Re: [RFC][PATCH] Improving directed yield scalability for PLE handler

[Andrew Jones]

On Fri, Sep 14, 2012 at 04:34:24PM -0400, Konrad Rzeszutek Wilk wrote:
> > The concern I have is that even though we have gone through changes to
> > help reduce the candidate vcpus we yield to, we still have a very poor
> > idea of which vcpu really needs to run.  The result is high cpu usage in
> > the get_pid_task and still some contention in the double runqueue lock.
> > To make this scalable, we either need to significantly reduce the
> > occurrence of the lock-holder preemption, or do a much better job of
> > knowing which vcpu needs to run (and not unnecessarily yielding to vcpus
> > which do not need to run).
> 
> The patches that Raghavendra  has been posting do accomplish that.
> >
> > On reducing the occurrence:  The worst case for lock-holder preemption
> > is having vcpus of same VM on the same runqueue.  This guarantees the
> > situation of 1 vcpu running while another [of the same VM] is not.  To
> > prove the point, I ran the same test, but with vcpus restricted to a
> > range of host cpus, such that any single VM's vcpus can never be on the
> > same runqueue.  In this case, all 10 VMs' vcpu-0's are on host cpus 0-4,
> > vcpu-1's are on host cpus 5-9, and so on.  Here is the result:
> >
> > kvm_cpu_spin, and all
> > yield_to changes, plus
> > restricted vcpu placement:  8823 +/- 3.20%   much, much better
> >
> > On picking a better vcpu to yield to:  I really hesitate to rely on
> > paravirt hint [telling us which vcpu is holding a lock], but I am not
> > sure how else to reduce the candidate vcpus to yield to.  I suspect we
> > are yielding to way more vcpus than are prempted lock-holders, and that
> > IMO is just work accomplishing nothing.  Trying to think of way to
> > further reduce candidate vcpus....
> 
> ... the patches are posted -  you could try them out?

Radim and I have done some testing with the pvticketlock series. While we
saw a gain over PLE alone, it wasn't huge, and without PLE also enabled it
could hardly support 2.0x overcommit. spinlocks aren't the only place
where cpu_relax() is called within a relatively tight loop, so it's likely
that PLE yielding just generally helps by getting schedule() called more
frequently.

Drew
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