Inaccuracy of 'time' bash command

macdojo · 04-03-2008, 02:17 PM

Hi,

I am using 'time' command to see how long a process takes to complete. The command displays 'real', 'user', and 'sys'. The value of 'real' is always greater than the sum 'user' and 'sys'. My question is how can the difference can be explained. In the man page of 'time' under "accuracy", it's said that:

"The elapsed time is not collected atomically with the execution of
the program; as a result, in bizarre circumstances (if the `time'
command gets stopped or swapped out in between when the program being
timed exits and when `time' calculates how long it took to run), it
could be much larger than the actual execution time."

This kinda explains but it's said to happen in only 'bizarre' circumstances. then how do you account for the difference under 'normal' circumstances since 'real' is ALWAYS greater than the sum of the other two?

jailbait · 04-03-2008, 02:27 PM

Quote:

Originally Posted by macdojo

Hi,

how do you account for the difference under 'normal' circumstances since 'real' is ALWAYS greater than the sum of the other two?

Real is clock time. user and sys are CPU time. Under normal circumstances such as a file copy the CPU is only used a fraction of clock time.

--------------------
Steve Stites

macdojo · 04-03-2008, 03:40 PM

Let me see if I come to a correct conclusion from your provided facts.

1 cpu second is still 1 real second and real is clock time and user/sys are cpu time. real is the time for the process being timed to enter and exit but during which there are other higher priorty, may or may not be related, processes that run and real account for them in its value. user/sys are the exact time consumed by the CPU to carry out the process being timed.

Is this along the line of what's going on with 'time' command?

Thanks!

chrism01 · 04-03-2008, 06:59 PM

Very close. It's more like
cpu = time spent in cpu
user = time spent in user mode (ie not inc kernel work)
time = how long it took between pressing enter and completion, which, as you pointed out also includes time spent swapped out whilst other progs run.
you can also try using the cmd
\time prog
as this is a GNU variation that gives more info.

zail · 05-29-2008, 07:48 AM

Quote:

Originally Posted by macdojo

then how do you account for the difference under 'normal' circumstances since 'real' is ALWAYS greater than the sum of the other two?

Don't forget about SMP systems. In some cases 'real' stat can be smaller than 'usr'+'sys'.

This can be helpful:
http://www.ncsa.uiuc.edu/UserInfo/Re...gd/2365c62.htm