Use Tall Arrays on a Parallel Pool

If you have Parallel Computing Toolbox™, you can use tall arrays in your local MATLAB^® session, or on a local parallel pool. You can also run tall array calculations on a cluster if you have MATLAB Distributed Computing Server™ installed. This example uses the workers in a local cluster on your machine. You can develop code locally, and then scale up, to take advantage of the capabilities offered by Parallel Computing Toolbox and MATLAB Distributed Computing Server without having to rewrite your algorithm. See also Big Data Workflow Using Tall Arrays and Datastores.

Create a datastore and convert it into a tall table.

ds = datastore('airlinesmall.csv');
varnames = {'ArrDelay', 'DepDelay'};
ds.SelectedVariableNames = varnames;
ds.TreatAsMissing = 'NA';

If you have Parallel Computing Toolbox installed, when you use the tall function, MATLAB automatically starts a parallel pool of workers, unless you turn off the default parallel pool preference. The default cluster uses local workers on your machine.

Note

If you want to turn off automatically opening a parallel pool, change your parallel preferences. If you turn off the Automatically create a parallel pool option, then you must explicitly start a pool if you want the tall function to use it for parallel processing. See Specify Your Parallel Preferences.

If you have Parallel Computing Toolbox, you can run the same code as the MATLAB tall table example (MATLAB) and automatically execute it in parallel on the workers of your local machine.

Create a tall table tt from the datastore.

tt = tall(ds)

Starting parallel pool (parpool) using the 'local' profile ... connected to 4 workers.

tt =

  M×2 tall table 

    ArrDelay    DepDelay
    ________    ________

     8          12      
     8           1      
    21          20      
    13          12      
     4          -1      
    59          63      
     3          -2      
    11          -1      
    :           :
    :           :

The display indicates that the number of rows, M, is not yet known. M is a placeholder until the calculation completes.

Extract the arrival delay ArrDelay from the tall table. This action creates a new tall array variable to use in subsequent calculations.

a = tt.ArrDelay;

You can specify a series of operations on your tall array, which are not executed until you call gather. Doing so enables you to batch up commands that might take a long time. For example, calculate the mean and standard deviation of the arrival delay. Use these values to construct the upper and lower thresholds for delays that are within 1 standard deviation of the mean.

m = mean(a,'omitnan');
s = std(a,'omitnan');
one_sigma_bounds = [m-s m m+s];

Use gather to calculate one_sigma_bounds, and bring the answer into memory.

sig1 = gather(one_sigma_bounds)

Evaluating tall expression using the Parallel Pool 'local':
Evaluation completed in 0 sec

sig1 =

  -23.4572    7.1201   37.6975

You can specify multiple inputs and outputs to gather if you want to evaluate several things at once. Doing so is faster than calling gather separately on each tall array . As an example, calculate the minimum and maximum arrival delay.

[max_delay, min_delay] = gather(max(a),min(a))

Evaluating tall expression using the Parallel Pool 'local':
- Pass 1 of 1: Completed in 1 sec
Evaluation completed in 1 sec

max_delay =

        1014

min_delay =

   -64

If you want to develop in serial and not use local workers or your specified cluster, enter the following command.

mapreducer(0);

If you use mapreducer to change the execution environment after creating a tall array, then the tall array is invalid and you must recreate it. To use local workers or your specified cluster again, enter the following command.

mapreducer(gcp);

Note

One of the benefits of developing algorithms with tall arrays is that you only need to write the code once. You can develop your code locally, and then use mapreducer to scale up to a cluster, without needing to rewrite your algorithm. For an example, see Use Tall Arrays on a Spark Enabled Hadoop Cluster.

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