit

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Date:   Wed, 5 Oct 2016 13:36:24 -0700
From:   Shaohua Li <shli@...com>
To:     Paolo Valente <paolo.valente@...more.it>
CC:     Tejun Heo <tj@...nel.org>, Vivek Goyal <vgoyal@...hat.com>,
        <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...com>, <Kernel-team@...com>,
        <jmoyer@...hat.com>, Mark Brown <broonie@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH V3 00/11] block-throttle: add .high limit

On Wed, Oct 05, 2016 at 09:57:22PM +0200, Paolo Valente wrote:
> 
> > Il giorno 05 ott 2016, alle ore 21:08, Shaohua Li <shli@...com> ha scritto:
> > 
> > On Wed, Oct 05, 2016 at 11:30:53AM -0700, Shaohua Li wrote:
> >> On Wed, Oct 05, 2016 at 10:49:46AM -0400, Tejun Heo wrote:
> >>> Hello, Paolo.
> >>> 
> >>> On Wed, Oct 05, 2016 at 02:37:00PM +0200, Paolo Valente wrote:
> >>>> In this respect, for your generic, unpredictable scenario to make
> >>>> sense, there must exist at least one real system that meets the
> >>>> requirements of such a scenario.  Or, if such a real system does not
> >>>> yet exist, it must be possible to emulate it.  If it is impossible to
> >>>> achieve this last goal either, then I miss the usefulness
> >>>> of looking for solutions for such a scenario.
> >>>> 
> >>>> That said, let's define the instance(s) of the scenario that you find
> >>>> most representative, and let's test BFQ on it/them.  Numbers will give
> >>>> us the answers.  For example, what about all or part of the following
> >>>> groups:
> >>>> . one cyclically doing random I/O for some second and then sequential I/O
> >>>> for the next seconds
> >>>> . one doing, say, quasi-sequential I/O in ON/OFF cycles
> >>>> . one starting an application cyclically
> >>>> . one playing back or streaming a movie
> >>>> 
> >>>> For each group, we could then measure the time needed to complete each
> >>>> phase of I/O in each cycle, plus the responsiveness in the group
> >>>> starting an application, plus the frame drop in the group streaming
> >>>> the movie.  In addition, we can measure the bandwidth/iops enjoyed by
> >>>> each group, plus, of course, the aggregate throughput of the whole
> >>>> system.  In particular we could compare results with throttling, BFQ,
> >>>> and CFQ.
> >>>> 
> >>>> Then we could write resulting numbers on the stone, and stick to them
> >>>> until something proves them wrong.
> >>>> 
> >>>> What do you (or others) think about it?
> >>> 
> >>> That sounds great and yeah it's lame that we didn't start with that.
> >>> Shaohua, would it be difficult to compare how bfq performs against
> >>> blk-throttle?
> >> 
> >> I had a test of BFQ. I'm using BFQ found at
> >> https://urldefense.proofpoint.com/v2/url?u=http-3A__algogroup.unimore.it_people_paolo_disk-5Fsched_sources.php&d=DQIFAg&c=5VD0RTtNlTh3ycd41b3MUw&r=X13hAPkxmvBro1Ug8vcKHw&m=zB09S7v2QifXXTa6f2_r6YLjiXq3AwAi7sqO4o2UfBQ&s=oMKpjQMXfWmMwHmANB-Qnrm2EdERzz9Oef7jcLkbyFg&e= . version is
> >> 4.7.0-v8r3. It's a LSI SSD, queue depth 32. I use default setting. fio script
> >> is:
> >> 
> >> [global]
> >> ioengine=libaio
> >> direct=1
> >> readwrite=randread
> >> bs=4k
> >> runtime=60
> >> time_based=1
> >> file_service_type=random:36
> >> overwrite=1
> >> thread=0
> >> group_reporting=1
> >> filename=/dev/sdb
> >> iodepth=1
> >> numjobs=8
> >> 
> >> [groupA]
> >> prio=2
> >> 
> >> [groupB]
> >> new_group
> >> prio=6
> >> 
> >> I'll change iodepth, numjobs and prio in different tests. result unit is MB/s.
> >> 
> >> iodepth=1 numjobs=1 prio 4:4
> >> CFQ: 28:28 BFQ: 21:21 deadline: 29:29
> >> 
> >> iodepth=8 numjobs=1 prio 4:4
> >> CFQ: 162:162 BFQ: 102:98 deadline: 205:205
> >> 
> >> iodepth=1 numjobs=8 prio 4:4
> >> CFQ: 157:157 BFQ: 81:92 deadline: 196:197
> >> 
> >> iodepth=1 numjobs=1 prio 2:6
> >> CFQ: 26.7:27.6 BFQ: 20:6 deadline: 29:29
> >> 
> >> iodepth=8 numjobs=1 prio 2:6
> >> CFQ: 166:174 BFQ: 139:72  deadline: 202:202
> >> 
> >> iodepth=1 numjobs=8 prio 2:6
> >> CFQ: 148:150 BFQ: 90:77 deadline: 198:197
> > 
> > More tests:
> > 
> > iodepth=8 numjobs=1 prio 2:6, group A has 50M/s limit
> > CFQ:51:207  BFQ: 51:45  deadline: 51:216
> > 
> > iodepth=1 numjobs=1 prio 2:6, group A bs=4k, group B bs=64k
> > CFQ:25:249  BFQ: 23:42  deadline: 26:251
> > 
> 
> A true proportional share scheduler like BFQ works under the
> assumption to be the only limiter of the bandwidth of its clients.
> And the availability of such a scheduler should apparently make
> bandwidth limiting useless: once you have a mechanism that allows you
> to give each group the desired fraction of the bandwidth, and to
> redistribute excess bandwidth seamlessly when needed, what do you need
> additional limiting for?
> 
> But I'm not expert of any possible system configuration or
> requirement.  So, if you have practical examples, I would really
> appreciate them.  And I don't think it will be difficult to see what
> goes wrong in BFQ with external bw limitation, and to fix the
> problem.

I think the test emulates a very common configuration. We assign more IO
resources to high priority workload. But such workload doesn't always dispatch
enough io. That's why I set a rate limit. When this happend, we hope low
priority workload uses the disk bandwidth. That's the whole point of disk
sharing.

Thanks,
Shaohua