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

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

Hi!

Some updates on the integration. The main culprit was to allow pasta
to keep reading packets using the regular read() on the tap device. I
thought that part was completely disabled, but I guess the kernel is
able to omit the notification on tap as long as the userspace does not
read it.

My scenario: in different CPUs, all in the same NUMA. I run iperf
server on CPU 11 with "iperf3 -A 11 -s". All odds CPUs are isolated
with isolcpus=1,3,... nohz=on nohz_full=1,3,...

With vanilla pasta isolated to CPUs 1,3 with taskset, and just
--config-net option, and running iperf with "iperf3 -A 5 -c
10.6.68.254 -w 8M":
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  30.7 GBytes  26.4 Gbits/sec    0             sender
[  5]   0.00-10.04  sec  30.7 GBytes  26.3 Gbits/sec                  receiver

Now trying with the vhost patches we get a slightly worse performance:
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  25.5 GBytes  21.9 Gbits/sec    0             sender
[  5]   0.00-10.04  sec  25.5 GBytes  21.8 Gbits/sec                  receiver

Now vhost patch still lacks optimizations like disabling notifications
or batch more rx available buffer notifications. At the moment it
refills the rx buffers in each iteration, and does not set the
no_notify bit which makes the kernel skip the used buffer
notifications if pasta is actively checking the queue, which is not
optimal.

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), 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. 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. 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 :).

Thanks!

[1] Notes to reproduce it, I'm able to see it with top -H and then set
with taskset. Either the latest changes on the module or the way pasta
behaves does not allow me to see in classical ps output.