Re: [Xen-devel] [PROPOSAL] Event channel for SMP-VMs: per-vCPU or per-OS?

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Oct 29, 2013 at 10:43:34PM +0800, Luwei Cheng wrote:
> On Tue, Oct 29, 2013 at 10:30 PM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> 
> > On Tue, Oct 29, 2013 at 10:20:46PM +0800, Luwei Cheng wrote:
> > [...]
> > > >
> > > > If events are no longer assigned to a single CPU there's no guarantee
> > > > that the CPU you deliver the event to is the one that's actually going
> > > > to handle it, another CPU might be already in the event channel upcall
> > > > and stole it from under your feet (or event worse, the event could be
> > > > fired on several CPUs at the same time, at least with the current
> > > > implementation).
> > > >
> > > > The goal is: to process the event asap. So, if the event is indeed
> > stolen
> > > by
> > > another vCPU, we should be happy about it because it means that the event
> > > can be processed "fasterâ, before the targeted vCPU picks it:)
> > >
> > > With current implementation, the upcall only happens when the processor
> > > switches from the hypervisor world to the guest world. It seems that the
> > > likelihood that, such"switch" happens on multiple CPUs at the same time,
> > is
> > > very small.
> > > Even if the event fires on several vCPUs, what is the negative effect..?
> > > Is the guest OS able to tolerate it (reentrant IRQ handler)?
> > >
> >
> > As Jan said, it depends. It is sure that unnecessary call to handlers
> > introduce overhead (however small).
> >
> > Furthurmore, with your proposed scheme, it looks like you would need to
> > introduce locks to protect critical regions if there's any. This can
> > introduce overhead as well.
> >
> > Wei.
> >
> >
> Thanks Wei for your comment. Let's compare the cons with pros:
> 
> [Benefit]:
> avoid long vCPU scheduling delays (10x ms), without introducing additional
> reschedule operations
> 
> [Negative effect, possible]:
> the latency due to unnecessary call to handlers on other vCPUs
> (micro-second or nano-second?)
> 

What I mean is that you will introduce latency / performance penalty
from locks to protect critical sections. Say, if several CPUs contents
for same event, overall performance might downgrade.

> So, ... which side we should prefer?
> 
> Thanks,
> Luwei

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