Messages in this thread | | | Date | Sat, 11 Sep 2010 15:57:08 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC patch 1/2] sched: dynamically adapt granularity with nr_running |
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* Peter Zijlstra (peterz@infradead.org) wrote: > On Sat, 2010-09-11 at 13:37 -0400, Mathieu Desnoyers wrote: > > Its not at all clear what or why you're doing what exactly. > > What we used to have is: > > period -- time in which each task gets scheduled once > > This period was adaptive in that we had an ideal period > (sysctl_sched_latency), but since keeping to this means that each task > gets latency/nr_running time. This is undesired in that it means busy > systems will over-schedule due to tiny slices. Hence we also had a > minimum slice (sysctl_sched_min_granularity). > > This yields: > > period := max(sched_latency, nr_running * sched_min_granularity) > > [ where we introduce the intermediate: > nr_latency := sched_latency / sched_min_granularity > in order to avoid the multiplication where possible ] > > Now you introduce a separate preemption measure, sched_gran as: > > sched_std_granularity; nr_running <= 8 > sched_gran := { > max(sched_min_granularity, sched_latency / nr_running) > > Which doesn't make any sense at all, because it will either be larger or > as large as the current sched_min_granularity. > > And you break the above definition of period by replacing nr_latency by > 8. > > Not at all charmed, this look like random changes without conceptual > integrity.
Err.. I think the preemption measure you are describing does not match my code, so let's try to figure this one out. Here is what I am doing:
nr_latency is still 3. I introduce nr_latency_max (8).
sched_min_granularity is now sched_latency / nr_latency_max sched_std_granularity is sched_latency / nr_latency
sched_std_granularity is the granularity effective when there are 3 tasks or less running. This is the exact same behavior as the current kernel.
For more than 8 tasks, the behavior is the same as the current kernel (we increase the scheduling period, ergo the latency); we are using the new "sched_min_granularity" (which is now sched_latency / 8 rather than sched_latency /3).
The interesting part is in the range from 4 to 8 tasks. I diminish the scheduler granularity as the number of tasks increases rather than increasing latency. This leads to more scheduler preemptions than usual, but only in the 4-8 running tasks range.
We could possibly fine-tune nr_latency_max to a value that would keep an appropriate sched_min_granularity (that would not cause an insane rate of scheduler events).
The major interest in the approach I propose (rather than just increasing nr_latency and decreasing sched_min_granularity) is that I don't have to change the scheduler granularity when there are only few tasks running. So the extra scheduler overhead is only taken when we are running more tasks.
I hope my explanation clarifies things a bit,
Thanks,
Mathieu
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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