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aleksr9
Journeyman III

Problem with performance counters on Opteron 6172

Hi,

I've been trying to analyze certain applications with performance counters on a Opteron 6172, running Red Hat Enterprise Linux Workstation release 6.2 (Santiago).

I'm using PAPI v4.1.3.0 which uses the AMD native events CPU_CLK_UNHALTED for counting total cycles and DATA_CACHE_ACCESSES for counting L1 Data cache accesses.

http://support.amd.com/us/Processor_TechDocs/31116.pdf

- CPU_CLK_UNHALTED

The number of clocks that the CPU is not in a halted state (due to STPCLK or a HLT instruction). Note: this

event allows system idle time to be automatically factored out from IPC (or CPI) measurements, providing the

OS halts the CPU when going idle. If the OS goes into an idle loop rather than halting, such calculations are

influenced by the IPC of the idle loop.

- DATA_CACHE_ACCESSES

The number of accesses to the data cache for load and store references. This may include certain microcode

scratchpad accesses, although these are generally rare. Each increment represents an eight-byte access,

although the instruction may only be accessing a portion of that. This event is a speculative event.

The problems I've been experiencing is that the number of cache accesses have been higher than the total number of cycles in some cases. A cache access does not halt the cpu, to my understanding, so it should fit within the total cycles. Also when dividing the total cycles by the clock frequency of the Opteron 6172 I get a pretty accurate estimate of the runtime, which makes me think that the total cycles is ok and the problem has to be with the counting of the data cache accesses.

Any help or reason to why this can occur is greatly appreciated, thanks in advance


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4 Replies
aleksr9
Journeyman III

Re: Problem with performance counters on Opteron 6172

Also it would be nice if I could get some information of exactly which events halt the Opteron 6172 cpu.

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kcarney
Staff
Staff

Re: Problem with performance counters on Opteron 6172

Since you haven't gotten any replies yet to your question on the software developers forum, below are some other resources that you should follow up with to get your question answered.

If you don't get your question answered by either of these 2 resources, will you please let me know?

Thanks!

Kristen

aleksr9
Journeyman III

Re: Problem with performance counters on Opteron 6172

Will do, thanks for the reply!

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rocky67
Journeyman III

Re: Problem with performance counters on Opteron 6172

What happens if the cache access is denied or proves to be higher relative to the cycles in the given cases because it will not stop the computer.

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