Why is sys time 0 here?

Question

When I do $(time sleep 1), I got:

real    0m1.001s
user    0m0.000s
sys     0m0.000s

I'm not sure why sys time was 0, so nanosleep is called from userspace ?

Also, does the real time equals to context switch time + real sleep time (1s) ?

Answer 1

Looking at the man page for time, we can see that,

1. real : Length of time the program took to terminate, calculated as :

   (time at end of program - time at star of program)

2. user : The sum of the tms_utime and tms_cutime values in a struct tms as returned by times(2)

3. sys : The sum of the tms_stime and tms_cstime values in a struct tms as returned by times(2)

So the result you are getting effectively means that your program lasted for 1 second and in that 1 second it used 0 seconds as user time and 0 seconds as system time. Which is exactly what we should expect from the program.

When a program is sleeping it just means the kernel will not schedule it for processing, it won't even run no-ops, it just wont run so no system time is consumed.

Answer 2

Because the process is not executing kernel-space code while it is sleeping. The kernel is executing other processes, or idling, neither of which is influenced by the sleeping process.