Does the Intel RDTSC instruction (or equivalent) count clocks only in your task, or globally?

Question

Related to earlier question: making mistake in inline assembler in gcc

in the RDTSC or similar instruction, is the count only for your process, or is it a global count? If there is a context switch, is the only inaccuracy the cost of the context switch itself, or is the entire time the CPU is on a different job counted?

score 2 · Accepted Answer · answered Dec 17 '10 at 18:31

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The count is for the whole system (including your running process and every other process). The instruction counts the number of ticks occurred from reset.

answered Dec 17 '10 at 18:31

gusbro

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so is there any standard library to catch the context switch and save the clock number in and out? – Dov Dec 17 '10 at 18:35
You'd have to modify the kernel. – wallyk Dec 17 '10 at 18:38

score 1 · Answer 2 · answered Dec 17 '10 at 18:42

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It counts instructions per core(hence why you shouldn't use it for timing on a multicore system). The intel developer guides should be your first stop when a query arises.

answered Dec 17 '10 at 18:42

Necrolis

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Actually on newer CPUs it is synchronized. There's a CPUID bit that tells you (you can find the details in the Linux implementation of the VDSO for gettimeofday). – Ben Jackson Dec 17 '10 at 18:45
True, however, if the cpu doesn't, your sol, which is why you should stick with higher level api's, like microsofts performance counter – Necrolis Dec 17 '10 at 18:53

Does the Intel RDTSC instruction (or equivalent) count clocks only in your task, or globally?

2 Answers2