Re: vhost-kernel net on pasta: from 26 to 37Gbit/s

[Eugenio Perez Martin <eperezma@redhat.com>]

On Fri, Jun 6, 2025 at 6:37 PM Stefano Brivio <sbrivio@redhat.com> wrote:
>
> On Fri, 6 Jun 2025 16:32:38 +0200
> Eugenio Perez Martin <eperezma@redhat.com> wrote:
>
> > On Wed, May 21, 2025 at 12:35 PM Eugenio Perez Martin
> > <eperezma@redhat.com> wrote:
> > >
> > > On Wed, May 21, 2025 at 12:09 PM Stefano Brivio <sbrivio@redhat.com> wrote:
> > > >
> > > > On Tue, 20 May 2025 17:09:44 +0200
> > > > Eugenio Perez Martin <eperezma@redhat.com> wrote:
> > > >
> > > > > [...]
> > > > >
> > > > > Now if I isolate the vhost kernel thread [1] I get way more
> > > > > performance as expected:
> > > > > - - - - - - - - - - - - - - - - - - - - - - - - -
> > > > > [ ID] Interval           Transfer     Bitrate         Retr
> > > > > [  5]   0.00-10.00  sec  43.1 GBytes  37.1 Gbits/sec    0             sender
> > > > > [  5]   0.00-10.04  sec  43.1 GBytes  36.9 Gbits/sec                  receiver
> > > > >
> > > > > After analyzing perf output, rep_movs_alternative is the most called
> > > > > function in the three iperf3 (~20%Self), passt.avx2 (~15%Self) and
> > > > > vhost (~15%Self)
> > > >
> > > > Interesting... s/most called function/function using the most cycles/, I
> > > > suppose.
> > > >
> > >
> > > Right!
> > >
> > > > So it looks somewhat similar to
> > > >
> > > >   https://archives.passt.top/passt-dev/20241017021027.2ac9ea53@elisabeth/
> > > >
> > > > now?
> > > >
> > >
> > > Kind of. Below tcp_sendmsg_locked I don't see sk_page_frag_refill but
> > > skb_do_copy_data_nocache. Not sure if that means something, as it
> > > should not be affected by vhost.
> > >
> > > > > But I don't see any of them consuming 100% of CPU in
> > > > > top: pasta consumes ~85% %CPU, both iperf3 client and server consumes
> > > > > 60%, and vhost consumes ~53%.
> > > > >
> > > > > So... I have mixed feelings about this :). By "default" it seems to
> > > > > have less performance, but my test is maybe too synthetic.
> > > >
> > > > Well, surely we can't ask Podman users to pin specific stuff to given
> > > > CPU threads. :)
> > > >
> > >
> > > Yes but maybe the result changes under the right schedule? I'm
> > > isolating the CPUs entirely, which is not the usual case for pasta for
> > > sure :).
> > >
> > > > > There is room for improvement with the mentioned optimizations so I'd
> > > > > continue applying them, continuing with UDP and TCP zerocopy, and
> > > > > developing zerocopy vhost rx.
> > > >
> > > > That definitely makes sense to me.
> > > >
> > >
> > > Good!
> > >
> > > > > With these numbers I think the series should not be
> > > > > merged at the moment. I could send it as RFC if you want but I've not
> > > > > applied the comments the first one received, POC style :).
> > > >
> > > > I don't think it's really needed for you to spend time on
> > > > semi-polishing something just to have an RFC if you're still working on
> > > > it. I guess the implementation will change substantially anyway once
> > > > you factor in further optimisations.
> > > >
> > >
> > > Agree! I'll keep iterating on this then.
> > >
> >
> > Actually, if I remove all the taskset etc, and trust the kernel
> > scheduler, vanilla pasta gives me:
