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General Discussions

HelloGeek
Journeyman III
a week ago

The Op Cache miss ratio is 100% in VM with SMT enabled

Hi everyone, I deployed a Windows VM on a Linux host using KVM, and the serve CPU model is AMD EPYC 9654. Then I met the following problem.

I used some CPU benchmark tools to test the performance of CPU, and I found that after enabling SMT, the single-thread performance of CPU decreased significantly.

I confirmed that there was almost no SMT contention (I turned off the SMT sibling core when testing). In addition, I also found some strange phenomena:

  • The single-thread CPU performance is not changed in host whether SMT is enabled or not;
  • The single-thread CPU performance is not changed in Linux VM whether SMT is enabled or not.

I used perf tool to analyze the reason (perf kvm stat command), and found the op_cache_hit_miss ratio reached 100% only in SMT+Windows VM.

I also confirmed the bit 5 MSR 0xc0011021 register is 0, so the opcache is not closed all the time.

Does anyone know what the reason is?

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1 Reply
elstaci
MVP
Friday

AMD used to have a Server Processor AMD Forum for all EPYC processors and programs like uProf.

 

I was going to mention to copy this thread to that forum but I see that it is missing from the Communities but it is only listed under "BLOGS" which isn't a Forum to upload threads too.

 

AMD Forums is in the process of changing its AMD Forum Format so need to wait to see if AMD Forum's Server Processor Forum is again enabled in the new AMD format.

elstaci
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