How to measure the ACTUAL number of clock cycles elapsed on modern x86?

Question

On recent x86, RDTSC returns some pseudo-counter that measures time instead of clock cycles.

Given this, how do I measure actual clock cycles for the current thread/program?

Platform-wise, I prefer Windows, but a Linux answer works too.

Answer 1

This is not simple. Such a thing is described in the Intel® 64 and IA-32 Architectures Developer's Manual: Vol. 3B:

Here is the behaviour:

• For Pentium M processors; for Pentium 4 processors, Intel Xeon processors; and for P6 family processors: the time-stamp counter increments with every internal processor clock cycle. The internal processor clock cycle is determined by the current core-clock to bus-clock ratio. Intel® SpeedStep® technology transitions may also impact the processor clock.
• For Pentium 4 processors, Intel Xeon processors; for Intel Core Solo and Intel Core Duo processors; for the Intel Xeon processor 5100 series and Intel Core 2 Duo processors; for Intel Core 2 and Intel Xeon processors; for Intel Atom processors: the time-stamp counter increments at a constant rate. That rate may be set by the maximum core-clock to bus-clock ratio of the processor or may be set by the maximum resolved frequency at which the processor is booted. The maximum resolved frequency may differ from the processor base frequency. On certain processors, the TSC frequency may not be the same as the frequency in the brand string.

Here is the advise for your use-case:

To determine average processor clock frequency, Intel recommends the use of performance monitoring logic to count processor core clocks over the period of time for which the average is required. See Section 18.17, “Counting Clocks on systems with Intel Hyper-Threading Technology in Processors Based on Intel NetBurst® Microarchitecture,” and Chapter 19, “Performance- Monitoring Events,” for more information.

The bad news is that AFAIK performance counters are often not portable between AMD and Intel processors. Thus, you certainly need to check which performance counters to use in the AMD documentation. There are also complications: you cannot easily measure the number of of cycle taken by any arbitrary code. For example, the processor can be halted or enter in sleep mode for a short period of time (see C-state) or the OS can executing some protected code that cannot be profiled without high privileges (for sake of security). This method is fine as long as you need to measure the number of cycle of a numerically-intensive code taking relatively-long time (at least several dozens of cycles). On top of all of that, the documentation and usage of MSR is pretty complex and it has some restrictions.

Performance counters like CPU_CLK_UNHALTED.THREAD and CPU_CLK_UNHALTED.REF_TSC seems a good start for what you want to measure. Using library to read such performance counter is generally a very good idea (unless you like having a headache for at least few days). PAPI might be enough to do the job for this.

---

Here is some interesting related posts:

• Lost Cycles on Intel? An inconsistency between rdtsc and CPU_CLK_UNHALTED.REF_TSC
• How to read performance counters by rdpmc instruction?