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Comment by iamcreasy | Hacker News Reader

Can someone explain how to read the readings from Instruction Latency(InstLatX86).

For example how to read the following line,

  201 X86   :INC r8                      L:  13.36ns=  1.0c  T:   6.68ns=  0.50c

This instruction is incrementing the value of r8 general purpose register. But I am not sure how to parse L: and T: section.

Also, does this number change proportionally if it is in low power mode or if the CPU is undervolted?

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L is the latency of the instruction, so 1 clock cycle after the INC r8 instruction another instruction can operate on the result in r8. T is the reciprocal throughput, meaning in this case that the CPU can sustain 2 INC instructions per clock cycle, if there are no data dependencies of course.

In general these numbers will stay the same when counted in clock cycles, regardless of frequency.

To add: with pipelined processors, it doesn’t make sense to list “clocks per instruction” as a single number. Since the advent of superscalar processors, it makes even less sense. Hence why there’s “latency” and “throughput”.

There are some explanation in Comment page:

  In this dump latency means the time that it takes for the next dependent same-type instruction to start. Throughput means the time that it takes for the next independent same-type instruction to start:

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