Re: [Xen-devel] apic-v reduce network performance in my test case

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Feb 02, 2015 at 09:58:57PM +0800, Liuqiming (John) wrote:
> Hi Jan,
> Thanks for the reply.
> 
> On 2015/2/2 18:12, Jan Beulich wrote:
> >>>> On 31.01.15 at 11:29, <john.liuqiming@xxxxxxxxxx> 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.

It sounds like it brings latency disruption, not neccessarily performance 
reduction.

What happens if you run with the cpufreq turned to performance, or as Jan 
suggested - 
with disabling C-states?

> >
> >.
> >
> 
> 
> 
> 
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