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distributed

Create distributed array from data in the client workspace or a datastore

Description

example

D = distributed(ds) creates a distributed array from a datastore ds. D is a distributed array stored in parts on the workers of the open parallel pool. You operate on the entire array as a single entity, however, workers operate only on their part of the array, and automatically transfer data between themselves when necessary.

To retrieve the distributed array elements from the pool back to an array in the MATLAB® workspace, use gather.

example

D = distributed(X) creates a distributed array from an array X.

Constructing a distributed array from local data this way is appropriate only if the MATLAB client can store the entirety of X in its memory. To construct large distributed arrays, use one of the constructor methods such as ones(___,'distributed'), zeros(___,'distributed'), etc. For a list, see Constructor.

If the input argument is already a distributed array, the result is the same as the input.

example

D = distributed(C,dim) creates a distributed array from a Composite array C, with the entries of C concatenated and distributed along the dimension dim. If you omit dim, then the first dimension is the distribution dimension.

All entries of the Composite array must have the same class. Dimensions other than the distribution dimension must match.

Examples

Create Distributed Arrays

Create a small array and distribute it.

Nsmall = 50;
D1 = distributed(magic(Nsmall));

Create a large distributed array directly, using a build method.

Nlarge = 1000;
D2 = rand(Nlarge,'distributed');

Retrieve elements of a distributed array, and note where the arrays are located by their Class.

D3 = gather(D2);
whos
  Name           Size           Bytes  Class

  D1            50x50             733  distributed
  D2          1000x1000           733  distributed
  D3          1000x1000       8000000  double
  Nlarge         1x1                8  double
  Nsmall         1x1                8  double

Create a Distributed Array from a Datastore

This example shows how to create and load distributed arrays using datastore. You first create a datastore using an example data set. This data set is too small to show equal partitioning of the data over the workers. To simulate a large data set, artificially increase the size of the datastore using repmat.

files = repmat({'airlinesmall.csv'}, 10, 1);
ds = tabularTextDatastore(files);

Select the example variables.

ds.SelectedVariableNames = {'DepTime','DepDelay'};
ds.TreatAsMissing = 'NA';

Create a distributed table by reading the datastore in parallel. Partition the datastore with one partition per worker. Each worker then reads all data from the corresponding partition. The files must be in a shared location accessible from the workers.

dt = distributed(ds);
Starting parallel pool (parpool) using the 'local' profile ... connected to 4 workers.

Finally, display summary information about the distributed table.

summary(dt) 
Variables:

    DepTime: 1,235,230×1 double
        Values:

            min          1
            max       2505
            NaNs    23,510

    DepDelay: 1,235,230×1 double
        Values:

            min      -1036
            max       1438
            NaNs    23,510

Create a Distributed Array from a Composite Array

Start a parallel pool of workers and create a Composite array by using spmd.

p = parpool("local",4);
Starting parallel pool (parpool) using the 'local' profile ...
Connected to the parallel pool (number of workers: 4).
spmd
C = rand(3,labindex-1);
end
C
 
C =
 
   Lab 1: class = double, size = [3  0]
   Lab 2: class = double, size = [3  1]
   Lab 3: class = double, size = [3  2]
   Lab 4: class = double, size = [3  3]
 

To create a distributed array out of the Composite array, use the distributed function. For this example, distribute the entries along the second dimension.

d = distributed(C,2)
d =

    0.6383    0.9730    0.2934    0.3241    0.9401    0.1897
    0.5195    0.7104    0.1558    0.0078    0.3231    0.3685
    0.1398    0.3614    0.3421    0.9383    0.3569    0.5250
spmd
    d
end
Lab 1: 
  
  This worker does not store any elements of d.
  
Lab 2: 
  
  This worker stores d(:,1).
  
          LocalPart: [3x1 double]
      Codistributor: [1x1 codistributor1d]
  
Lab 3: 
  
  This worker stores d(:,2:3).
  
          LocalPart: [3x2 double]
      Codistributor: [1x1 codistributor1d]
  
Lab 4: 
  
  This worker stores d(:,4:6).
  
          LocalPart: [3x3 double]
      Codistributor: [1x1 codistributor1d]
  

When you are finished with the computations, delete the parallel pool.

delete(p);

Tips

  • A distributed array is created on the workers of the existing parallel pool. If no pool exists, distributed starts a new parallel pool unless the automatic starting of pools is disabled in your parallel preferences. If there is no parallel pool and distributed cannot start one, the result is the full array in the client workspace.