![]() On Linux, this can run purely in user-space. Normal std::chrono implementations will use an OS-provided function like POSIX clock_gettime. that on older CPUs it actually did count core clock cycles, and didn't tick during sleep states, so was less useful for wall-clock time.) (This is the counter you get from rdtsc, or _rdtsc() in C/C++, see this for more details, e.g. (I actually don't know the details when I wrote this I thought everything was just based on rdtsc but I don't think that's correct.) If networking is available, often NTP is used to correct the scale factors to keep the system time correct.įine-grained timing comes from a fixed-frequency counter that counts "reference cycles" regardless of turbo, power-saving, or clock-stopped idle. On a modern x86, the timesource used by the kernel (internally and for clock_gettime and other system calls) is typically a timer interrupt, or hardware timer (e.g. Question 3: Is the "cycle count" measured by cpu and gpus varying depending on the hardware frequency? If yes, then how std::chrono deal with it? If not, what does a cycle correspond to (like what is the "fundamental" time)? Is there a way to access the conversion at compile-time? Is there a way to access the conversion at runtime?Ĭounting cycles, yes, but cycles of what? In that case, it would mean that the number of cycles would vary during a second. Some hardware have varying frequencies (for example idle mode, and turbo modes). If that is the case, because the way a computer count cycles will never be as precise as an atomic clock, it means that a "second" ( period = std::ratio) for a computer can be actually shorter or bigger than an actual second, causing differences in the long run for time measurements between the computer clock and let's say GPS. Question 1: Does a cpu or a gpu has any other way to evaluate time than by counting cycles? At the same time, I guess the only way a cpu can evaluate time, is by counting cycles. Std::chrono offer several clocks to measure times. ![]()
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