Re: [patch v5 09/15] sched: add power aware scheduling infork/exec/wake
From: Peter Zijlstra
Date: Fri Feb 22 2013 - 03:54:59 EST
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
> > 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),
> > 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.
> 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.
> 2, the power policy don't care load weight.
Then its broken, it should very much still care about weight.
> 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
> 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
enough, you have to have at least a general idea (but much preferable a
very good idea) _why_ things work.
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