# Improving performance of parallel code sections

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A.B. on 31 Jan 2024
Edited: Matt J on 1 Feb 2024
Consider the following test code which benchmarks different methods of parallelization within MATLAB:
function partest()
delete(gcp("nocreate"));
njob = pool.NumWorkers;
ndim = 10000000;
state = rand(ndim, njob);
dummy = 0;
% parfor
tic
func = zeros(njob, 1);
parfor ijob = 1 : njob
func(ijob) = getFunc(state(:, ijob));
end
disp(" parfor = " + toc + " seconds.")
dummy = dummy + sum(func);
% spmd
tic
func = zeros(njob, 1);
spmd
indx = spmdIndex;
func(indx) = getFunc(state(:, indx));
end
disp(" spmd = " + toc + " seconds.")
%dummy = dummy + sum([func{:}]);
% parfeval
tic
clear func
fout(1 : njob) = parallel.FevalFuture;
for ijob = 1 : njob
fout(ijob) = parfeval(pool, @getFunc, 1, state(:, ijob));
end
func = fetchOutputs(fout);
disp("parfeval = " + toc + " seconds.")
dummy = dummy + sum(func);
% serial
tic
func = zeros(njob, 1);
for ijob = 1 : njob
func(ijob) = getFunc(state(:, ijob));
end
disp(" serial = " + toc + " seconds.")
dummy = dummy + sum(func);
disp(dummy)
delete(gcp("nocreate"));
end
function func = getFunc(state)
%func = -sum(state.^2);
func = 0;
for idim = 1 : length(state)
func = func - state(idim)^2;
end
end
Here is the benchmark output:
parfor = 0.092802 seconds.
spmd = 0.044583 seconds.
parfeval = 0.032457 seconds.
serial = 0.050159 seconds.
Firstly, why is parfeval outperforming all others?
Secondly, are there anything that could be done to any of these parallel constructs to improve their performance against the last (serial) case?
Edric Ellis on 1 Feb 2024
• What OS are you using?
• Which release of MATLAB?
• How many physical cores do you have - what's the result of maxNumCompThreads.
A.B. on 1 Feb 2024
WSL, 2023b, 12 per cpu, 12

Matt J on 31 Jan 2024
Edited: Matt J on 31 Jan 2024
The task is too small for parallelization to have any meaningful effect. Similarly, the relative performance numbers for the different methods is not meaningful. You need a task that is at least 30 seconds long serially for any of the comparisons to make any sense.
A.B. on 1 Feb 2024
No, virtually. That was the response to the strangeness issue I mentioned and you disregaded as normal. The real issue which prompted this post is the performance difference between the constructs, which of course diminishes with workload size. I just moved on with what's pragmatic. I'd appreciate your explanation of the phenomenon if you have any.
Matt J on 1 Feb 2024
Edited: Matt J on 1 Feb 2024
What about my earlier comment about having more than 1 loop iteration per worker? It makes intuitive sense to me that parFeval would be optimal when you have only 1 function call per worker. Otherwise, if parfor were optimal for everything, why would they provide parFeval?

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