Re: sched: Performance of Trade workload running inside VM

[Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>]

* Peter Zijlstra <a.p.zijlstra@chello.nl> [2012-02-15 18:45:02]:

> I'm still waiting for a problem description that isn't a book.
> 
> What does the load-balancer do,

select_idle_sibling() tries finding a core (sched group) which is fully idle
failing which will let task wakeup on prev_cpu.

> why is it wrong,

Take this scenario. wakeup source for task is cpu0 while its prev_cpu is
cpu7 (which is in another cache domain).

		cache_dom0		cache_dom1
		(0, 1) (2, 3)		(4, 5) (6, 7)
nr_running->	 1  1   1  1     	 0, 1   1, 2

In this case we let task wakeup on cpu7, resulting in it incurring some
wait time before being scheduled. A better choice would have been cpu4 (whose
core is partially idle) or in general any other less-loaded cpu which is in same
cache domain?

> why does your patch sort it etc.

The patch does result in a hunt for "least" busy cpu when the target cpu
returned by select_idle_sibling() is not idle - thus resulting in better
scheduling latencies for the task (and in turn better benchmark scores).

Another variant of the patch could be to have select_idle_sibling() look
for any idle cpu that is in same cache domain (rather than looking for a
whole group of cpus to be idle)?

- vatsa