MATLAB: Is SUM not multithreaded on a dual core machine in MATLAB 7.8 (R2009a)

Question

The MEAN function is multithreaded via SUM. However, executing MEAN in multithreading mode performs similar to single threading mode.

>> x = rand(1000);
>> tic; mean(mean(x)); toc
Elapsed time is 0.002357 seconds.

Using ‘-singleCompThread’:

>> x = rand(1000);
>> tic; mean(mean(x)); toc
Elapsed time is 0.002660 seconds.

Answer 1

SUM is memory bound. The issue directly relates to throughput on the CPU(s). The following results illustrate this:

1. Two cores share one BUS

>> x = rand(1000);
>> tic; mean(mean(x)); toc
Elapsed time is 0.002357 seconds.

2. Two cores with two BUSES

>> x = rand(1000);
>> tic; mean(mean(x)); toc
Elapsed time is 0.001802 seconds.

3. Using ‘-singleCompThread’:

>> x = rand(1000);
>> tic; mean(mean(x)); toc
Elapsed time is 0.002660 seconds.