Is user + sys time equal to real/wall clock time?

Question

In Perl:

my ($usr,$sys,$cusr,$csys) = times;
printf "real %s, user %s, sys %s\n", $usr + $sys, $usr, $sys;

Is this true? I read some other questions where people did things like saving the current time away in a variable and collecting it later, but why wouldn't this work?

Answer 1

No, it is not. User and system time account only for used CPU time. While the program is asleep, real time still passes, but the user and system time stand still. Also, on multi-CPU systems, it may well happen that user+system is greater than real time.

Answer 2

C:\Temp> cat tt.pl
sleep 5;
my ($user, $system) = times;
print "u = $user s = $system\n";


C:\Temp> tt
u = 0.015 s = 0.015