Messages in this thread | | | Date | Tue, 5 Oct 2021 11:24:23 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to nr_running |
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On Tue, Sep 21, 2021 at 02:32:54PM +0200, Mike Galbraith wrote: > On Tue, 2021-09-21 at 11:36 +0100, Mel Gorman wrote:
> > FAIR_SLEEPERS primarily affects tasks that just became runnable and the > > new task is trying to fit in without causing too much disruption based > > on sysctl_sched_latency. > > No, fair sleepers is all about sleeper wakeup preemption, I think > you're thinking of fork initial placement.
Butting in in the middle of the thread (and I know there's still lots to read)...
So the FAIR_SLEEPERS thing is about giving tasks that have slept a while some extra credit to run sooner.
The classical example has always been a task that run 50% combined with a task that runs 100%, what's fair? a 1:2 or 1:1 ratio? Strict fair (runnable) scheduling will get you the 1:2, while intuitively having two tasks with 100% combined CPU utilization 1:1 would be 'fair'.
FAIR_SLEEPERS gets you towards that 1:1, *provided* the period of that 50% is near sched_latency/2.
Another important factor for wakeup preemption has always been desktop usage; can you still get responsive terminals while building a kernel, how does firefox scroll during a kernel build etc..
(fwiw, firefox should start scrolling responsively and then bog down if you keep on scrolling because it becomes a hog and has exhausted the inital boost)
Also, I think the ChromeOS people have interactivity measures these days.
All our traditinoal benchmarks miss out here; they're mostly throughput oriented, and it is really easy to totally wreck interactivity while getting great througput :/
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