Error Found an interactive session. You cannot have multiple interactive sessions open simultaneously. To terminate the existing session, use 'delete(gc​p('nocreat​e'))'

Hello everybody,

I want to create a pool but Matlab returns that I have a pool opened. I make sure that if a pool is open close it before open the new one. I mean:

delete(gcp('nocreate')); %delete the current pool 

poolobj = gcp;
if isempty(poolobj)
    poolsize = 1;
else
    poolsize = poolobj.NumWorkers;
end

parpool(poolsize,'IdleTimeout',Inf);

This is my code. I check if a pool is opened and then open a new one.

What is wrong? It always shows that issue.

Thanks!

Javi

[pengfei ZHAO]

Close all the MATLAB sessions running on your computer and try again.

Answer 1

[Javier Naranjo]

I am going to answer my own question if this helps to someone else.

It is not possible to open a pool with gcp and the open again with parpool.

That was the problem

[Baban Kumar]

Then what is solution?

[zhaorong huang]

delete(gcp('nocreate'))

Answer 2

[Hernan]

Hello Javier,

I arrived here looking for something else, but I think maybe what you wanted to do is something like this:

poolobj = gcp('nocreate'); % get the pool object, and do it avoiding creating a new one.

if isempty(poolobj) % check if there is not a pool.
    poolsize = 1;
else
    poolsize = poolobj.NumWorkers;
    delete( gcp('nocreate')); % delete the current pool object.
end

parpool( poolsize, 'IdleTimeout',Inf); % create a new pool with the previous poolsize and new specs.

In that way your code works.

And what I was looking for was just to delete in case the current number of workers is different that the amount of workers that I want.

So, to replace a parpool( workers, 'IdleTimeout', idletimeout) at the begining of my code, that would drop the error here in subject, I defined the following function:

function [ poolobj ] = check_my_parpool( workers, idletimeout)

    if nargin < 1
        workers = 4; % default number of workers that I want.
    end
    if nargin < 2
        idletimeout = 20; % default time in minutes of being idle before shut itself down.
    end
    
    poolobj = gcp('nocreate'); % get the pool object.

    if ~isempty(poolobj) % check if there IS actually a pool:

        if poolobj.NumWorkers ~= workers % it has a different number of workers???:
            delete( poolobj); % delete the current pool object.
        end
    end
    
    if isempty( gcp('nocreate')) % finally, if there is not a pool:
        poolobj = parpool( workers, 'IdleTimeout', idletimeout); % create a new pool.
    end
    
end

So, as example, add that function to your workspace, and then just replace:

parpool( 4);

with:

check_my_parpool( 4);

I hope this helps you,

Best,

Hernán.