Re: Make yield_task_fair more efficient

Mike Galbraith wrote:
Hi,

On Thu, 2008-02-21 at 15:01 +0530, Balbir Singh wrote:
Ingo Molnar wrote:
* Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:

If you insist that sched_yield() is bad, I might agree, but how does
my patch make things worse. [...]
it puts new instructions into the hotpath.

[...] In my benchmarks, it has helped the sched_yield case, why is
that bad? [...]
I had the same cache for the rightmost task in earlier CFS (it's a
really obvious thing) but removed it. It wasnt a bad idea, but it hurt
the fastpath hence i removed it. Algorithms and implementations are a
constant balancing act.
This is more convincing, was the code ever in git? How did you measure the
overhead?

Counting enqueue/dequeue cycles on my 3GHz P4/HT running a 60 seconds
netperf test that does ~85k/s context switches shows:

sched_cycles: 7198444348 unpatched
vs
sched_cycles: 8574036268 patched


Thanks for the numbers! I am very convinced that the patch should stay out until
we can find a way to reduce the overhead. I'll try your patch and see what the
numbers look like as well.

--
Warm Regards,
Balbir Singh
Linux Technology Center
IBM, ISTL
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