Dot indexing is not supported for variables of this type??? Parfor loop, and global variable

I'm trying to run a parfor loop for my simulation. But I got an error "Dot indexing is not supported for variables of this type", beside on this line, I still got the similar error and I couldn't figure out what went wrong. In the test_simdata function, there is a global variable and I'm bypassing it using a function, not sure if this is the cause.

%Main Function
clear
clc
iCellNum = 1;
activeUeNums = 1;

parfor ttiNum = 1 : 20  

            timeDomainSignal{ttiNum} = test3(iCellNum, ttiNum, activeUeNums);
    
end

%test3
function test = test3(iCellNum, iTtiNum, iActiveUeList)

g = test_global;
simData = test_simData;

substr.simData = struct();
substr.simData.outputFolder         = g.G_SIMULATION_OUTPUT_PATH;
substr.simData.cellId               = simData.cell(iCellNum).cellId;
substr.simData.chanBW               = simData.cell(iCellNum).bw;
substr.simData.simLen               = simData.cell(iCellNum).totalSf;
substr.simData.txAnt                = simData.cell(iCellNum).txAnt;
substr.simData.pairedSpectrum       = simData.cell(iCellNum).pairedSpectrum;
substr.simData.ssBlockNumerology    = simData.cell(iCellNum).ssBlockNumerology;
numerologyIndex = substr.simData.ssBlockNumerology + g.G_MATLAB_START_INDEX_OFFSET;
substr.simData.ssNumerologyOneOpt   = simData.cell(iCellNum).ssNumerologyOneOpt;
substr.simData.subCarrierSpace      = getSubCarrierSpace(simData.LuT, substr.simData.ssBlockNumerology);
substr.simData.gridConfig           = simData.cell(iCellNum).grid;
substr.simData.gridOffsetForNumerology = substr.simData.gridConfig(numerologyIndex).offsetNsc;


end

%test_simData
function y = test_simData

global simData
y = simData;

end

Answer 1

[Walter Roberson]

You can use parallel.pool.Constant to send a copy of the data to all workers, and you can use parfevalOnAll to run a function on the workers to copy the constant into the (per-worker) global variable.

[Yi Xien Yap]

I'm not familiar with parfevalOnAll but I assume it is like this. First, I have the temp_simData to store the copy to all the worker. Then I call the parfevalOnAll to run the function, but I still get the error message from the worker. And, how do you retrieve the output from the parfevalOnAll?

iCellNum = 1;
activeUeNums = 1;
global simData;
temp_simData = parallel.pool.Constant(simData);

for ttiNum = 1 : 20  
%     timeDomainSignal{ttiNum} = test3(iCellNum, ttiNum, activeUeNums, temp_simData.Value);   
    timeDomainSignal{ttiNum} = parfevalOnAll(@test3, 1 , iCellNum, ttiNum, activeUeNums, temp_simData.Value);
end

[Walter Roberson]

iCellNum = 1;
activeUeNums = 1;
simData_constant = parallel.pool.Constant(simData);

parfevalOnAll(@() store_simData(simdata_constant));

max_iter = 20;
timeDomainSignal = cell(max_iter,1);

parfor ttiNum = 1 : 20  
  timeDomainSignal{ttiNum} = test3(iCellNum, ttiNum, activeUeNums);    
end

function test = test3(iCellNum, iTtiNum, iActiveUeList)
    g = test_global;
    simData = test_simData;
    
    etc
end

function store_simData(simData_constant)
  global simData
  simData = simData_constant.Value;
end

%test_simData
    function y = test_simData
    global simData
    y = simData;
end

[Walter Roberson]

OR, more simply if you do not really need simData to be stored as a global,

iCellNum = 1;
activeUeNums = 1;
simData_constant = parallel.pool.Constant(simData);

parfevalOnAll(@() store_simData(simdata_constant));

max_iter = 20;
timeDomainSignal = cell(max_iter,1);

parfor ttiNum = 1 : 20  
  timeDomainSignal{ttiNum} = test3(iCellNum, ttiNum, activeUeNums, simData_constant);    
end

function test = test3(iCellNum, iTtiNum, iActiveUeList, simData_constant)
    simData = simData_constant.Value;
    
    etc
end

[Yi Xien Yap]

Hi, I don't quite understand the approach. What will happpen when running this line? It says 'Not enough input arguments' when I run it.

parfevalOnAll(@() store_simData(simdata_constant));

I see that you created a function function store_simData(simData_constant), how will this be process in the parfevalOnAll??

[Bruno Luong]

@Yi Xien Yap please explain your intended design pattern with global variable? Why you need it? MATLAB cannot handle global variable in parallel computing, but have some mechanism of communication between workers, or between worker/client.

If you explain clearly what you want to do, we might be able to guide you to have alternative design.

[Yi Xien Yap]

I'm trying to move the current code I have to a parallel programming. Most of the global variable are replaced by the local constant. However, I have this issue where the simData contains a large structure, and this was updating across multiple files. I'm trying to localize it before it gets to the 'test3' function, so it can be use with parfor loop.

[Bruno Luong]

The only variable that can be updated in parallel is sliced variable. You have to allocate/initialize such variable (not global), then you can update in parallel.

Sorry but such restriction is to avoid collision in parallel. You just have to dig in and design your data to be able to handle it.

[Edric Ellis]

@Bruno Luong parfor can also emit "reduction variables". (Probably not relevant here, just thought I'd mention it).

[Yi Xien Yap]

Thanks guys, I think I know where the problem is

[Walter Roberson]

parfevalOnAll(@() store_simData(simdata_constant), 0);

Answer 2

[Bruno Luong]

The global variable is not accessible by the workers, and it will return EMPTY when you are trying tp access to it. Soo if you assume it's a structure, your code will crash.

[Yi Xien Yap]

What are the option I have to allocate the variable to all the worker? Can I call the global variable before the parfor loop and assign the global variable as the function input?

[Bruno Luong]

Yes you can assign to function input, but then it becomes specific by worker so no longer a global variable in th sense that any modification of this variable is not visible by other workers.

[Yi Xien Yap]

For example in the main function,

iCellNum = 1;
activeUeNums = 1;
global simData;
temp_simData = simData;
parfor ttiNum = 1 : 20  
            timeDomainSignal{ttiNum} = test3(iCellNum, ttiNum, activeUeNums, temp_simData);
    
end

Then in the test3 function

function test = test3(iCellNum, iTtiNum, iActiveUeList,simData)
...
end

In this case, the temp_simData will not do any modification and can be use by the workers I pressume. But I still got error on this.

[Bruno Luong]

It might be something more basic : I don't see where do you initialize simData, if you replace parfor with for you code might still crash.

[Yi Xien Yap]

The simData is a struct and it was declare on other file. After I have the simData initialize, will the save and load function solve this problem?

save('temp_simData.mat','simData');
simData_load = load('temp_simData.mat');

parfor ttiNum = 1 : 20
    etc...
end

[Bruno Luong]

"After I have the simData initialize"

But I think the probleme is it is NOT initialize. That is why the value is an empty array and not a struct.

[Yi Xien Yap]

Thanks Bruno.