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Restriction of functions run under parfeval

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Bruno Luong
Bruno Luong on 23 Sep 2021
Edited: Bruno Luong on 27 Sep 2021
I just see parfeval is available for R2021b allowing to run parallel function (in background) even without the parallel toolbox, which is great feature.
However when I play with it, the screen output displaying/plotting seems do nothing as illustrate in this script.
My question is is there a document that list the functions that are innactive when runing in such parallel thread?
% script bar.m to be run under R2021b
% run function foo in normal way
res=foo(10)
% run function foo in background way
f = parfeval(backgroundPool,@foo,1,10);
fprintf('Wait for background function foo to finish...')
while true
if strcmp(f.State,'finished')
break
end
pause(0.1);
end
fprintf('\n');
res=fetchOutputs(f)
function res = foo(n)
res = 0;
for i=1:n
res = res + i;
fprintf('%d\n', i); % this statement does nothing when foo is runing in background
pause(1);
end
end
>> bar
1
2
3
4
5
6
7
8
9
10
res =
55
Wait for background function foo to finish...
res =
55

Accepted Answer

Raymond Norris
Raymond Norris on 24 Sep 2021
fprintf isn't inactive/not working, as much backgroundPool doesn't capture the "diary" of parfeval. This has the same behaviour as threaded pools
f = parfeval(parpool('threads'),@foo,1,10);
And it has a similar behaviour as process pools
f = parfeval(parpool('local'),@foo,1,10);
with one exception. With a process pool, in the while-loop you could add the call
f.Diary
It won't do exactly what you want -- it'll print the entire diary each time. For example
ans =
1
2
3
4
5
6
7
8
9
ans =
1
2
3
4
5
6
7
8
9
10
But to your point, if instead of calling
f = parfeval(backgroundPool,@foo,1,10);
you called
parfor i = 1:1
res = foo(10);
end
you would see the fprintf statements. I'll check with Development, but it would appear as thought screen I/O would be the gating functions.
  3 Comments
Bruno Luong
Bruno Luong on 25 Sep 2021
"Would you want your diary interspersed with the for-loop (assuming that could even happen -- I'm just illustrating a bunch of code running with a display while foo is running)?"
Honestly if I was the designer, I would prefer to have a freedom to use or not printf in the background function (non blocking function). I would not expect the manager to interfer and blocks screen output. I can deal to make the screen messy or not, that's easier than do not have possibility to achieve something.

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