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

[Stefano Brivio <sbrivio@redhat.com>]

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.

So it looks somewhat similar to

  https://archives.passt.top/passt-dev/20241017021027.2ac9ea53@elisabeth/

now?

> 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. :)

> 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.

> 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.

-- 
Stefano