Re: AMD EPYC virtual network performances

[Elliott Mitchell <ehem+xen@xxxxxxx>]

On Wed, Aug 14, 2024 at 08:15:38AM +0200, Jürgen Groß wrote:
> On 14.08.24 00:36, Elliott Mitchell wrote:
> > On Tue, Aug 13, 2024 at 08:55:42PM +0200, Jürgen Groß wrote:
> > > On 13.08.24 19:49, Elliott Mitchell wrote:
> > > > 
> > > > There is a possibility spurious interrupts are being turned into 
> > > > spurious
> > > > events by the back-end drivers.
> > > 
> > > No, I don't think so.
> > > 
> > > > Jürgen Groß, what is the performance impact of "iommu=debug"?  Seems to
> > > > mostly cause more reporting and have minimal/no performance effect.
> > > 
> > > I guess you are referring to the Xen option? I'm no expert in this
> > > area.
> > 
> > Drat.  I haven't noticed much, which would match with simply enabling a
> > bunch of debugging printk()s (alas I'm not monitoring performance closely
> > enough to be sure).  Guess I wait for Andrei Semenov to state a comfort
> > level with trying "iommu=debug".
> 
> You didn't answer my question.

I guess I did not explicitly do so.  I was referring to the Xen
command-line option.  I was asking in the hope it might be reasonable
for Andrei Semenov to run with it on for some time.


Come to think of it, I now wonder whether "iommu=no-intremap" is part of
the recipe for having the messages I've seen show up.

I can now state they appear to correlate with activity in the HVM domain
(which is associated with network/vif activity) which also increases
Linux's spurious interrupt counters.
I cannot yet state the HVM domain is the exclusive source of these
messages.  I am able to run with that domain shutdown, so I've done so.
If no new messages show up that will seem to attribute those to the HVM
domain.

The other interesting bit is "iommu=no-intremap" seems to have made the
messages associated with the software RAID1 issue disappear (possibly
partially, but not fully mitigated it too).  My observations seem
insufficient to indicate whether or not these are distinct issues
though.


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