Intel BX80623I52500K Specification page 72

15.10.9 COUNTING CLOCKS
The count of cycles, also known as clockticks, forms a basis for measuring how long a
program takes to execute. Clockticks are also used as part of efficiency ratios like
cycles per instruction (CPI). Processor clocks may stop ticking under circumstances
like the following:
• The processor is halted when there is nothing for the CPU to do. For example, the
processor may halt to save power while the computer is servicing an I/O request.
When Hyper-Threading Technology is enabled, both logical processors must be
halted for performance-monitoring counters to be powered down.
• The processor is asleep as a result of being halted or because of a power-
management scheme. There are different levels of sleep. In the some deep sleep
levels, the time-stamp counter stops counting.
There are three ways to count processor clock cycles to monitor performance. These
are:
• Non-halted clockticks — Measures clock cycles in which the specified logical
processor is not halted and is not in any power-saving state. When Hyper-
Threading Technology is enabled, these ticks can be measured on a per-logical-
processor basis.
• Non-sleep clockticks — Measures clock cycles in which the specified physical
processor is not in a sleep mode or in a power-saving state. These ticks cannot
be measured on a logical-processor basis.
• Time-stamp counter — Some processor models permit clock cycles to be
measured when the physical processor is not in deep sleep (by using the time-
stamp counter and the RDTSC instruction). Note that such ticks cannot be
measured on a per-logical-processor basis. See Section 10.8 for detail on
processor capabilities.
The first two methods use performance counters and can be set up to cause an
interrupt upon overflow (for sampling). They may also be useful where it is easier for
a tool to read a performance counter than to use a time stamp counter (the
timestamp counter is accessed using the RDTSC instruction).
For applications with a significant amount of I/O, there are two ratios of interest:
• Non-halted CPI — Non-halted clockticks/instructions retired measures the CPI
for phases where the CPU was being used. This ratio can be measured on a
logical-processor basis when Hyper-Threading Technology is enabled.
• Nominal CPI — Time-stamp counter ticks/instructions retired measures the CPI
over the duration of a program, including those periods when the machine halts
while waiting for I/O.
15.10.9.3 Incrementing the Time-Stamp Counter
The time-stamp counter increments when the clock signal on the system bus is active
and when the sleep pin is not asserted. The counter value can be read with the
RDTSC instruction.
The time-stamp counter and the non-sleep clockticks count may not agree in all
cases and for all processors. See Section 10.8 for more information on counter
operation.
72
Specification Clarifications
Specification Update