Matlab is a numerical computing and programming environment with a broad range of functionality (matrix manipulation, numerical linear algebra, general-purpose graphics, etc.). Additionally, special application areas are served by a large number of optional toolboxes. Matlab version 2011b is install on the cluster and can be accessed here: /usr/local/bin/matlab

When running Matlab interactive, users should avoid running it on the head node. Instead, they should request an interactive session through the PBS job scheduler.

[<username@headnode> ~]$ qsub -I -X -l nodes=1:ppn=1 qsub: waiting for job <some job name> to start qsub: job <some job name> ready [<username@compute node> ~]$

You can then run Matlab and any other interactive commands you would like:

[username@<compute node> ~]$ pwd

/home/<username>

[username@<compute node> ~]$ matlab

When you are done with your interactive commands, you can use the exit command to end the job:

[username@<compute node> ~]$ exit

Matlab interactive mode can be used to take advantage of the integrated environment. But, it can also be run in a non-interactive mode for "batch processing" and to take advantage of cluster computing resources. If fact, the preferred mode of operation for Matlab on our cluster is non-interactive.

One way to run Matlab non-interactively is through

- re-directing the standard input and output when invoking Matlab and
- invoking Matlab from a submission script, submitted to the queue via the PBS scheduler.

Here is an example demonstrating a non-interactive Matlab program. A program file named `mystats.m`

that contains a main function, `mystats`

, and two local functions, `mymean`

and `mymedian`

.

function [avg, med] = mystats(x) n = length(x); avg = mymean(x,n); med = mymedian(x,n); end function a = mymean(v,n) % MYMEAN Example of a local function. a = sum(v)/n; end function m = mymedian(v,n) % MYMEDIAN Another example of a local function. w = sort(v); if rem(n,2) == 1 m = w((n + 1)/2); else m = (w(n/2) + w(n/2 + 1))/2; end end

The job is sent to the queue and executed on a backend node using the following PBS file provided and the command **qsub run.sh**. The script **run.sh** contains the following line to run the Matlab script:

matlab -nodisplay -nosplash < mystats.m > run.log

The flag **nodisplay** instructs Matlab to run without the GUI, while **nosplash** prevents the display of the Matlab logo. The **<** redirection operator ensures that Matlab runs the script **main.m**, while the **>** operator re-directs the standard output (normally to the terminal) to **run.log** file.

The example is run in batch mode with the command **qsub run.sh**, using the following PBS file:

#!/bin/bash #PBS -V #PBS -l nodes=1 #PBS -l walltime=0:05:00 #PBS -N matlab_test cd $PBS_O_WORKDIR matlab -nodisplay -nosplash < main.m > run.log

Notice how MATLAB is instructed to not load the interactive window.

**Note**: do not turn java off when lauching MATLAB (*i.e.* do not invoke **matlab -nojvm**); **matlabpool** uses the Java Virtual Machine.

After the job finishes, the CPU times spent executed the loops in **main.m** can be found in **timings.dat**, showing a clear speed-up of the execution in parallel.

Matlab can also be run where you can take advantage of parallel hardware in at least two ways.

The first is a built-in feature of Matlab, which "naturally" exploits multi-core processing via the underlying multi-threaded libraries Intel MKL and FFTW. Thus, linear algebra operations (such as the solution to a linear system **A\b** or matrix products **A*B**) and FFT operations (using the function **fft**) are implicitly multi-threaded and make use of all the cores available on a multi-core system without user intervention or special extra programming. Some of the vectorised operations in Matlab are also multi-threaded. However, this type of operations are only a part of Matlab programming and the vast proportion of the Matlab functionality are scripts or functions that can only use a single core.

You can also take advantage of parallel processing through a series of explicit programming techniques. The following techniques are:

- using the Matlab toolbox
**Parallel Computing Toolbox**; - trivial parallelism exploited through independent Matlab processes;
- multi-threaded MEX programming.

The functionality of the Parallel Computing Toolbox is extended from single cluster node processing to distributed processing across multiple nodes by the Distributed Computing Server. To learn more about the product, please visit the Distributed Computing Server webpage.

An easy way to exploit multi-core systems is to split the workflow into parts that can be processed completely independently. The typical example in this category is a parameter sweep, where the same Matlab script is run a large number of times using different inputs; these runs are indepent from each other and can be carried out concurrently. Thus, the entire workflow can be scheduled in jobs that group 8 independent runs to match the 8 cores available per compute node. This strategy is best coupled with the use of the Matlab **mcc** compiler in order to avoid an excessive use of licenses.

Yet another way to exploit multi-core systems is via multi-threaded Mex programming. Mex (**M**atlab **EX**ecutable) files are dynamically linked subroutines compiled from C, C++ or Fortran source code that can be run from within Matlab in the same way as M-files or built-in functions. These guidelines assume knowledge of serial Mex programming and provide an example of how to augment serial execution with multi-threading through OpenMP. Coupled with OpenMP multi-threading, Mex files become a powerful method to accelerate key parts of a Matlab program.

The main reason to write Mex files in C or Fortran (thus abandoning the high-level abstracted Matlab programming) is to gain speed of execution in computationally intensive operations that otherwise become a bottleneck in an application. Typically, this is done to replace a function that is identified through profiling as being slow and/or called a large number of times. Nevertheless, this programming effort is rewarded to various degrees, with the greatest relative benefits normally met when a Mex replaces a Matlab script (M-file). At the other extreme, Matlab operations that rely on performance libraries like FFTW (*e.g.* **fftn**) or BLAS/LAPACK (*e.g.* solution of a dense linear systems, **A\b**), which are highly optimised have nothing or very little to benefit from Mex programming. The best source for learning Mex programming is the Mathworks webpages

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