Using symbolic subs call inside parfor loop

Question

Antonio Bilotta 2020-10-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/612301-using-symbolic-subs-call-inside-parfor-loop

评论： Antonio Bilotta 2020-10-14

I have the following pieces of code where the symbolic expressions to be evaluated are quite heavy.

classdef T4
    properties (SetAccess = private)
        % symbolic properties
        xe % vector of symbolic variables
        ge % vector or matrix of symbolic expressions depending on xe
        
        % numeric properties
        se % numeric vector or matrix
        Xe % numeric values to be assigned
    end
    methods
        % all other methods
        function T = computeG(T);
            T.se = double(subs(T.ge,T.xe,T.Xe));
        end
    end
end
nT = 5;
Tv(nT,1) = T4;
for i=1:nT
    Tv(i) = computeG(Tv(i));
end

I do not get any gain, in fact it is worse if I use instead

parfor i=1:nT
    Tv(i) = computeG(Tv(i));
end

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Antonio Bilotta 2020-10-13

Thank you very much for your answer. I don't know how to check on this issue and how to allocate, if necessary, one MuPAD engine for each worker.

Walter Roberson 2020-10-14

My tests show that one mupad kernel process is allocated for each worker. (The kernels use a lot less memory than I might have guessed. They also have about 39 megabytes of shared memory; I am not sure what that is used for.)

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Answer 1

Raymond Norris 2020-10-13

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https://ww2.mathworks.cn/matlabcentral/answers/612301-using-symbolic-subs-call-inside-parfor-loop#answer_512556

Hi Antonio,

You're just not giving each of the workers enough work to do. Maybe you only need to run nT=5, but see what happens when you set it to 100, 200, or 1000. You should then start to see improvements.

Thanks,

Raymond

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Walter Roberson 2020-10-14

My test showed that a separate mupad kernel is launched for each parfor worker, so there would be no need to rewrite the symbolic functions themselves: each worker gets its own (single-threaded) engine.

It is not possible to rewrite the internals of symbolic functions to use parfor / spmd, as the internals of symbolic functions live inside the MuPAD engine, either in the form of binary files are in the form of MuPAD code.

Antonio Bilotta 2020-10-14

在 MATLAB Online 中打开

Maybe the situation is not so plain. I am performing some tests with nT=40. In my code the for loop is called several times and in the case of the normal for I get the following times which change only the first time but then they remain quite stable

In the case of the parfor I get the following times which increase as analysis proceeds (I performed only 3 steps but usually more steps are required).

To be more precise the exact code is not so much different (the class function contains 2 "subs" calls).

function T = computeG(T);
    T.se = double(subs(T.ge,T.xe,T.Xe)); % evaluation of a vector
    T.Ke = double(subs(T.Je,T.xe,T.Xe)); % evaluation of a matrix
end

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