Hello, Shaohua. On Fri, Jan 22, 2016 at 09:57:10AM -0800, Shaohua Li wrote: > > Let's say per-cgroup buffer budget B is calculated as, say, 100ms > > worth of IO cost (or bandwidth or iops) available to the cgroup. In > > practice, this may have to be adjusted down depending on the number of > > cgroups performing active IOs. For a given cgroup, B can be > > distributed among the CPUs that are actively issuing IOs in that > > cgroup. It will degenerate to round robin of small budget if there > > are too many active for the budget available but for most cases this > > will cut down most of cross-CPU traffic. > > The cgroup could be a single thread. It uses cpu0's per-cpu budget B-1, > move to cpu1 and use another B - 1, and so on
Sure, just ensure that the total cached is bound by B and expire if not used over a certain amount of time. The thing is as long as we can go through percpu cache most of the time, it's all fine. We can spend a lot of processing budget for corner cases. > > cost = F + R * size > > F could be IOPS. and the real cost becomes R. How do you get R? We can't > simply use R(4k) = 1, R(8k) = 2 .... I tried the idea several years ago: > https://lwn.net/Articles/474164/ > The idea is the same. But the reality is we can't get R. I don't want to > have a random math working for one SSD but not for another. Yeah, it'll have to be adaptive. We can't use fixed values; however, note that using bandwidth means that we assume F == 0 and R == 1, which wouldn't be appropriate for most devices. > One possible solution is we benchmark the device at startup and get > corresponding proportion of size. That would only work for IO read. And > how to choose the benchmark is another challenge. Hmmm... yeah, that can be one option although I think it'd still have to be adjusted dynamically. Let's think more about it. Thanks. -- tejun
