Re: [RFC PATCH 0/4] Implement multiqueue virtio-net

Michael S. Tsirkin
2010-09-13T11:57:03Z

On Mon, Sep 13, 2010 at 09:42:22AM +0530, Krishna Kumar2 wrote:
> "Michael S. Tsirkin" <mst@redhat.com> wrote on 09/12/2010 05:10:25 PM:
>
> > > SINGLE vhost (Guest -> Host):
> > > 1 netperf: BW: 10.7% SD: -1.4%
> > > 4 netperfs: BW: 3% SD: 1.4%
> > > 8 netperfs: BW: 17.7% SD: -10%
> > > 16 netperfs: BW: 4.7% SD: -7.0%
> > > 32 netperfs: BW: -6.1% SD: -5.7%
> > > BW and SD both improves (guest multiple txqs help). For 32
> > > netperfs, SD improves.
> > >
> > > But with multiple vhosts, guest is able to send more packets
> > > and BW increases much more (SD too increases, but I think
> > > that is expected).
> >
> > Why is this expected?
>
> Results with the original kernel:
> _____________________________
> # BW SD RSD
> ______________________________
> 1 20903 1 6
> 2 21963 6 25
> 4 22042 23 102
> 8 21674 97 419
> 16 22281 379 1663
> 24 22521 857 3748
> 32 22976 1528 6594
> 40 23197 2390 10239
> 48 22973 3542 15074
> 64 23809 6486 27244
> 80 23564 10169 43118
> 96 22977 14954 62948
> 128 23649 27067 113892
> ________________________________
>
> With higher number of threads running in parallel, SD
> increased. In this case most threads run in parallel
> only till __dev_xmit_skb (#numtxqs=1). With mq TX patch,
> higher number of threads run in parallel through
> ndo_start_xmit. I *think* the increase in SD is to do
> with higher # of threads running for larger code path
> >From the numbers I posted with the patch (cut-n-paste
> only the % parts), BW increased much more than the SD,
> sometimes more than twice the increase in SD.

Service demand is BW/CPU, right? So if BW goes up by 50%
and SD by 40%, this means that CPU more than doubled.

> N# BW% SD% RSD%
> 4 54.30 40.00 -1.16
> 8 71.79 46.59 -2.68
> 16 71.89 50.40 -2.50
> 32 72.24 34.26 -14.52
> 48 70.10 31.51 -14.35
> 64 69.01 38.81 -9.66
> 96 70.68 71.26 10.74
>
> I also think SD calculation gets skewed for guest->local
> host testing.

If it's broken, let's fix it?

> For this test, I ran a guest with numtxqs=16.
> The first result below is with my patch, which creates 16
> vhosts. The second result is with a modified patch which
> creates only 2 vhosts (testing with #netperfs = 64):

My guess is it's not a good idea to have more TX VQs than guest CPUs.

I realize for management it's easier to pass in a single vhost fd, but
just for testing it's probably easier to add code in userspace to open
/dev/vhost multiple times.

>
> #vhosts BW% SD% RSD%
> 16 20.79 186.01 149.74
> 2 30.89 34.55 18.44
>
> The remote SD increases with the number of vhost threads,
> but that number seems to correlate with guest SD. So though
> BW% increased slightly from 20% to 30%, SD fell drastically
> from 186% to 34%. I think it could be a calculation skew
> with host SD, which also fell from 150% to 18%.

I think by default netperf looks in /proc/stat for CPU utilization data:
so host CPU utilization will include the guest CPU, I think?

I would go further and claim that for host/guest TCP
CPU utilization and SD should always be identical.
Makes sense?

>
> I am planning to submit 2nd patch rev with restricted
> number of vhosts.
>
> > > Likely cause for the 1 stream degradation with multiple
> > > vhost patch:
> > >
> > > 1. Two vhosts run handling the RX and TX respectively.
> > > I think the issue is related to cache ping-pong esp
> > > since these run on different cpus/sockets.
> >
> > Right. With TCP I think we are better off handling
> > TX and RX for a socket by the same vhost, so that
> > packet and its ack are handled by the same thread.
> > Is this what happens with RX multiqueue patch?
> > How do we select an RX queue to put the packet on?
>
> My (unsubmitted) RX patch doesn't do this yet, that is
> something I will check.
>
> Thanks,
>
> - KK

You'll want to work on top of net-next, I think there's
RX flow filtering work going on there.

--
MST