Re: [RFC PATCH 0/3] generic hypercall support

["David S. Ahern" <daahern@cisco.com>]

Avi Kivity wrote:
> David S. Ahern wrote:
>> I ran another test case with SMT disabled, and while I was at it
>> converted TSC delta to operations/sec. The results without SMT are
>> confusing -- to me anyways. I'm hoping someone can explain it.
>> Basically, using a count of 10,000,000 (per your web page) with SMT
>> disabled the guest detected a soft lockup on the CPU. So, I dropped the
>> count down to 1,000,000. So, for 1e6 iterations:
>>
>> without SMT, with EPT:
>>     HC:   259,455 ops/sec
>>     PIO:  226,937 ops/sec
>>     MMIO: 113,180 ops/sec
>>
>> without SMT, without EPT:
>>     HC:   274,825 ops/sec
>>     PIO:  247,910 ops/sec
>>     MMIO: 111,535 ops/sec
>>
>> Converting the prior TSC deltas:
>>
>> with SMT, with EPT:
>>     HC:    994,655 ops/sec
>>     PIO:   875,116 ops/sec
>>     MMIO:  439,738 ops/sec
>>
>> with SMT, without EPT:
>>     HC:    994,304 ops/sec
>>     PIO:   903,057 ops/sec
>>     MMIO:  423,244 ops/sec
>>
>> Running the tests repeatedly I did notice a fair variability (as much as
>> -10% down from these numbers).
>>
>> Also, just to make sure I converted the delta to ops/sec, the formula I
>> used was cpu_freq / dTSC * count = operations/sec
>>
>>   
> 
> The only think I can think of is cpu frequency scaling lying about the
> cpu frequency.  Really the test needs to use time and not the time stamp
> counter.
> 
> Are the results expressed in cycles/op more reasonable?
> 

Power settings seem to be the root cause. With this HP server the SMT
mode must be disabling or overriding a power setting that is enabled in
the bios. I found one power-based knob that gets non-SMT performance
close to SMT numbers. Not very intuitive that SMT/non-SMT can differ so
dramatically.

david