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sippanspojk

February 6, 2020 08:47

What does reduce do in parallel computing?

Hi foamers,

I am adapting a code I wrote to work in parallel and I have some basic problems.

First of all, I don't understand what the reduce command is doing. Scatter and Gather I think I understand more or less; scatter divides my information and spread it on my procs and gather collects the information and assemble it on my master proc, correct? But reduce however I have no clue of what this is doing. Can someone help me here?

In my code I am reading the pressure values of each face of some specified patches, doing some computations using those values and then finally updating a custom defined scalarField.

I was then thinking that I should to do the following: gather the pressure field, read the pressure values, perform my computations, update my custom filed and lastly scatter the custom field before the next time step is taken. Is this correct?

I am really fumbling in the dark here so any kind of clarification would be very much appreciated.

Thanks,
David

HPE	February 7, 2020 13:46

I think it is just OpenMPI/MPI reduce command: reduce.

For example, let's assume you have a label (int):

Code:

label myBeautifulLabel = 0;

You perform something on this label on each processor

Code:

myBeautifulLabel = functionReturningProcessorID();

And you finally want to obtain the total value of all `myBeautifulLabel`s across all processors:

Code:

reduce(myBeautifulLabel, sumOp<label>());

So the value of `myBeautifulLabel` is the total magnitude of all `myBeautifulLabel`s, and this value is now known in all processors locally.

If you do coding, you may want to use/search Doxygen time to time (or all the time). It provides good information on function signatures, and where they have been used etc etc.

dimasbarile

May 11, 2022 16:10

Hi! I was wondering, ¿What happens if i don't use the reduce commnad?

I'm trying to print the value of U in a specific cell. Then i do the following:

Code:



posCellId = mesh().findCell(mappingPos_);



uProbeVector = vector(1000,1000,1000);



uProbeVector = U[posCellId];

OpenFOAM does this in all processor. Then if i use the reduce command:

Code:

reduce(uProbeVector, minOp<vector>());

My question is, if i don't use the reduce coomand and i ask OpenFOAM for the value of "uProbeVector", ¿What value does OpenFOAM return? I did the test and the result was similar (not equal) to the one with the reduce command, so i was wondering how it got to that value. Thanks!

olesen

May 12, 2022 12:38

You probably have a bigger problem. With findCell() you return the index of the cell that encloses that particular position. In parallel this means that only one processor will deliver an index and all of the others will return -1. Don't use a -1 to access any arrays, unless you like SEGFAULT.

So which value would you use for processors where the position is not found? Generally you would do something like initialise with vector::uniform(GREAT) instead of your value of 1000. If you want to know your uProbeVector globally (ie, not just on the proc where it is found) you need to transmit that information somehow. This is what the reduce is about. The name is actually slightly misleading since it is actually an MPI_Allreduce. So after this operation all processors will have the same value.
If that is not what you want, then don't do a reduce.

dimasbarile

May 12, 2022 12:46

Thanks a lot for the reply! it has become much clearer now

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