Il giorno 15/apr/2016, alle ore 17:08, Tejun Heo <tj@xxxxxxxxxx> ha scritto: > Hello, Paolo. > > On Fri, Apr 15, 2016 at 04:20:44PM +0200, Paolo Valente wrote: >>> It's actually a lot more difficult to answer that with bandwidth >>> scheduling. Let's say cgroup A has 50% of disk time. Sure, there are >>> inaccuracies, but it should be able to get close to the ballpark - >>> let's be lax and say between 30% and 45% of raw sequential bandwidth. >>> It isn't ideal but now imagine bandwidth based scheduling. Depending >>> on what the others are doing, it may get 5% or even lower of the raw >>> sequential bandwidth. It isn't isolating anything. >> >> Definitely. Nevertheless my point is still about the same: we have to >> consider one system at a time. If the workload of the system is highly >> variable and completely unpredictable, then it is hard to provide any >> bandwidth guarantee with any solution. > > I don't think that is true with time based scheduling. If you > allocate 50% of time, it'll get close to 50% of IO time which > translates to bandwidth which is lower than 50% but still in the > ballpark. But this is the same minimal service guarantee that you get with BFQ in any case. I'm sorry for being so confusing to not make this central point clear :( > That is very different from "we can't guarantee anything if > the other workloads are highly variable”. > If you have 50% of the time, but . you don’t know anything about your workload properties, and . the device speed can vary by two orders of magnitude, then you can't provide any bandwidth guarantee, with any scheduler. Of course I'm neglecting the minimal, trivial guarantee "getting a fraction of the minimum possible speed of the device". If you have 50% of the time allocated for a quasi-sequential workload, then bandwidth and latencies may vary by an uncontrollable 30 or 40%, depending on what you and the other groups do. With the same device, if you have 50% of the bandwidth allocated with BFQ for a quasi-sequential workload, then you can provide bandwidth and latencies that may vary at most by a (still uncontrollable) 3 or 4%, depending on what you and the other groups do. This improvement is shown, e.g., in my--admittedly boring--numerical example, and is confirmed by my experimental results so far. > So, I get that for a lot of workload, especially interactive ones, IO > patterns are quasi-sequential and bw based scheduling is beneficial > and we don't care that much about fairness in general; however, it's > problematic that it would make the behavior of proportional control > quite surprising. If I have somehow convinced you with what I wrote above, then I hope we might agree that a surprising behavior of BFQ with cgroups would be just a matter of bugs. Thanks, Paolo > >>> As I wrote before, as fairness isn't that important for normal >>> scheduling, if empirical data show that bandwidth based scheduling is >>> beneficial for most common workloads, that's awesome especially given >>> that CFQ has plenty of issues. I don't think cgroup case is workable >>> as currently implemented tho. >> >> I was thinking about some solution to achieve both goals. An option is >> probably to let BFQ work in a double mode: sector-based within groups >> and time-based among groups. However, I find it a little messy and >> confusing. >> >> Other ideas/solutions? I have no better proposal at the moment :( > > No idea. I don't think isolation could work without time based > scheduling at some level tho. :( > > Thanks. > > -- > tejun -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
