Re: AMD EPYC virtual network performances

[Jürgen Groß <jgross@xxxxxxxx>]

On 13.08.24 19:49, Elliott Mitchell wrote:
> On Tue, Aug 13, 2024 at 01:16:06PM +0200, Jürgen Groß wrote:
> > On 13.08.24 03:10, Elliott Mitchell wrote:
> > > On Tue, Jul 09, 2024 at 11:37:07AM +0200, Jürgen Groß wrote:
> > > > In both directories you can see the number of spurious events by looking
> > > > into the spurious_events file.
> > > >
> > > > In the end the question is why so many spurious events are happening. Finding
> > > > the reason might be hard, though.
> > > Hopefully my comments on this drew your attention, yet lack of response
> > > suggests otherwise.  I'm wondering whether this is an APIC misprogramming
> > > issue, similar to the x2APIC issue which was causing trouble with recent
> > > AMD processors.
> > >
> > > Trying to go after the Linux software RAID1, my current attempt is
> > > "iommu=debug iommu=no-intremap".  I'm seeing *lots* of messages from
> > > spurious events in `xl dmesg`.  So many I have a difficult time believing
> > > they are related to hardware I/O.
> > Seeing them in `xl dmesg` means those spurious events are seen by the
> > hypervisor, not by the Linux kernel.
> Indeed.  Yet this seems to be pointing at a problem, whereas most other
> information sources merely indicate there is a problem.
>
> I'm unable to resolve those to hardware.  This could mean those are being
> synthesized by software and when crossing some interface they get
> reinterpreted as hardware events.  This could mean those are hardware
> events, but somewhere inside Xen information is being corrupted and the
> information displayed is unrelated to the original event (x2APIC
> misinterpretation?).
>
>
> > > In which case could the performance problem observed by Andrei Semenov
> > > be due to misprogramming of [x2]APIC triggering spurious events?
> > I don't see a connection here, as spurious interrupts (as seen by the
> > hypervisor in your case) and spurious events (as seen by Andrei) are
> > completely different (hardware vs. software level).
> The entries seem to appear at an average of about 1/hour.  Could be most
> events are being dropped and 10x that number are occuring.  If so, those
> extras could be turning into spurious events seen by various domains.
Even 10 spurious events per hour should not have a measurable impact
on performance.

> 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.


Juergen