On 02/25/2013 01:51 AM, Preeti U Murthy wrote: > Hi, > > On 02/24/2013 02:57 PM, Alex Shi wrote: >> On 02/22/2013 04:54 PM, Peter Zijlstra wrote: >>> On Thu, 2013-02-21 at 22:40 +0800, Alex Shi wrote: >>>>> The name is a secondary issue, first you need to explain why you >>>> think >>>>> nr_running is a useful metric at all. >>>>> >>>>> You can have a high nr_running and a low utilization (a burst of >>>>> wakeups, each waking a process that'll instantly go to sleep again), >>>> or >>>>> low nr_running and high utilization (a single process cpu bound >>>>> process). >>>> >>>> It is true in periodic balance. But in fork/exec/waking timing, the >>>> incoming processes usually need to do something before sleep again. >>> >>> You'd be surprised, there's a fair number of workloads that have >>> negligible runtime on wakeup. >> >> will appreciate if you like introduce some workload. :) >> BTW, do you has some idea to handle them? >> Actually, if tasks is just like transitory, it is also hard to catch >> them in balance, like 'cyclitest -t 100' on my 4 LCPU laptop, vmstat >> just can catch 1 or 2 tasks very second. >>> >>>> I use nr_running to measure how the group busy, due to 3 reasons: >>>> 1, the current performance policy doesn't use utilization too. >>> >>> We were planning to fix that now that its available. >> >> I had tried, but failed on aim9 benchmark. As a result I give up to use >> utilization in performance balance. >> Some trying and talking in the thread. >> https://lkml.org/lkml/2013/1/6/96 >> https://lkml.org/lkml/2013/1/22/662 >>> >>>> 2, the power policy don't care load weight. >>> >>> Then its broken, it should very much still care about weight. >> >> Here power policy just use nr_running as the criteria to check if it's >> eligible for power aware balance. when do balancing the load weight is >> still the key judgment. >> >>> >>>> 3, I tested some benchmarks, kbuild/tbench/hackbench/aim7 etc, some >>>> benchmark results looks clear bad when use utilization. if my memory >>>> right, the hackbench/aim7 both looks bad. I had tried many ways to >>>> engage utilization into this balance, like use utilization only, or >>>> use >>>> utilization * nr_running etc. but still can not find a way to recover >>>> the lose. But with nr_running, the performance seems doesn't lose much >>>> with power policy. >>> >>> You're failing to explain why utilization performs bad and you don't >>> explain why nr_running is better. That things work simply isn't good >> >> Um, let me try to explain again, The utilisation need much time to >> accumulate itself(345ms). Whenever with or without load weight, many >> bursting tasks just give a minimum weight to the carrier CPU at the >> first few ms. So, it is too easy to do a incorrect distribution here and >> need migration on later periodic balancing. > > Why can't this be attacked in *either* of the following ways: > > 1.Attack this problem at the source, by ensuring that the utilisation is > accumulated faster by making the update window smaller.
It is a double blade sword. Small period will response quickly, but loses lots of history record. A extreme short period is just same as current instant utilization. > > 2.Balance on nr->running only if you detect burst wakeups. > Alex, you had released a patch earlier which could detect this right? Yes, the patch is here: https://lkml.org/lkml/2013/1/11/45 One of problem is the how to decide the criteria of the burst? If we set 5 waking up/ms is burst, we will lose 4 waking up/ms. another problem is the burst detection cost, we need tracking a period history info of the waking up, better on whole group. but that give the extra cost in burst. solution candidates: https://lkml.org/lkml/2013/1/21/316 After talk with MikeG, I remove the runnable load avg in performance load balance. Using nr_running as instant utilization may narrow the power policy suitable situation. -- consider for power consumption, a light but cpu intensive task will cost much more power than a heavy load but run occasionally task. And it fit all my benchmarks aim7/hackbench/kbuild/cyclitest/netperf etc. > Instead of balancing on nr_running all the time, why not balance on it > only if burst wakeups are detected. By doing so you ensure that > nr_running as a metric for load balancing is used when it is right to do > so and the reason to use it also gets well documented. > > Regards > Preeti U Murthy > -- Thanks Alex -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
