Time gets % CPU time for a task wrong for long jobs

[Richard Earnshaw]

Ubuntu OS version: oneiric
Description: Linaro 12.01
Release: 12.01

Package: time
Priority: standard
Section: utils
Installed-Size: 148
Maintainer: Ubuntu Developers <email address hidden>
Architecture: armel
Version: 1.7-23.1
Depends: libc6 (>= 2.4)

When time is used to measure the resources used for a long job the results returned do not match up; the manual says that the %-age CPU used is the elapsed time divided by the (user+system) time consumed by the job.
The example below shows that for a job that took 34 hours to run the claimed CPU utilization was only 25%, but in reality it should be 95%.

93186.56user 24134.59system 34:07:55elapsed 25%CPU (0avgtext+0avgdata 3268608maxresident)k
0inputs+0outputs (11010major+702900386minor)pagefaults 0swaps
$ expr 93186 + 24134
117320
$ expr 117320 / 36
3258

ie ~32 1/2 hours

Shorter jobs (a few minutes) seem to show the correct %-age, so I suspect an overflow somewhere.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Richard Earnshaw (richard-earnshaw)]

34548.03user 2220.37system 10:26:49elapsed 97%CPU (0avgtext+0avgdata 1651664maxresident)k

Which looks fine.

Interestingly
10 * 3600 is 0x8ca0
while
20 * 3600 is 0x11940

That might suggest the overflow starts to happen at about 18:12:16

[Richard Earnshaw (richard-earnshaw)]

Erm, not quite. If time captures the time in milliseconds, overflow in the percentage calculation will occur after 2^32 ms, which is 11:55:49.67

[Richard Earnshaw (richard-earnshaw)]

Still not quite there. it's 11:55:49.67 is 2^32 / 100 ms