Computing time on Linux: granularity and precision

Question

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I am using different kind of clocks to get the time on Linux systems:

rdtsc, gettim         


        

Answer 1

• Precision is the amount of information, i.e. the number of significant digits you report. (E.g. I am 2 m, 1.8 m, 1.83 m, and 1.8322 m tall. All those measurements are accurate, but increasingly precise.)

• Accuracy is the relation between the reported information and the truth. (E.g. "I'm 1.70 m tall" is more precise than "1.8 m", but not actually accurate.)

• Granularity or resolution are about the smallest time interval that the timer can measure. For example, if you have 1 ms granularity, there's little point reporting the result with nanosecond precision, since it cannot possibly be accurate to that level of precision.

On Linux, the available timers with increasing granularity are:

• clock() from <time.h> (20 ms or 10 ms resolution?)

• gettimeofday() from Posix <sys/time.h> (microseconds)

• clock_gettime() on Posix (nanoseconds?)

In C++, the <chrono> header offers a certain amount of abstraction around this, and std::high_resolution_clock attempts to give you the best possible clock.