Re: [PATCH 5/6] kvm: qemu: virtio-net: handle all tx in I/O thread without timer

[Mark McLoughlin <markmc@redhat.com>]

On Tue, 2008-11-04 at 08:23 -0700, David S. Ahern wrote:
> 
> Mark McLoughlin wrote:
> 
> > Note also that when tuning for a specific workload, which CPU
> > the I/O thread is pinned to is important.
> > 
> 
> Hi Mark:
> 
> Can you give an example of when that has a noticeable affect?
> 
> For example, if the guest handles network interrupts on vcpu0 and it is
> pinned to pcpu0 where should the IO thread be pinned for best performance?

Basically, the I/O thread is where packets are copied too and from host
kernel space at the moment.

If there are other copies of the packets anywhere, you want those to
copy from a cache.

With my netperf guest->host benchmark, you actually have four copies
going on:

  1) netperf process in guest copying to guest kernel space

  2) qemu process in the host copying between internal buffers

  3) qemu process in the host copying to host kernel space

  4) netserver process in the host copying into its buffers

My machine has four CPUs, with two 6Mb L2 caches - each cache is shared
between two of the CPUs, so I set things up as follows:

  pcpu#3 - netserver, I/O thread, vcpu#0
  pcup#4 - vcpu#1, virtio_net irq, netperf

which (hopefully) ensures that we're only doing one copy using RAM and
the rest are using the L1/L2 caches.

Cheers,
Mark.