MATLAB: Cputime not working as expected

Question

It seems like cputime does not work as advertised on Mac OSX 10.11.6 and Matlab 2016b or Windows 7 Matlab 2016a. cputime should be in seconds, but running this command, then waiting for many seconds, and then running it again. Here's some example code, comparing it to tic/toc. The numbers aren't even close …

t1 = tic; t2 = cputime; pause(5); t3 = cputime-t2; t4 = toc(t1); disp(t3); disp(t4);

I don't use cputime() normally, but I was exploring another issue with a mismatch between tic/toc and some internally timed c code (will post as another issue) and I wanted to see how cputime compared.

Answer 1

cputime measures the # of clock cycles (summed across all threads) that the Matlab process has used since Matlab started. Units are in seconds. cputime does not measure elapsed "wall time" like tic/toc does. Most often "elapsed" cputime (i.e. the difference between two cputime()) calls is less than wall time, because processors aren't using every clock cycle to process Matlab commands. However, since this is summed across threads, it is possible to have elapsed cputime be larger than wall time, in cases in which multiple threads are executing Matlab commands at the same time.