Описание тега rdtsc

RDTSC is the x86 read time stamp counter instruction often used for high resolution timing.

See How to Benchmark Code Execution Times on Intel® IA-32 and IA-64 Instruction Set Architectures.

Get CPU cycle count? has info on various caveats of using it: on modern x86, it measures reference cycles, not actual core clock cycles. (And also shows how to access it from C++.)

The earliest CPUs to support RDTSC had fixed clock frequency, and some OSes found it was more useful as a low-overhead time source time-of-day functions, so CPU vendors eventually changed it to be how it is now: a fixed-frequency nonstop counter.

It can be out-of-sync across different cores. (Some CPUs avoid that for cores in the same physical package.)