On Wed, Jan 20, 2016 at 04:11:00PM -0500, Vivek Goyal wrote: > 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 > > > > It would be nice if you also update the documentation. What are the max > and min for weight values. What does it mean if a group has weight 200. > While others have not been configured. What % of disk share this cgroup > will get. > > I am still wrapping my head around the patches but it looks like this is > another way of coming up automatically with bandwidth limit for a cgroup > based on weight. So user does not have to configure absolute values > for read/write bandwidth. They can configure the weight and that will > automatically control the bandwidth of cgroup dynamically. > > What I am not clear is that once I apply weight on one cgroup, what happes > to rest of peer cgroups which are still not configured. If I don't apply > rules to them, then adding weight to one cgroup does not mean much. > > Ideally, I might help that we assign default weights to cgroup and have > a per device switch to enable weight based controller. That way user > space can enable it per device as needed and all the cgroup get their > fair share without any extra configuration. If the overhead of this > mechanism is ultra low, then a global switch to enable it by default > for all devices should be useful too. That way user space has to toggle > just that switch and by default all IO cgroups on all block devices get > their fair share.
I haven't thought about the interface too much yet. This version mainly demonstrates the idea. Your suggestions look reasonable. A single control to enable weight/bandwidth with proper default setting is convenient. Will add it in next post. Thanks, Shaohua
