The single program multiple data (spmd) language construct allows
seamless interleaving of serial and parallel programming. The spmd
statement
lets you define a block of code to run simultaneously on multiple
workers. Variables assigned inside the spmd
statement
on the workers allow direct access to their values from the client
by reference via Composite objects.
This chapter explains some of the characteristics of spmd
statements and Composite
objects.
The “single program” aspect of spmd means that
the identical code runs on multiple workers. You run one program in
the MATLAB® client, and those parts of it labeled as spmd
blocks
run on the workers. When the spmd block is complete, your program
continues running in the client.
The “multiple data” aspect means that even though
the spmd
statement runs identical code on all workers,
each worker can have different, unique data for that code. So multiple
data sets can be accommodated by multiple workers.
Typical applications appropriate for spmd
are
those that require running simultaneous execution of a program on
multiple data sets, when communication or synchronization is required
between the workers. Some common cases are:
Programs that take a long time to execute — spmd
lets
several workers compute solutions simultaneously.
Programs operating on large data sets — spmd
lets
the data be distributed to multiple workers.
The general form of an spmd
statement is:
spmd <statements> end
If a parallel pool is not running, spmd
creates
a pool using your default cluster profile, if your parallel preferences
are set accordingly.
The block of code represented by <statements>
executes
in parallel simultaneously on all workers in the parallel pool. If
you want to limit the execution to only a portion of these workers,
specify exactly how many workers to run on:
spmd (n) <statements> end
This statement requires that n
workers run
the spmd
code. n
must be less
than or equal to the number of workers in the open parallel pool.
If the pool is large enough, but n
workers are
not available, the statement waits until enough workers are available.
If n
is 0, the spmd
statement
uses no workers, and runs locally on the client, the same as if there
were not a pool currently running.
You can specify a range for the number of workers:
spmd (m,n) <statements> end
In this case, the spmd
statement requires
a minimum of m
workers, and it uses a maximum of n
workers.
If it is important to control the number of workers that execute
your spmd
statement, set the exact number in the
cluster profile or with the spmd
statement, rather
than using a range.
For example, create a random matrix on three workers:
spmd (3) R = rand(4,4); end
All subsequent examples in this chapter assume that a parallel
pool is open and remains open between sequences of spmd
statements.
Unlike a parfor
-loop, the workers used for
an spmd
statement each have a unique value for labindex
. This lets you specify code
to be run on only certain workers, or to customize execution, usually
for the purpose of accessing unique data.
For example, create different sized arrays depending on labindex
:
spmd (3) if labindex==1 R = rand(9,9); else R = rand(4,4); end end
Load unique data on each worker according to labindex
,
and use the same function on each worker to compute a result from
the data:
spmd (3) labdata = load(['datafile_' num2str(labindex) '.ascii']) result = MyFunction(labdata) end
The workers executing an spmd
statement operate
simultaneously and are aware of each other. As with a communicating
job, you are allowed to directly control communications between the
workers, transfer data between them, and use codistributed arrays
among them.
For example, use a codistributed array in an spmd
statement:
spmd (3) RR = rand(30, codistributor()); end
Each worker has a 30-by-10 segment of the codistributed array RR
.
For more information about codistributed arrays, see Working with Codistributed Arrays.
When running an spmd
statement on a parallel
pool, all command-line output from the workers displays in the client
Command Window. Because the workers are MATLAB sessions without
displays, any graphical output (for example, figure windows) from
the pool does not display at all.
All workers executing an spmd
statement must
have the same MATLAB search path as the client, so that they
can execute any functions called in their common block of code. Therefore,
whenever you use cd
, addpath
, or rmpath
on
the client, it also executes on all the workers, if possible. For
more information, see the parpool
reference
page. When the workers are running on a different platform than the
client, use the function pctRunOnAll
to
properly set the MATLAB path on all workers.
When an error occurs on a worker during the execution of an spmd
statement,
the error is reported to the client. The client tries to interrupt
execution on all workers, and throws an error to the user.
Errors and warnings produced on workers are annotated with the
worker ID (labindex
) and displayed in the client’s
Command Window in the order in which they are received by the MATLAB client.
The behavior of lastwarn
is
unspecified at the end of an spmd
if used within
its body.
Inside a function, the body of an spmd
statement
cannot make any direct reference to a nested function (MATLAB).
However, it can call a nested function by means of a variable defined
as a function handle to the nested function.
Because the spmd
body executes on workers,
variables that are updated by nested functions called inside an spmd
statement
do not get updated in the workspace of the outer function.
The body of an spmd
statement cannot define
an anonymous
function (MATLAB). However, it can reference an anonymous function
by means of a function handle.
The body of an spmd
statement cannot directly
contain another spmd
. However, it can call a
function that contains another spmd
statement.
The inner spmd
statement does not run in parallel
in another parallel pool, but runs serially in a single thread on
the worker running its containing function.
The body of a parfor
-loop cannot contain
an spmd
statement, and an spmd
statement
cannot contain a parfor
-loop.
The body of an spmd
statement cannot contain break
or return
statements.
The body of an spmd
statement cannot contain global
or persistent
variable
declarations.