MATLAB: Why is mex parfor slower them mex for

MATLABmatlab codermex parforParallel Computing Toolbox

I am starting to work with the Parallel Computing Toolbox, and just constructed an FIR filter example to compare for and parfor
coefs =  [-0.00393617608745112 -5.95945405003999e-05...] length 1x10498
values = [30.3750000000000 30.3760000000000...] length 1x131000
tic; 
outVal = FIRMP(coefs,values);  
%outVal = FIRMP_mex(coefs,values);  
time = toc;
with function FIRMP
function [result] = FIRMP(coefs, values)
  coefLen = length(coefs);
  valLen = length(values);
  result = zeros(size(values));
  (par)for I = 1 : valLen - coefLen;
    suma = 0;
    for J = 1 : coefLen
      suma = suma + coefs(J)*values(I + J);
    end
    result(I) = suma;
  end
end
I used 4 threads and got this results
for   : time= 13.5s
parfor: time = 5.5s
It is OK, but if I create C++ mex (matlab CODER) and run again, the result has changed
for   : time = 3.1s
parfor: time = 4.3s
why is the 'parfor' in C++ mex slower than 'for'?

Best Answer

When I try your example on Linux (Debian 9) using GCC I see a good speedup with parfor in generated MEX:
for    : time = 1.3s
parfor : time = 0.4s
On Windows 10 using Microsoft Visual Studio 2017, I see a much more modest speedup:
for    : time = 1.3s
parfor : time = 1.0s
What compiler and OS are you using?
One thing that may be happening for certain compilers is that each of the parfor loop iterations are very fast. When this is the case, the overhead of managing threads can dominate the loop execution time. This can ruin any possible parallelism gains.
The Coder documentation covers this in some detail:
https://www.mathworks.com/help/coder/ug/acceleration-of-matlab-algorithms-using-parallel-for-loops-parfor.html#btfitu2-2
as does the MATLAB parfor documentation:
https://www.mathworks.com/help/distcomp/decide-when-to-use-parfor.html
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