linux-kernel - Re: [RFC 0/3] block: proportional based blk-throttling

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Message-ID: <20160120195433.GD10553@redhat.com>
Date:	Wed, 20 Jan 2016 14:54:33 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Shaohua Li <shli@...com>
Cc:	linux-kernel@...r.kernel.org, axboe@...nel.dk, tj@...nel.org,
	jmoyer@...hat.com, Kernel-team@...com, linux-block@...r.kernel.org
Subject: Re: [RFC 0/3] block: proportional based blk-throttling

On Wed, Jan 20, 2016 at 11:43:27AM -0800, Shaohua Li wrote:
> On Wed, Jan 20, 2016 at 02:40:13PM -0500, Vivek Goyal wrote:
> > On Wed, Jan 20, 2016 at 11:34:48AM -0800, Shaohua Li wrote:
> > > On Wed, Jan 20, 2016 at 02:05:35PM -0500, Vivek Goyal wrote:
> > > > On Wed, Jan 20, 2016 at 09:49:16AM -0800, Shaohua Li wrote:
> > > > > Hi,
> > > > > 
> > > > > Currently we have 2 iocontrollers. blk-throttling is bandwidth based. CFQ is
> > > > > weight based. It would be great there is a unified iocontroller for the two.
> > > > > And blk-mq doesn't support ioscheduler, leaving blk-throttling the only option
> > > > > for blk-mq. It's time to have a scalable iocontroller supporting both
> > > > > bandwidth/weight based control and working with blk-mq.
> > > > > 
> > > > > blk-throttling is a good candidate, it works for both blk-mq and legacy queue.
> > > > > It has a global lock which is scaring for scalability, but it's not terrible in
> > > > > practice. In my test, the NVMe IOPS can reach 1M/s and I have all CPU run IO. Enabling
> > > > > blk-throttle has around 2~3% IOPS and 10% cpu utilization impact. I'd expect
> > > > > this isn't a big problem for today's workload. This patchset then try to make a
> > > > > unified iocontroller. I'm leveraging blk-throttling.
> > > > > 
> > > > > The idea is pretty simple. If we know disk total bandwidth, we can calculate
> > > > > cgroup bandwidth according to its weight. blk-throttling can use the calculated
> > > > > bandwidth to throttle cgroup. Disk total bandwidth changes dramatically per IO
> > > > > pattern. Long history is meaningless. The simple algorithm in patch 1 works
> > > > > pretty well when IO pattern changes.
> > > > > 
> > > > > This is a feedback system. If we underestimate disk total bandwidth, we assign
> > > > > less bandwidth to cgroup. cgroup will dispatch less IO and finally lower disk
> > > > > total bandwidth is estimated. To break the loop, cgroup bandwidth calculation
> > > > > always uses (1 + 1/8) * disk_bandwidth. Another issue is cgroup could be
> > > > > inactive. If inactive cgroup is accounted in, other cgroup will be assigned
> > > > > less bandwidth and so dispatch less IO, and disk total bandwidth drops further.
> > > > > To avoid the issue, we periodically check cgroups and exclude inactive ones.
> > > > > 
> > > > > To test this, create two fio jobs and assign them different weight. You will
> > > > > see the jobs have different bandwidth roughly according to their weight.
> > > > 
> > > > Patches look pretty small. Nice to see an implementation which will work
> > > > with faster devices and get away from dependency on cfq.
> > > > 
> > > > How does one switch between weight based vs bandwidth based throttling?
> > > > What's the default. 
> > > > 
> > > > So this has been implemented at throttling layer. By default is weight 
> > > > based throttling enabled or one needs to enable it explicitly.
> > > 
> > > So in current implementation, only one of weight/bandwidth can be
> > > enabled. After one is enabled, switching to the other is forbidden. It
> > > should not be hard to enable switching. But mixing the two in one
> > > hierarchy sounds not trivial.
> > 
> > So is this selection per device? Would be good if you also provide steps
> > to test it. I am going through code now and will figure out ultimately,
> > just that if you give steps, it makes it little easier.
> 
> Just uses:
> echo "8:16 200" > $TEST_CG/blkio.throttle.weight
> 
> 200 is the weight

Ok. So by default this mechanism is off. And the moment I assign the
weight to any of the cgroups on a device, weight based mechanism
kicks in? And what happens to other cgroups where I have not assigned
any weight which are doing IO?

I am doing cc to linux-block mailing list also.

Thanks
Vivek