How can I tell if my threaded or OpenMP program used multiple cpus?
Run the program with /usr/bin/time and compare the user and elapsed time reported by /usr/bin/time and note also the %CPU [comparing cputime and wall clock time from “qhist jobid” would also work]. Here’s sample output from /usr/bin/time for a job that requested 4 cpus and ran with 4 threads. The batch script contained a line like this: /usr/bin/time a.out The stderr file contained this output from /usr/bin/time: 176.04user 0.28system 0:44.93elapsed 392%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+4569minor)pagefaults 0swaps Note that 176.04user/44.93elapsed=3.918 which corresponds to the 392 %CPU. The sample program used 392/400 or about 98% of its 4 cpus. Qhist for the job showed: … Usage: wall clock: 00:00:56 cputime: 00:02:59 That works out to be (2min*60sec+59sec)/56sec = 3.20 . The wall clock time for a short job contains some batch system overhead, so the ratio is a little smaller than it should be for the test job, but the qhist data confirm that >3 cpus wer
