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Re: [Xen-devel] [PATCH V4 0/3] xen-netback: switch to NAPI + kthread 1:1 model

On Tue, Aug 06, 2013 at 03:17:46PM +0100, Andrew Bennieston wrote:
> On 06/08/13 14:29, David Vrabel wrote:
> >On 06/08/13 14:16, Pasi Kärkkäinen wrote:
> >>On Tue, Aug 06, 2013 at 10:06:00AM +0100, Wei Liu wrote:
> >>>
> >>>IRQs are distributed to 4 cores by hand in the new model, while in the
> >>>old model vifs are automatically distributed to 4 kthreads.
> >>>
> >>
> >>Hmm.. so with these patches applied is is *required* to do manual 
> >>configuration in dom0 to get good performance?
> >
> >This should be irqbalanced's job.  The existing version doesn't do a
> >good enough job yet though.  Andrew Bennieston may have more details.
> >
> >David
> >
> irqbalance 1.0.6 [1] includes a patch [2] from Wei Liu [3] that adds
> support for balancing `xen-dyn-event' interrupts. When I have compiled
> this version and run it under Xen(Server) I noticed that the
> interrupts are indeed moving between cores, but not necessarily in
> what I would call an obvious or optimal way (e.g. several VIF
> interrupts are being grouped onto a single dom0 VCPU at times). I plan
> on investigating this further when time permits.
> I also noticed that, from time to time, the irqbalance process
> disappears. I tracked this down to a segfault that occurs when a VM
> shuts down and an IRQ disappears during one of irqbalance's periodic
> rescans. I'm hoping to be able to narrow this down sufficiently to
> identify the cause and ideally fix it, but I don't have a lot of time
> to work on this at the moment.
> As for the impact on Wei's patches, without irqbalance it would be
> trivial to automatically assign (via a script, on VM start) the
> interrupts for a particular VIF to a particular dom0 vCPU in a
> round-robin fashion, just as VIFs were previously assigned to netback
> kthreads. This would result in broadly the same performance as before,
> while an improved irqbalanced should give better performance and
> fairness when two different VIFs would otherwise be competing for the
> same resources.

So can I conclude that this model doesn't incur severe performance
regression, on the other hand it has its advantage on fairness so it's
worth upstreaming?

If so I will post another series shortly with all comments addressed.


> Andrew.
> [1] https://code.google.com/p/irqbalance/downloads
> [2] 
> https://code.google.com/p/irqbalance/source/detail?r=736a91b739018530c44e59d9b17252f88227fe51
> [3] https://code.google.com/p/irqbalance/issues/detail?id=46

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