In
computer science
Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...
, implicit parallelism is a characteristic of a programming language that allows a
compiler
In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...
or
interpreter
Interpreting is translation from a spoken or signed language into another language, usually in real time to facilitate live communication. It is distinguished from the translation of a written text, which can be more deliberative and make use o ...
to automatically exploit the
parallelism inherent to the computations expressed by some of the language's constructs. A pure implicitly parallel language does not need special directives, operators or functions to enable parallel execution, as opposed to
explicit parallelism.
Programming languages with implicit parallelism include
Axum
Axum, also spelled Aksum (), is a town in the Tigray Region of Ethiopia with a population of 66,900 residents (as of 2015). It is the site of the historic capital of the Aksumite Empire.
Axum is located in the Central Zone of the Tigray Re ...
,
BMDFM
Binary Modular Dataflow Machine (BMDFM) is a software package that enables running an application in parallel on shared memory symmetric multiprocessing (SMP) computers using the multiple processors to speed up the execution of single applications ...
,
HPF,
Id,
LabVIEW,
MATLAB M-code,
NESL,
SaC,
SISAL
Sisal (, ; ''Agave sisalana'') is a species of flowering plant native to southern Mexico, but widely cultivated and naturalized in many other countries. It yields a stiff fibre used in making rope and various other products. The sisal fiber is ...
,
ZPL, and pH.
Example
If a particular problem involves performing the same operation on a group of numbers (such as taking the
sine
In mathematics, sine and cosine are trigonometric functions of an angle. The sine and cosine of an acute angle are defined in the context of a right triangle: for the specified angle, its sine is the ratio of the length of the side opposite th ...
or
logarithm
In mathematics, the logarithm of a number is the exponent by which another fixed value, the base, must be raised to produce that number. For example, the logarithm of to base is , because is to the rd power: . More generally, if , the ...
of each in turn), a language that provides implicit parallelism might allow the programmer to write the instruction thus:
numbers = 1 2 3 4 5 6 7
result = sin(numbers);
The compiler or interpreter can calculate the sine of each element independently, spreading the effort across multiple processors if available.
Advantages
A programmer that writes implicitly parallel code does not need to worry about task division or process communication, focusing instead on the problem that his or her program is intended to solve. Implicit parallelism generally facilitates the design of parallel programs and therefore results in a substantial improvement of programmer productivity.
Many of the constructs necessary to support this also add simplicity or clarity even in the absence of actual parallelism. The example above, of
list comprehension
A list comprehension is a syntactic construct available in some programming languages for creating a list based on existing lists. It follows the form of the mathematical '' set-builder notation'' (''set comprehension'') as distinct from the use o ...
in the
sin() function, is a useful feature in of itself. By using implicit parallelism, languages effectively have to provide such useful constructs to users simply to support required functionality (a language without a decent
for loop
In computer science, a for-loop or for loop is a control flow Statement (computer science), statement for specifying iteration. Specifically, a for-loop functions by running a section of code repeatedly until a certain condition has been satisfi ...
, for example, is one few programmers will use).
Disadvantages
Languages with implicit parallelism reduce the control that the programmer has over the parallel execution of the program, resulting sometimes in less-than-optimal
parallel efficiency. The makers of the
Oz programming language
Oz is a multiparadigm programming language, developed in the Programming Systems Lab at Université catholique de Louvain, for programming-language education. It has a canonical textbook: Concepts, