Why do I get different results with parfor and for loops?

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Pegah Hosseini 2012-9-3

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https://ww2.mathworks.cn/matlabcentral/answers/47305-why-do-i-get-different-results-with-parfor-and-for-loops

评论： Pegah Hosseini 2014-5-30

Hi,

I am trying to run a specific code 15 times, each time for a new set of data so my iterations are not sequential. I use SPM toolbox and am trying to speed up the calculations using matlabpool and parfor. The results of parfor loop and for-loop are different even when I just load one dataset instead of 15 datasets i.e. parfor i=1:1. If I close matlabpool then the results are the same. My code looks something like the following. I have not included all the details of defining my structure. This code is exactly the same for for-loop.

I define some parameters in structure variable DCM. The name of the dataset that each parfor iteration should load is saved in DCM.xY.Dfile. Then, DCM is sent to spm_dcm_erp. Inside this function, the dataset whose name is saved in DCM.xY.Dfile is loaded and some parameters are calculated until the model converges which may take from 1 to 64 iterations. Then, some parameters will be added to DCM and at the end of spm_dcm_erp (and inside the function not in parfor loop) the updated DCM will be saved on the disk with the name defined in DCM.name. It is this saved DCM that I check for the two models (with for and parfor) and is different in the 2 conditions. * I even ran parfor for just one of my datasets (instead of i=1:15 I put i=1) and the result is still different from the one from for-loop. * The funny thing is when I run for-loop the model converges at iteration 62 but when I run parfor it converges at iteration 11. * I tried saving the workspace inside my spm_dcm_erp and the parameters of the two conditions (for and parfor) are exactly the same before the for-loop of spm_dcm_erp function starts to calculate the parameters of the model so I made sure that parfor does not change my variable in any way before sending it to the spm_dcm_erp function.

    clc
    clear all
    DCM=[];
    parfor i=1
       spm('defaults','EEG');
       DCM=[];
       % paths to data, etc.
       %-------------------------------------------------------
       Pbase     = pwd;        % directory with your data,
       Pdata     = fullfile(Pbase, '.'); % data directory in Pbase
       Panalysis = fullfile(Pbase, '.'); % analysis directory in Pbase
       % the data (mismatch negativity ERP SPM file from SPM-webpages)
       % -------------------------------------------------------------
       DCM.xY.Dfile = ['mansour_trials_' num2str(i)];
       DCM.name = ['DCM_mansour_trials_' num2str(i) '_10'];
       DCM      = spm_dcm_erp(DCM);
    end

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Pegah Hosseini 2012-9-5

I was in contact with SPM coders for a while and it seems that there might be a problem in the interaction between the two toolboxes (PCT and SPM). I will let you know if this is the case.

Pegah Hosseini 2014-5-30

Although I don't think you are interested in this trend anymore, just so it has a final answer, I am updating it. There really was a problem in the interaction between PCT and SPM and it was solved in the new version. At least that is what I was told by the group. By the time the problem was solved, I had already moved on from SPM so I did not go back to check if it worked for me or it still had any problems.

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