Re: [RFT] IRQ sharing for assigned devices - method selection

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 2011-01-07 at 10:33 +0100, Jan Kiszka wrote:
> Hi,
> 
> to finally select the approach for adding overdue IRQ sharing support
> for PCI pass-through, I hacked up two versions based on Thomas' patches
> and his suggestion to use a timeout-based mode transition:
> 
>     git://git.kiszka.org/linux-kvm.git queues/dev-assign.notify
>     git://git.kiszka.org/linux-kvm.git queues/dev-assign.timeout
> 
>     git://git.kiszka.org/qemu-kvm.git queues/dev-assign
> 
> Both approaches work, but I'm either lacking a sufficiently stressing
> test environment to tickle out a relevant delta, even between masking at
> irqchip vs. PCI config space level - or there is none... Yes, there are
> differences at micro level but they do not manifest in measurable (ie.
> above the noise level) load increase or throughput/latency decrease in
> my limited tests here. I that actually turns out to be true, I would
> happily bury all this dynamic mode switching again.
> 
> So, if you have a good high-bandwidth test case at hand, I would
> appreciate if you could give this a try and report your findings. Does
> switching from exclusive to shared IRQ mode decrease the throughput or
> increase the host load? Is there a difference to current kvm?

I think any sufficiently high bandwidth device will be using MSI and or
NAPI, so I wouldn't expect we're going to see much change there.
Perhaps you can simply force a 1GbE device to use INTx and do some
netperf TCP_RR tests to try to expose any latency differences.  Thanks,

Alex