Re: [Xen-devel] performace issue when turn on apicv

["Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx>]

Jan Beulich wrote on 2015-06-18:
>>>> On 18.06.15 at 10:02, <john.liuqiming@xxxxxxxxxx> wrote:
>> When using FIO  to test the performance of SSD passthroughed in vm
>> the result show that: When the apicv is on, each
>> EXIT_REASON_MSR_WRITE event spent more time than apicv is off.
>> 
>> Following is the xentrace result:
>> 
>> apicv on:
>> 
>>    VMExitCode                            VMExitReason
> VMExitCnt         VMExitTicks        VMExitTicks/VMExitCnt
>> 
>> 0x0000000001          EXIT_REASON_EXTERNAL_INTERRUPT 270334         
>> 2730912532      10101.99432 0x0000000012                     
>> EXIT_REASON_VMCALL 20              438736      21936.8 0x000000001c    
>>               EXIT_REASON_CR_ACCESS 381340          1096174160     
>> 2874.532333 0x000000001e              EXIT_REASON_IO_INSTRUCTION 413   
>>         32958356      79802.31477 0x0000000020                 
>> EXIT_REASON_MSR_WRITE 111830           818317724      7317.515193
>> 0x000000002d                 EXIT_REASON_EOI_INDUCED 58944          
>> 234914864      3985.390608 0x0000000030              
>> EXIT_REASON_EPT_VIOLATION 10              298368      29836.8
>> 
>> Total 822891          4914014740
>> 
>> apicv off:
>> 
>>    VMExitCode                            VMExitReason
> VMExitCnt         VMExitTicks VMExitTicks/VMExitCnt
>> 
>> 0x0000000001          EXIT_REASON_EXTERNAL_INTERRUPT 237100         
>> 2419717824     10205.47374 0x0000000007          
>> EXIT_REASON_PENDING_VIRT_INTR 792             2324824     2935.383838
>> 0x0000000012                      EXIT_REASON_VMCALL 19             
>> 415168     21850.94737 0x000000001c                  
>> EXIT_REASON_CR_ACCESS 406848          1075393292     2643.231113
>> 0x000000001e              EXIT_REASON_IO_INSTRUCTION 413           
>> 39433068     95479.58354 0x000000001f                   
>> EXIT_REASON_MSR_READ 48              150528    3136 0x0000000020       
>>            EXIT_REASON_MSR_WRITE 229609          1004000084   
>> 4372.651264 0x0000000030               EXIT_REASON_EPT_VIOLATION 10    
>>          249172     24917.2
>> 
>> Total
> 874839             4541683960
> 
> And did you drill into _which_ MSR(s) are requiring this much longer
> to have their writes handled? After all, that's the relevant thing,
> provided the increase of this indeed has anything to do with the
> performance issue you're seeing (the absolute increase of 200M ticks
> there doesn't mean much for the performance effect without knowing what the 
> total execution time was).

IIRC, APICv doesn't impact the WRMSR's behavior (except EOI, self-IPI and TPR). 
Cannot understand why APICv will impact it.

> 
> Apart from that I notice that the EXIT_REASON_EOI_INDUCED handling
> also adds about the same number of ticks...
> 
> Jan


Best regards,
Yang



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