Intel® 64 and IA-32 Architectures Optimization Reference Manual

USING PERFORMANCE MONITORING EVENTSB.1.2Counting ClocksThe count of cycles (known as clock ticks) forms a fundamental basis for measuringhow long a program takes to execute. The count is also used as part of efficiencyratios like cycles-per-instruction (CPI). Some processor clocks may stop tickingunder certain circumstances:• The processor is halted (for example: during I/O). There may be nothing for theCPU to do while servicing a disk read request and the processor may halt to savepower. When HT Technology is enabled, both logical processors must be haltedfor performance-monitoring-related counters to be powered down.• The processor is asleep, either as a result of being halted for a while or as part ofa power-management scheme. There are different levels of sleep. In the deepersleep levels, the time-stamp counter stops counting.Three mechanisms to count processor clock cycles for monitoring performance are:• Non-Halted Clock Ticks — Clocks when the specified logical processor is nothalted nor in any power-saving states. These can be measured on a per-logicalprocessorbasis, when HT Technology is enabled.• Non-Sleep Clock Ticks — Clocks when the physical processor is not in any ofthe sleep modes, nor power-saving states. These cannot be measured on a perlogical-processor basis.• Time-stamp Counter — Clocks when the physical processor is not in deepsleep. These cannot be measured on a per-logical-processor basis.The first two metrics use performance counters and can cause an interrupt uponoverflow for sampling. They may also be useful for cases where it is easier for a toolto read a performance counter instead of the time-stamp counter. The time-stampcounter is accessed using an RDTSC instruction.For applications with a significant amount of I/O, there are two ratios of interest:• Non-Halted CPI — Non-halted clock ticks/instructions retired measures the CPIfor the phases where the CPU was being used. This ratio can be measured on aper- logical-processor basis, when HT Technology is enabled.• Nominal CPI — Time-stamp counter ticks/instructions retired measures the CPIover the entire duration of the program, including those periods the machine ishalted while waiting for I/O.The distinction between the two CPI is important for processors that support HTTechnology. Non-halted CPI should use the “non-halted clock ticks” performancemetric in the numerator. Nominal CPI should use “non-sleep clock ticks” in thenumerator. “non-sleep clock ticks” is the same as the “clock ticks” metric in previouseditions of this manual.B-3