Re: [Qemu-devel] [PATCH 5/6] virtio-pci: introduce pio notification capability for modern device

[Jason Wang <jasowang@redhat.com>]

On 08/25/2015 07:48 PM, Michael S. Tsirkin wrote:
> On Fri, Aug 21, 2015 at 05:05:49PM +0800, Jason Wang wrote:
>> > We used to use mmio for notification. This could be slow on some arch
>> > (e.g on x86 without EPT). So this patch introduces pio bar and a pio
>> > notification cap for modern device. This ability is enabled through
>> > property "modern-pio-notify" for virtio pci devices and was disabled
>> > by default. Management can enable when it thinks it was needed.
>> > 
>> > Benchmarks shows almost no obvious difference with legacy device.
>> > Thanks Wenli Quan <wquan@redhat.com> for the benchmarking.
>> > 
>> > Cc: Michael S. Tsirkin <mst@redhat.com>
>> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> I don't really care much about non-EPT hosts, but if you propose
> a patch to optimize them, it should be accompanied by numbers
> showing the performance difference.
>

According to the test, PIO is a little bit faster than fast mmio in some
specific TCP_RR case:

modern device fast mmio vs modern device pio:

TCP_RR:

size/session/+transaction rate%/+cpu%/-+per cpu%/
64/1/[+1.5646%]/+5.6604%/-4.3415%/  
64/25/+0.3003%/-0.4517%/+0.7486%/
...
256/1/[+1.0046%]/[-6.5238%]/[+7.0673%]/

So the improvement is almost as much as previous patch.