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Why CPU_CLK_UNHALTED.REF_TSC is lower than CPU_CLK_UNHALTED.THREAD on CPU without Turbo

Alexander_Alexeev
407 Views

Hello 

Almost whole my question is in subject. Question is simple and straight forward.

Why reference frequency might be significantly lower ( ~13% ) than thread frequency on CPU with SKU without Turbo.

E.g.

Package               CPU_CLK_UNHALTED.REF_TSC  CPU_CLK_UNHALTED.THREAD

package_0          350,845,000,000                398,688,400,000

CPU: Intel(R) Xeon(R) CPU E5-2407 0 @ 2.20GHz

SMT (Hyper-threading) is disabled

My ideas are limited to two cases, either something wrong with VTune or PMU. Are there any other possibilities?

Thanks,

Alexander

 

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7 Replies
Bernard
Black Belt
407 Views

If I understood it properly  CPU_CLK_UNHALTED.THREAD could have spent less time in halt state hence its counter value is higher.

Partial info is available  in this thread https://software.intel.com/en-us/forums/topic/285232

Alexander_Alexeev
407 Views

Both counters are collected for the same conditions, so from my point of view it would be strange if one counter would observe by 13% more unhalted state than another.

Collection is run with target type "profile system" so it global and includes execution of whole SW stack.

 

Patrick_F_Intel1
Employee
407 Views
Hello Alexander, I assume you are collecting both clocktick events simultaneously. Is this true? If so, then it looks like turbo is not truly disabled. How are you disabling turbo mode? Pat
Alexander_Alexeev
407 Views

Accordingly to this page http://ark.intel.com/products/64614  Turbo Boost isn't available for the SKU E5-2407 (Sandy Bridge). There is nothing to disable.

Yes, counters were collected simultaneously with VTune 2013 U14

Alexander_Alexeev
407 Views

I have to apologize! SKU was incorrectly identified. The real one is   E5-2430  that has Turbo.

Thank you for your comments.

 

Patrick_F_Intel1
Employee
407 Views
What is the line from the movie? "What are you going to believe, me or your lying eyes?" ... this is what the guy says to his girlfriend after the girl catches the boyfriend cheating... Not sure that the line really applies here but I've always liked the line anyway. In any case, from the counters, I'd say it looks like your system DOES support turbo. Can you check in your bios to see if there is a cpu option to enable/disable turbo? And if so, can you disable turbo and rerun the measurement? Pat
Bernard
Black Belt
407 Views

Probably as Patrick said Turbo is enabled and you are seeing CPU_CLK_UNHALTED.THREAD  variation as a function of current load.

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