Why is CLOCKS_PER_SEC not the actual number of clocks per second?
I have just written this short C++ program to approximate the actual number of clock ticks per second.
#include
#include
usi
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From the man page of
clock(3):POSIX requires that CLOCKS_PER_SEC equals 1000000 independent of the actual resolution.
Your implementation seems to follow POSIX at least in that respect.
Running your program here, I get
Actual clocks per second = 980000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 1000000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 990000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 1000000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 1000000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 1000000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 1000000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 1000000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 1000000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 1000000 CLOCKS_PER_SEC = 1000000or similar output on an idle machine, and output like
Actual clocks per second = 50000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 600000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 530000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 580000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 730000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 730000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 600000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 560000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 600000 CLOCKS_PER_SEC = 1000000 Actual clocks per second = 620000 CLOCKS_PER_SEC = 1000000on a busy machine. Since
clock()measures the (approximate) time spent in your program, it seems that you tested on a busy machine, and your program got only about 60% of the CPU time.
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