In general, it is not safe to have multiple workers attempting to save to a single file. The results are likely to be unpredictable, as you have observed - that's because if two workers happen to attempt to save at precisely the same moment, then it's possible that only one of the saves will actually occur. I would recommend saving to unique files. The simplest way is to derive the filename from the parfor loop index, and then post-process on the client to amalgamate the data if required. A slightly more sophisticated approach would use a single file per worker by basing the file name on the task ID. (See: getCurrentTask).
% Clean up prior runs
delete data_*.mat
% Main loop - save to one file per worker
parfor i = 1:200
varName = sprintf('X_%d', i);
varValue = magic(i);
fname = getFname();
doSave(fname, varName, varValue);
end
% Client loop - amalgamate into a single file
fnames = dir('data_*.mat');
for i = 1:numel(fnames)
s = load(fnames(i).name);
if i == 1
args = {};
else
args = {'-append'};
end
save('result.mat', '-struct', 's', args{:});
end
% Simple wrapper around SAVE to allow it to be used in PARFOR
function doSave(fname, varName, varValue)
s.(varName) = varValue;
if exist(fname, 'file')
args = {'-append'};
else
args = {};
end
save(fname, '-struct', 's', args{:});
end
% Choose a unique file name per worker
function fname = getFname()
t = getCurrentTask();
if ~isempty(t)
fname = sprintf('./data_%d.mat', t.ID);
else
% Get here on the client
fname = './data_0.mat';
end
end
