Restriction of functions run under parfeval

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

Answer 1

[Raymond Norris]

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.

[Bruno Luong]

Thanks, parfor does the trick.

So parfeval does not capture tha dairy and parfror does? What is the rational reason of the different behavior?

[Raymond Norris]

parfor is blocking (you can't run any other code while it's running), so it makes sense to capture the diary and display it. parfeval is non-blocking, which allows you to spawn the task (which could take hours to run) while running other code. In your example, you've created a barrier with the while-loop, but take the following scenario

f = parfeval(backgroundPool,@foo,1,10);
for sidx = 1:50
    [U, S, V] = svd(magic(sidx))
end

While your spawned task is running, I'm also displaying the output the values U, S, & V (50 times). 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)?

Another task that won't be "captured" (parfor as well) is plotting. Try any one of these

f = parfeval(gcp,@plot,0,rand(100));
f = parfeval(backgroundPool,@plot,0,rand(100));
parfor idx = 1:1, plot(rand(100)), end

The plotting is happening on a remote process, not forwarded back to the client. Rather, look at DataQueue (https://www.mathworks.com/help/parallel-computing/parallel.pool.dataqueue.html), it shows good examples of sending data back to the client to plot for both parfor and parfeval.

[Bruno Luong]

"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.

[Alexander Graf]

Is there already an update on the parfeval fct that allows to decide if the printf-commands are executed or blocked?

[Raymond Norris]

No update other than to say R2023b continues to behave in the same manner.

[Thomas Falch]

Just one clarification, the Diary property also works for thread pools and the background pool:

f = parfeval(backgroundPool, @() disp("Text"), 0);

f.Diary

ans =

'Text

'