> > [pasta@virtlab716 ~]$ /home/passt/pasta --config-net iperf3 -c 10.6.68.254 -w 8M
> > Connecting to host 10.6.68.254, port 5201
> > [  5] local 10.6.68.20 port 40408 connected to 10.6.68.254 port 5201
> > [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
> > [  5]   0.00-1.00   sec  3.11 GBytes  26.7 Gbits/sec    0   25.4 MBytes
> > [  5]   1.00-2.00   sec  3.11 GBytes  26.7 Gbits/sec    0   25.4 MBytes
> > [  5]   2.00-3.00   sec  3.12 GBytes  26.8 Gbits/sec    0   25.4 MBytes
> > [  5]   3.00-4.00   sec  3.11 GBytes  26.7 Gbits/sec    0   25.4 MBytes
> > [  5]   4.00-5.00   sec  3.10 GBytes  26.6 Gbits/sec    0   25.4 MBytes
> > [  5]   5.00-6.00   sec  3.11 GBytes  26.7 Gbits/sec    0   25.4 MBytes
> > [  5]   6.00-7.00   sec  3.11 GBytes  26.7 Gbits/sec    0   25.4 MBytes
> > [  5]   7.00-8.00   sec  3.09 GBytes  26.6 Gbits/sec    0   25.4 MBytes
> > [  5]   8.00-9.00   sec  3.08 GBytes  26.5 Gbits/sec    0   25.4 MBytes
> > [  5]   9.00-10.00  sec  3.10 GBytes  26.6 Gbits/sec    0   25.4 MBytes
> > - - - - - - - - - - - - - - - - - - - - - - - - -
> > [ ID] Interval           Transfer     Bitrate         Retr
> > [  5]   0.00-10.00  sec  31.0 GBytes  26.7 Gbits/sec    0             sender
> > [  5]   0.00-10.04  sec  31.0 GBytes  26.5 Gbits/sec                  receiver
> >
> > And with vhost-net :
> > [pasta@virtlab716 ~]$ /home/passt/pasta --config-net iperf3 -c 10.6.68.254 -w 8M
> > ...
> > Connecting to host 10.6.68.254, port 5201
> > [  5] local 10.6.68.20 port 46720 connected to 10.6.68.254 port 5201
> > [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
> > [  5]   0.00-1.00   sec  4.17 GBytes  35.8 Gbits/sec    0   11.9 MBytes
> > [  5]   1.00-2.00   sec  4.17 GBytes  35.9 Gbits/sec    0   11.9 MBytes
> > [  5]   2.00-3.00   sec  4.16 GBytes  35.7 Gbits/sec    0   11.9 MBytes
> > [  5]   3.00-4.00   sec  4.14 GBytes  35.6 Gbits/sec    0   11.9 MBytes
> > [  5]   4.00-5.00   sec  4.16 GBytes  35.7 Gbits/sec    0   11.9 MBytes
> > [  5]   5.00-6.00   sec  4.16 GBytes  35.8 Gbits/sec    0   11.9 MBytes
> > [  5]   6.00-7.00   sec  4.18 GBytes  35.9 Gbits/sec    0   11.9 MBytes
> > [  5]   7.00-8.00   sec  4.19 GBytes  35.9 Gbits/sec    0   11.9 MBytes
> > [  5]   8.00-9.00   sec  4.18 GBytes  35.9 Gbits/sec    0   11.9 MBytes
> > [  5]   9.00-10.00  sec  4.18 GBytes  35.9 Gbits/sec    0   11.9 MBytes
> > - - - - - - - - - - - - - - - - - - - - - - - - -
> > [ ID] Interval           Transfer     Bitrate         Retr
> > [  5]   0.00-10.00  sec  41.7 GBytes  35.8 Gbits/sec    0             sender
> > [  5]   0.00-10.04  sec  41.7 GBytes  35.7 Gbits/sec                  receiver
> >
> > If I  go the extra mile and disable notifications (it might be just
> > noise, but...)
> > [pasta@virtlab716 ~]$ /home/passt/pasta --config-net iperf3 -c 10.6.68.254 -w 8M
> > ...
> > Connecting to host 10.6.68.254, port 5201
> > [  5] local 10.6.68.20 port 56590 connected to 10.6.68.254 port 5201
> > [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
> > [  5]   0.00-1.00   sec  4.19 GBytes  36.0 Gbits/sec    0   12.4 MBytes
> > [  5]   1.00-2.00   sec  4.18 GBytes  35.9 Gbits/sec    0   12.4 MBytes
> > [  5]   2.00-3.00   sec  4.18 GBytes  35.9 Gbits/sec    0   12.4 MBytes
