Discrepancy in Linux time command output

Question

I am aware that the output of the time command can show greater time under the user section than the real section for multi-processor cases, but recently, I was trying to profile a program when I saw that real was substantially greater than user + sys.

$ time ./test.o

real    0m5.576s
user    0m1.270s
sys     0m0.540s

Can anybody explain why such a behaviour is caused?

Answer 1

That's the normal behavior.

"Real" is is the wall-clock time. In your example, it literally took 5.576 seconds to run './test.o'

'user' is the User CPU time, or (roughly) CPU time used by user-space processes. This is essentially the time your CPU spent actually executing './test.o'. 1.270 seconds.

And finally, 'sys' is System CPU time, or (roughly) CPU time used by your kernel. 0.540 seconds.

If you add sys + user, you get the amount of time your CPU had to spend executing the program.

real - (user + sys) is, then, the time spent not running your program. 3.766 seconds were spent between invocation and termination not running your program--probably waiting for the CPU to finish running other programs, waiting on disk I/O, etc.

Answer 2

Time your process spends sleeping (e.g., waiting for I/O) is not counted by either "user" or "system", but "real" time still elapses.

Try:

time cat

...then wait 10 seconds and hit ctrl-D.