Il giorno 15/apr/2016, alle ore 21:29, Tejun Heo <tj@xxxxxxxxxx> ha scritto: > Hello, Paolo. > > On Fri, Apr 15, 2016 at 06:17:55PM +0200, Paolo Valente wrote: >>> 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 :( > > lol sorry about being dumb. > dumb? my problem is your remaining patience ... >>> 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". > > Oh, the guarantee is about "getting close to half of the available IO > resource", what that translates to depends on the underlying hardware > and the workload of course. > yes >> 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. > > Yes, may be but it won't dive to 5% depending on what others are > doing. > exact >> 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. > > I don't think the above is true. Are you saying that the following > two cases would lead to the same outcome for cgroup A? > > cgroup A (50) cgroup B (50) > case 1 sequential sequential > case 2 sequential random (to a certain degree) > > The aggregate bandwidths for case 1 and 2 would be wildly different > depending on the randomness of the second workload. What cgroup A > would be able to get would fluctuate accordingly, no? > Your example is definitely to the point. The answer to your question is no. In fact, in both cases cgroup A will get exactly the same service slots, and in each slot exactly the same number of sectors transferred. In particular, cgroup B will systematically hit the timeout in the second case. In other words, in case 2 cgroup A is guaranteed the same bandwidth that it would get, in case 1, if cgroup B was quasi-sequential and so slow to get served for a full time slice every time it got access to the resource. Maybe the source of confusion is the fact that a simple sector-based, proportional share scheduler always distributes total bandwidth according to weights. The catch is the additional BFQ rule: random workloads get only time isolation, and are charged for full budgets, so as to not affect the schedule of quasi-sequential workloads. So, the correct claim for BFQ is that it distributes total bandwidth according to weights (only) when all competing workloads are quasi-sequential. If some workloads are random, then these workloads are just time scheduled. This does break proportional-share bandwidth distribution with mixed workloads, but, much more importantly, saves both total throughput and individual bandwidths of quasi-sequential workloads. We could then check whether I did succeed in tuning timeouts and budgets so as to achieve the best tradeoffs. But this is probably a second-order problem as of now. >>> 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. > > I think I might still need more help. What am I missing? > I hope that what I wrote above did help. Thanks, Paolo > 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
