Intel PCM - What does nominal CPU mean of?

[GHui]

EXEC is instructions per nominal CPU cycle.
IPC is instructions per CPU cycle.
What does nominal CPU mean of?

[Roman_D_Intel]

GHui,

EXEC = number of retired instructions / number of nominal CPU cycles

the number of nominal CPU cycles during a time period is defined by the nominal frequency on the processor, which is advertized in the product. For example if you have an Intel Core i5 3.3 GHz processor, then during a second you have 3.300.000.000 nominal cycles.

However, the processor can be idle most of the time during this second such that the clock signal is not running (in apower saving state). Therefore to adjust to this fact that processor was in power saving state it makes sense to have a differentmetric that counts only time (cycles) with active clock signal. Thus the IPC metric divides the number of retired instructions by the number of active clockcycles.

Roman

[SergeyKostrov]

Hi Roman,

Quoting Roman Dementiev (Intel)

...
For example if you have an Intel Core i5 3.3 GHz processor, then during a second you have 3.300.000.000 nominal cycles.

Is it what RDTSC instruction returns? Correct?

Best regards,
Sergey

[Roman_D_Intel]

Hi Sergey,

yes, on this processor thevalue returned by theRDTSC instruction runs at a constant invariant rate defined by thenominal core frequency.

Roman

[Patrick_F_Intel1]

To add a little more info to Roman's response.
You can read more about the rdtsc, rdtscp instruction behavior in the SDM instruction manual vol 2 and the SDM, vol 3,section 17.12, October 2011.
Pat

[SergeyKostrov]

Thank you for the explanation.

Best regards,
Sergey

[SergeyKostrov]

Hi Patrick,

A link to Intel's manuals that Iused for a long time is broken now:

http://developer.intel.com/products/processor/manuals/index.htm

I just found another one:

http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html?wapkw=Manuals

Iwould like to suggest to create a Sticky Post inthe "Software Tuning, Performance Optimization, ..."
forumwith the most useful information, like links tomanuals, references, sources, articles,etc.

Best regards,
Sergey

[bobjc]

thanks for sharing this post and thanks to all for giving explainatios and suggestion, its really good...!