How to specify the number of labs in smpd?

My computer has 4 cores. When I use

smpd(3)

    c=labindex;
    if(c==1)
        a=1;
    elseif(c==2)
        b=1;
    elseif(c==3)
        d=2;
    end
end

Error: Could not create an SPMD block to match the requested size: 3. The parallel pool size is: 4, the active SPMD context is of size: 4.

So one can't specify the number of labs? Also smpd is based on shared memory like OpenMP?

Answer 1

[Edric Ellis]

You're only allowed to have a single spmd context active at any given time. An spmd context is created when you open an spmd block, and remains active after completion of that block if you create any Composite or distributed objects. In your case, I presume you have some of these objects from a previous spmd block. Here's how you might hit the problem:

parpool('local', 4);
% The next line creates an spmd context of size 4:
spmd, x = 1; end

% The following line will throw an error because the Composite "x" is
% keeping the spmd context active:
spmd(3), y = 2; end

% We can close the active spmd context by clearing "x":
clear x

% This will now work correctly:
spmd(3), y = 2; end

[micholeodon]

Also avoid syntax with tildas like:

[outvar1, ~] = some_function(arg1, arg2);      

do instead

outvar1 = some_function(arg1, arg2);  

because it can trigger context error as well.

Answer 2

[Walter Roberson]

The message might be hinting that you already have an spmd pool open, and there isn't room for 3 more workers.

[Edric Ellis]

That's not quite right - see my answer for an explanation of the spmd active context.