Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommitscenarios in PLE handler

[Andrew Theurer]

On Thu, 2012-09-27 at 14:03 +0200, Avi Kivity wrote:
> On 09/27/2012 01:23 PM, Raghavendra K T wrote:
> >>
> >> This gives us a good case for tracking preemption on a per-vm basis.  As
> >> long as we aren't preempted, we can keep the PLE window high, and also
> >> return immediately from the handler without looking for candidates.
> > 
> > 1) So do you think, deferring preemption patch ( Vatsa was mentioning
> > long back)  is also another thing worth trying, so we reduce the chance
> > of LHP.
> 
> Yes, we have to keep it in mind.  It will be useful for fine grained
> locks, not so much so coarse locks or IPIs.
> 
> I would still of course prefer a PLE solution, but if we can't get it to
> work we can consider preemption deferral.
> 
> > 
> > IIRC, with defer preemption :
> > we will have hook in spinlock/unlock path to measure depth of lock held,
> > and shared with host scheduler (may be via MSRs now).
> > Host scheduler 'prefers' not to preempt lock holding vcpu. (or rather
> > give say one chance.
> 
> A downside is that we have to do that even when undercommitted.
> 
> Also there may be a lot of false positives (deferred preemptions even
> when there is no contention).
> 
> > 
> > 2) looking at the result (comparing A & C) , I do feel we have
> > significant in iterating over vcpus (when compared to even vmexit)
> > so We still would need undercommit fix sugested by PeterZ (improving by
> > 140%). ?
> 
> Looking only at the current runqueue?  My worry is that it misses a lot
> of cases.  Maybe try the current runqueue first and then others.
> 
> Or were you referring to something else?
> 
> > 
> > So looking back at threads/ discussions so far, I am trying to
> > summarize, the discussions so far. I feel, at least here are the few
> > potential candidates to go in:
> > 
> > 1) Avoiding double runqueue lock overhead  (Andrew Theurer/ PeterZ)
> > 2) Dynamically changing PLE window (Avi/Andrew/Chegu)
> > 3) preempt_notify handler to identify preempted VCPUs (Avi)
> > 4) Avoiding iterating over VCPUs in undercommit scenario. (Raghu/PeterZ)
> > 5) Avoiding unnecessary spinning in overcommit scenario (Raghu/Rik)
> > 6) Pv spinlock
> > 7) Jiannan's proposed improvements
> > 8) Defer preemption patches
> > 
> > Did we miss anything (or added extra?)
> > 
> > So here are my action items:
> > - I plan to repost this series with what PeterZ, Rik suggested with
> > performance analysis.
> > - I ll go back and explore on (3) and (6) ..
> > 
> > Please Let me know..
> 
> Undoubtedly we'll think of more stuff.  But this looks like a good start.

9) lazy gang-like scheduling with PLE to cover the non-gang-like
exceptions  (/me runs and hides from scheduler folks)

-Andrew Theurer

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