Re: [Qemu-devel] [virtio-dev] Re: [virtio-dev] Re: [PATCH v2 00/16] Vhost-pci for inter-VM communication

[Jason Wang]

On 2017年05月18日 11:03, Wei Wang wrote:
> On 05/17/2017 02:22 PM, Jason Wang wrote:
> > On 2017年05月17日 14:16, Jason Wang wrote:
> > > On 2017年05月16日 15:12, Wei Wang wrote:
> > > > > >
> > > > >
> > > > > Hi:
> > > > >
> > > > > Care to post the driver codes too?
> > > > OK. It may take some time to clean up the driver code before post 
> > > > it out. You can first
> > > > have a check of the draft at the repo here:
> > > > https://github.com/wei-w-wang/vhost-pci-driver
> > > >
> > > > Best,
> > > > Wei
> > >
> > > Interesting, looks like there's one copy on tx side. We used to have 
> > > zerocopy support for tun for VM2VM traffic. Could you please try to 
> > > compare it with your vhost-pci-net by:
> We can analyze from the whole data path - from VM1's network stack to 
> send packets -> VM2's
> network stack to receive packets. The number of copies are actually 
> the same for both.

That's why I'm asking you to compare the performance. The only reason 
for vhost-pci is performance. You should prove it.

> vhost-pci: 1-copy happen in VM1's driver xmit(), which copes packets 
> from its network stack to VM2's
> RX ring buffer. (we call it "zerocopy" because there is no 
> intermediate copy between VMs)
> zerocopy enabled vhost-net: 1-copy happen in tun's recvmsg, which 
> copies packets from VM1's TX ring
> buffer to VM2's RX ring buffer.

Actually, there's a major difference here. You do copy in guest which 
consumes time slice of vcpu thread on host. Vhost_net do this in its own 
thread. So I feel vhost_net is even faster here, maybe I was wrong.

> That being said, we compared to vhost-user, instead of vhost_net, 
> because vhost-user is the one
> that is used in NFV, which we think is a major use case for vhost-pci.

If this is true, why not draft a pmd driver instead of a kernel one? And 
do you use virtio-net kernel driver to compare the performance? If yes, 
has OVS dpdk optimized for kernel driver (I think not)?

What's more important, if vhost-pci is faster, I think its kernel driver 
should be also faster than virtio-net, no?

> > > - make sure zerocopy is enabled for vhost_net
> > > - comment skb_orphan_frags() in tun_net_xmit()
> > >
> > > Thanks
> >
> > You can even enable tx batching for tun by ethtool -C tap0 rx-frames 
> > N. This will greatly improve the performance according to my test.
>
> Thanks, but would this hurt latency?
>
> Best,
> Wei

I don't see this in my test.

Thanks