Error using Parallel Computing Toolbox with INTLAB

Hi,

I am unable to convert code that uses interval arithmetic (based on the INTLAB toolbox) to be executed in parallel using the Parallel Computing toolbox. Here is a snippet of (otherwise useless) code which will lead to the error:

x=infsup(1,2);
pool=parpool(2);
parfor i=1:1
    2*x
end

Here is the error message that is received:

Starting parallel pool (parpool) using the 'local' profile ... connected to 2 workers.
Error using  *  (line 35)
Struct contents reference from a non-struct array object.
Error in spmd_test (line 8)
parfor i=1:1

Please note that the same code would work without issue if a standard "for" loop was used instead of "parfor". Any advice would be appreciated on why this doesn't work as expected.

Thank you,

Marc

Answer 1

[Marc Arsenault]

Many thanks to Michio and Edric for promptly responding to my query. I eventually found the issue to be due to the use of a global variable (INTLAB_CONST) within the Intlab toolbox and referred to by mostly all commands provided by the toolbox. Since the Parallel Computing toolbox doesn't have a good relationship with global variables, this led to the error (the error message itself was to the effect that the INTLAB_CONST variable, which is a structure, did not exist in the m-file being executed). A functional workaround is the following (for example):

% Main function excerpt (illustration only):
global INTLAB_CONST
parloop i=1:100
% Call to subroutine:
x=other_function(INTLAB_CONST,other_variables);
end

Within the subroutine being called by the parfor loop, the global variable is again declared and the value passed explicitly to the function is assigned to it, e.g.:

function x=other_function(INTLAB_CONST_TEMP,other_variables)
global INTLAB_CONST
INTLAB_CONST=INTLAB_CONST_TEMP
% Remainder of code...
end

I believe this allows (forces?) the Parallel Computing toolbox to categorize INTLAB_CONST as a variable and avoid the aforementioned difficulties.

Thanks again!

Marc

Answer 2

[michio]

There are some restrictions on variables you can use within parfor. Example: Use Objects and Handles in parfor-Loops

I am guessing that the following code would work without a problem.

x=1;
pool=parpool(2);
parfor i=1:1
    2*x
end

If so, getting to know what x=infsup(1,2) is should help. What kind of datatype does infsup returns? Could you try

x = infsup(1,2);
whos x

to see the datatype of x? Or could you simply get output without displaying the result?

x=infsup(1,2);
pool=parpool(2);
parfor i=1:1
    result(i) = 2*x;
end

Answer 3

[Edric Ellis]

I suspect this is related to the transfer of an object of type infsup. You could check this as follows:

x = infsup(1,2);
save tmp x
clear
load tmp
x * 2;

If that fails in the same way, then the problem lies with the class infsup. If that succeeds, then I'm not sure where the problem lies...