Hi Jan,
Thanks for the reply.
On 2015/2/2 18:12, Jan Beulich wrote:
>>> On 31.01.15 at 11:29, <john.liuqim...@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.
.
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