On 2023/11/13 20:44, Yuri Benditovich wrote:
On Sat, Nov 11, 2023 at 5:28 PM Akihiko Odaki <akihiko.od...@daynix.com
<mailto:akihiko.od...@daynix.com>> wrote:
On 2023/11/03 22:14, Yuri Benditovich wrote:
>
>
> On Fri, Nov 3, 2023 at 11:55 AM Akihiko Odaki
<akihiko.od...@daynix.com <mailto:akihiko.od...@daynix.com>
> <mailto:akihiko.od...@daynix.com
<mailto:akihiko.od...@daynix.com>>> wrote:
>
> On 2023/11/03 18:35, Yuri Benditovich wrote:
> >
> >
> > On Thu, Nov 2, 2023 at 4:56 PM Akihiko Odaki
> <akihiko.od...@daynix.com <mailto:akihiko.od...@daynix.com>
<mailto:akihiko.od...@daynix.com <mailto:akihiko.od...@daynix.com>>
> > <mailto:akihiko.od...@daynix.com
<mailto:akihiko.od...@daynix.com>
> <mailto:akihiko.od...@daynix.com
<mailto:akihiko.od...@daynix.com>>>> wrote:
> >
> > On 2023/11/02 19:20, Yuri Benditovich wrote:
> > >
> > >
> > > On Thu, Nov 2, 2023 at 11:33 AM Michael S. Tsirkin
> > <m...@redhat.com <mailto:m...@redhat.com>
<mailto:m...@redhat.com <mailto:m...@redhat.com>>
> <mailto:m...@redhat.com <mailto:m...@redhat.com>
<mailto:m...@redhat.com <mailto:m...@redhat.com>>>
> > > <mailto:m...@redhat.com <mailto:m...@redhat.com>
<mailto:m...@redhat.com <mailto:m...@redhat.com>>
> <mailto:m...@redhat.com <mailto:m...@redhat.com>
<mailto:m...@redhat.com <mailto:m...@redhat.com>>>>> wrote:
> > >
> > > On Thu, Nov 02, 2023 at 11:09:27AM +0200, Yuri
> Benditovich wrote:
> > > > Probably we mix two different patches in this
> discussion.
> > > > Focusing on the patch in the e-mail header:
> > > >
> > > > IMO it is not acceptable to fail QEMU run
for one
> feature
> > that we
> > > can't make
> > > > active when we silently drop all other
features in
> such a
> > case.
> > >
> > > If the feature is off by default then it seems more
> reasonable
> > > and silent masking can be seen as a bug.
> > > Most virtio features are on by default this is
why it's
> > > reasonable to mask them.
> > >
> > >
> > > If we are talking about RSS: setting it initially
off is the
> > development
> > > time decision.
> > > When it will be completely stable there is no reason to
> keep it
> > off by
> > > default, so this is more a question of time and of a
> readiness of
> > libvirt.
> >
> > It is not ok to make "on" the default; that will
enable RSS
> even when
> > eBPF steering support is not present and can result in
> performance
> > degradation.
> >
> >
> > Exactly as it is today - with vhost=on the host does not
suggest RSS
> > without eBPF.
> > I do not understand what you call "performance
degradation", can you
> > describe the scenario?
>
> I was not clear, but I was talking about the case of
vhost=off or peers
> other than tap (e.g., user). rss=on employs in-qemu RSS,
which incurs
> overheads for such configurations.
>
>
> So, vhost=off OR peers other than tap:
>
> In the case of peers other than tap (IMO) we're not talking about
> performance at all.
> Backends like "user" (without vnet_hdr) do not support _many_
> performance-oriented features.
> If RSS is somehow "supported" for such backends this is rather a
> misunderstanding (IMO again).
We do not need to ensure good performance when RSS is enabled by the
guest for backends without eBPF steering program as you say. In-QEMU
RSS
is only useful for testing and not meant to improve the performance.
However, if you set rss=on, QEMU will advertise the availability of RSS
feature. The guest will have no mean to know if it's implemented in a
way not performance-wise so it may decide to use the feature to improve
the performance, which can result in performance degradation.
Therefore,
it's better not to set rss=on for such backends.
I still do not understand what is the scenario where you see or suspect
the mentioned "performance degradation".
We can discuss whether such a problem exists as soon as you explain it.
The scenario is that:
- rss=on,
- A backend without eBPF steering support is in use, and
- The guest expects VIRTIO_NET_F_RSS has little overheads as hardware
RSS implementations do.
I consider the risk of the performance degradation in such a situation
is the reason why virtio-net emits a warning ("Can't load eBPF RSS -
fallback to software RSS") when in-QEMU RSS is in use.
