Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

[Ingo Molnar <mingo@kernel.org>]

* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Thu, 10 Oct 2013 09:27:57 +0200 Ingo Molnar <mingo@kernel.org> wrote:
> 
> > > > Should be fairly straightforward to test: the sys_sched_getaffinity() 
> > > > and sys_sched_setaffinity() syscalls both make use of 
> > > > get_online_cpus()/put_online_cpus(), so a testcase frobbing affinities 
> > > > on N CPUs in parallel ought to demonstrate scalability improvements 
> > > > pretty nicely.
> > > 
> > > Well, an in-kernel microbenchmark which camps in a loop doing get/put 
> > > would measure this as well.
> > > 
> > > But neither approach answers the question "how useful is this patchset".
> > 
> > Even ignoring all the other reasons cited, sys_sched_getaffinity() / 
> > sys_sched_setaffinity() are prime time system calls, and as long as 
> > the patches are correct, speeding them up is worthwhile.
> 
> That I would not have guessed.  What's the use case for calling 
> get/set_affinity at high frequency?

I don't think high-freq usage is common (at all).

It could happen in AIM7-like workloads that start up a ton of binaries in 
parallel, which can easily create parallel sched_getaffinity() calls 
during process startup?

Thanks,

	Ingo