> > [  5]   3.00-4.00   sec  4.20 GBytes  36.1 Gbits/sec    0   12.4 MBytes
> > [  5]   4.00-5.00   sec  4.21 GBytes  36.2 Gbits/sec    0   12.4 MBytes
> > [  5]   5.00-6.00   sec  4.21 GBytes  36.1 Gbits/sec    0   12.4 MBytes
> > [  5]   6.00-7.00   sec  4.20 GBytes  36.1 Gbits/sec    0   12.4 MBytes
> > [  5]   7.00-8.00   sec  4.23 GBytes  36.4 Gbits/sec    0   12.4 MBytes
> > [  5]   8.00-9.00   sec  4.24 GBytes  36.4 Gbits/sec    0   12.4 MBytes
> > [  5]   9.00-10.00  sec  4.21 GBytes  36.2 Gbits/sec    0   12.4 MBytes
> > - - - - - - - - - - - - - - - - - - - - - - - - -
> > [ ID] Interval           Transfer     Bitrate         Retr
> > [  5]   0.00-10.00  sec  42.1 GBytes  36.1 Gbits/sec    0             sender
> > [  5]   0.00-10.04  sec  42.1 GBytes  36.0 Gbits/sec                  receiver
> >
> > So I guess the best is to actually run performance tests closer to
> > real-world workload against the new version and see if it works
> > better?
>
> Well, that's certainly a possibility.
>
> I'd say the biggest value for vhost-net usage in pasta is reaching
> throughput figures that are comparable with veth, with or without
> multithreading (keeping an eye on bytes per cycle, of course), with or
> without kernel changes, so that users won't need to choose between
> rootless and performance anymore.
>
> It would also simplify things in Podman quite a lot (and to some extent
> in rootlesskit / Docker as well). We're pretty much there with virtual
> machines, just not quite with containers (which is somewhat ironic, but
> of course there's a good reason for that).
>
> If we're clearly wasting cycles in vhost-net (because of the bounce
> buffer, plus something else perhaps?) *and* there's a somewhat possible
> solution for that in sight *and* the interface would change anyway,
> running throughput tests and polishing up the current version with a
> half-baked solution at the moment sounds a bit wasteful to me.
>

My point is that I'm testing a very synthetic scenario. If everybody
agree this is close enough to real world ones, I'm ok to continue
improving the edges we see. If not, maybe we're picking the wrong
fruit even if it is low hand?

Getting a table like [1] would give us light about this, especially if
it is just a matter of running "make performance" or similar. Maybe we
need to include longer queues? Focus on a given scenario? UDP goes
better but TCP?

Now more points about this scenario:
1) I don't see 100% CPU usage in any element:
CPU%
84.2  passt.avx2
57.9 iperf3
57.2 iperf3
50.7 vhost-1805109

2) The most used (Self%) function in vhost is rep_movs_alternative,
called from skb_copy_datagram_iter, so yes, ZeroCopy should help a lot
here.

Now, is "iperf3 -w 8M" representative? I'm sure ZC helps in this
scenario, does it make it worse if we have small packets? Do we care?

I'm totally ok with continuing trying with ZC, I just want to make
sure we're not missing anything :).

Thanks!

[1] https://passt.top/passt/about/#performance_1

More details

> But if one of those assumptions doesn't hold, or if you feel the need to
> consolidate the current status, perhaps polishing up the current
> version right now and actually evaluating throughput (as well as
> overhead) makes sense to me, yes.
>
> --
> Stefano
>