decrease in CPU for Parallel processing job with time

I am running a longish (~6 hour) simulation using the parallel processing toolbox. The Activity Monitor on my 4-core i7 iMac shows User activity of 75% of available CPU time with a small amount of idle time (~20%) at the start, but over time, the User fraction drops to <20% and the idle time increases to >75%. Can anyone explain this slowdown please?

Thanks,

Andy

[Walter Roberson]

Is it possible that some of your iterations take much less time than others? Especially if those are mostly grouped together in the first third of the iteration range?

[Andy Daubenspeck]

I think all should take equal time--each is a simulation of a system with a different set of parameters.

[Walter Roberson]

Lower usage is expected close to the end, as the load balancing even for tasks of equal time is not perfect unless the number of iterations is an integer multiple of the number of workers.

[Andy Daubenspeck]

That makes sense though with only 4 cores working and 1000s of iterations left--I signal how far through the job along the way-- I would have expected a bit more attention to the task! The job did seem to take about as long as I had expected scaling up from previous, shorter runs, so your explanation makes sense.

Thank you!

[Walter Roberson]

The factor I refer to would typically not be relevant until more than 90 percent was done, except in cases where there were big differences in processing time.