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

[Shaohua Li]

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.
 
> What's the performance impact of new weight based throttling.

I haven't benchmarked yet, but this doesn't add too many code, I'd
expect the performance isn't changed. I'll do a test soon.

Thanks,
Shaohua