On 2023/11/14 2:26, Yuri Benditovich wrote:
On Mon, Nov 13, 2023 at 2:44 PM Akihiko Odaki <akihiko.od...@daynix.com
<mailto:akihiko.od...@daynix.com>> wrote:
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>
> <mailto: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>>
> > <mailto: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>>>
> > > <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
<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>>>>
> > > > <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 <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.
In a described scenario (vhost=off) I do not see why the performance
degradation should happen:
the SW RSS (if activated) will place each packet into proper queue (even
if the auto_mq in kernel is not able to do that) and such a way the
guest will not need to reschedule the packet to proper CPU
The scenario I'm concerned is that the guest has its own packet steering
mechanism which is feature-wise superior to RSS. For example, Linux has
such a mechanism called RPS, which has some advantages due to its
extensible nature according to:
https://www.kernel.org/doc/html/v6.6/networking/scaling.html#rps-receive-packet-steering
Such a guest may still prefer hardware RSS if available since hardware
RSS is expected to have less overheads. However, it is not true for
in-qemu RSS, and using in-QEMU RSS instead of the guest-side steering
mechanism may just hide useful features the guest-side steering
mechanism has and result in performance degradation.
Andrew, I appreciate if you also tell the rationale behind the warning
you put for software RSS ("Can't load eBPF RSS - fallback to software RSS").
