Re: [RFC PATCH 0/3] Sharing MSIX irq for tx/rx queue pairs

[Jason Wang <jasowang@redhat.com>]

On 01/04/2015 07:36 PM, Michael S. Tsirkin wrote:
> On Sun, Jan 04, 2015 at 04:38:17PM +0800, Jason Wang wrote:
>> On 12/28/2014 03:52 PM, Michael S. Tsirkin wrote:
>>> On Fri, Dec 26, 2014 at 10:53:42AM +0800, Jason Wang wrote:
>>>> Hi all:
>>>>
>>>> This series try to share MSIX irq for each tx/rx queue pair. This is
>>>> done through:
>>>>
>>>> - introducing virtio pci channel which are group of virtqueues that
>>>>   sharing a single MSIX irq (Patch 1)
>>>> - expose channel setting to virtio core api (Patch 2)
>>>> - try to use channel setting in virtio-net (Patch 3)
>>>>
>>>> For the transport that does not support channel, channel paramters
>>>> were simply ignored. For devices that does not use channel, it can
>>>> simply pass NULL or zero to virito core.
>>>>
>>>> With the patch, 1 MSIX irq were saved for each TX/RX queue pair.
>>>>
>>>> Please review.
>>> How does this sharing affect performance?
>>>
>> Patch 3 only checks more_used() for tx ring which in fact reduces the
>> effect of event index and may introduce more tx interrupts. After fixing
>> this issue, tested with 1 vcpu and 1 queue. No obvious changes in
>> performance were noticed.
>>
>> Thanks
> Is this with or without MQ?

Without MQ. 1 vcpu and 1 queue were used.
> With MQ, it seems easy to believe as interrupts are
> distributed between CPUs.
>
> Without MQ, it should be possible to create UDP workloads where
> processing incoming and outgoing interrupts
> on separate CPUs is a win.

Not sure. Processing on separate CPUs may only win when the system is
not busy. But if we processing a single flow in two cpus, it may lead
extra lock contention and bad cache utilization.
And if we really want to distribute the load, RPS/RFS could be used.