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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

> 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


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