Message-ID: <OF5A57530E.4E8C0A65-ON6525779D.00591022-6525779D.0059D5BB@in.ibm.com>
Date:	Mon, 13 Sep 2010 21:53:40 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	anthony@...emonkey.ws, avi@...hat.com, davem@...emloft.net,
	kvm@...r.kernel.org, netdev@...r.kernel.org, rusty@...tcorp.com.au,
	rick.jones2@...com
Subject: Re: [RFC PATCH 0/4] Implement multiqueue virtio-net

"Michael S. Tsirkin" <mst@...hat.com> wrote on 09/13/2010 05:20:55 PM:

> > 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.

I think the SD calculation might be more complicated,
I think it does it based on adding up averages sampled
and stored during the run. But, I still don't see how CPU
can double?? e.g.
	BW: 1000 -> 1500 (50%)
	SD: 100 -> 140 (40%)
	CPU: 10 -> 10.71 (7.1%)

> > 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.

Definitely, I will try to run tomorrow with more reasonable
values, also will test with my second version of the patch
that creates restricted number of vhosts and post results.

> 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?

It appears that way to me too, but the data above seems to
suggest the opposite...

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

It makes sense to me, but once again I am not sure how SD
is really done, or whether it is linear to CPU. Cc'ing Rick
in case he can comment....

>
> >
> > 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.

Thanks Michael, I will follow up on that for the RX patch,
plus your suggestion on tying RX with TX.

Thanks,

- KK

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