Re: apic-v reduce network performance in my test case

["Liuqiming (John)" <john.liuqiming@huawei.com>]

Hi Jan,
Thanks for the reply.

On 2015/2/2 18:12, Jan Beulich wrote:
> >>> On 31.01.15 at 11:29, <john.liuqiming@huawei.com> wrote:
> >       Recently I met an odd performance problem: when I turn on APIC
> > Virtualization feature (apicv=1), the network performance of a windows
> > guest become worse.
> >
> >       My test case like this: host only have one windows 2008 R2 HVM
> > guest running,and this guest has a SR-IOV VF network passthrough to it.
> > Guest using this network access a NAS device. No fontend or backend of
> > network and storage, all data transfered through network.
> >
> >       The xentrace data shows: the mainly difference between apicv and
> > non-apicv, is the way guest write apic registers, and
> > EXIT_REASON_MSR_WRITE vmexit cost much more time than
> > EXIT_REASON_APIC_WRITE, but when using WRMSR, the PAUSE vmexit is much
> > less than using APIC-v.
>
> There being heavier use of the pause VMEXIT doesn't by itself say
> anything, I'm afraid. It may suggest that you have a C-state exit
> latency problem - try lowering the maximum C-state allowed, or
> disabling use of C-states altogether.
Sorry, I forgot to mention  my test scenario:
Its a video test suite,I am not sure what the logic inside the tools exactly (not opensource tool).
The basic flow is:
      1) test suite start several thread to read video file from disk (from NAS through network in my case)
      2) decode these video data as a frame one by one
      3) if  any frame delay more than 40ms, then mark as lost

test result:
        apicv=1,  there can be 15 thread running at the same time without lost frame
        apicv=0,  there can be 22 thread running at the same time without lost frame

so when I'm saying apicv reduce the performance, I got the conclusion from the test result not from what xentrace shows.
>
> > In commit 7f2e992b824ec62a2818e64390ac2ccfbd74e6b7
> > "VMX/Viridian: suppress MSR-based APIC suggestion when having APIC-V",
> > msr based apic is disabled when apic-v is on, I wonder can they co-exist
> > in some way? seems for windows guest msr-based apic has better performance.
>
> The whole purpose is to avoid the costly MSR access exits. Why
> would you want to reintroduce that overhead?
>
> Jan
>
I agree to avoid the MSR access vmexit by using apicv, I just do not know what's the side effect.
Because from the test result,  apicv replacing  msr-based access brings performance reduction.
>
> .
>