Re: About the performance of hyper-v

[Liang Li]

> >> > Analyze events for all VMs, all VCPUs:
> >> >              VM-EXIT    Samples  Samples%     Time%    Min Time    Max
> >> > Time         Avg time
> >> >   EXTERNAL_INTERRUPT     471831    59.89%    68.58%      0.64us
> >> > 65.42us      2.34us ( +-   0.11% )
> >> >            MSR_WRITE     238932    30.33%    23.07%      0.48us
> >> > 41.05us      1.56us ( +-   0.14% )
> >> >
> >> > Total Samples:787803, Total events handled time:1611193.84us.
> >> >
> >> > I tried turning off hyper-v for the same workload and repeat the test,
> >> > the overall virtualization overhead reduced by about of 50%:
> >> >
> >> > -------
> >> >
> >> > Analyze events for all VMs, all VCPUs:
> >> >
> >> >              VM-EXIT    Samples  Samples%     Time%    Min Time    Max
> >> > Time         Avg time
> >> >           APIC_WRITE     255152    74.43%    50.72%      0.49us
> >> > 50.01us      1.42us ( +-   0.14% )
> >> >        EPT_MISCONFIG      39967    11.66%    40.58%      1.55us
> >> > 686.05us      7.27us ( +-   0.43% )
> >> >            DR_ACCESS      35003    10.21%     4.64%      0.32us
> >> > 40.03us      0.95us ( +-   0.32% )
> >> >   EXTERNAL_INTERRUPT       6622     1.93%     2.08%      0.70us
> >> > 57.38us      2.25us ( +-   1.42% )
> >> >
> >> > Total Samples:342788, Total events handled time:715695.62us.
> >> >
> >> > For this scenario,  hyper-v works really bad.  stimer works better
> >> > than hpet, but on the other hand, it relies on SynIC which has
> >> > negative effects for IPI intensive workloads.
> >> > Do you have any plans for improvement?
> >> >
> >>
> >> Hey,
> >>
> >> the above can be caused by the fact that when 'hv-synic' is enabled, KVM
> >> automatically disables APICv and this can explain the overhead and the
> >> fact that you're seeing more vmexits. KVM disables APICv because SynIC's
> >> 'AutoEOI' feature is incompatible with it. We can, however, tell Windows
> >> to not use AutoEOI ('Recommend deprecating AutoEOI' bit) and only
> >> inhibit APICv if the recommendation was ignored. This is implemented in
> >> the following KVM patch series:
> >> https://lore.kernel.org/kvm/20210518144339.1987982-1-vkuznets@redhat.com/
> >>
> >> It will, however, require a new 'hv-something' flag to QEMU. For now, it
> >> can be tested with 'hv-passthrough'.
> >>
> >> It would be great if you could give it a spin!
> >>
> >> --
> >> Vitaly
> >
> > It's great to know that you already have a solution for this. :)
> >
> > By the way,  is there any requirement for the version of windows or
> > windows updates for the new feature to work?
>
> AFAIR, 'Recommend deprecating AutoEOI' bit appeared in WS2012 so I'd
> expect WS2008 to ignore it completely (and thus SynIC will always be
> disabling APICv for it).
>

Hi Vitaly,
      I tried your patchset and found it's not helpful to reduce the
virtualization overhead.
here are some perfdata with the same workload

===============================
Analyze events for all VMs, all VCPUs:
             VM-EXIT    Samples  Samples%     Time%    Min Time    Max
Time         Avg time
           MSR_WRITE     924045    89.96%    81.10%      0.42us
68.42us      1.26us ( +-   0.07% )
           DR_ACCESS      44669     4.35%     2.36%      0.32us
50.74us      0.76us ( +-   0.32% )
  EXTERNAL_INTERRUPT      29809     2.90%     6.42%      0.66us
70.75us      3.10us ( +-   0.54% )
              VMCALL      17819     1.73%     5.21%      0.75us
15.64us      4.20us ( +-   0.33%

Total Samples:1027227, Total events handled time:1436343.94us.
===============================

The result shows the overhead increased.  enable the apicv can help to
reduce the vm-exit
caused by interrupt injection, but on the other side, there are a lot
of vm-exit caused by APIC_EOI.

When turning off the hyper-v and using the kvm apicv, there is no such
overhead. It seems turning
on hyper V related features is not always the best choice for a windows guest.

Thanks!
Liang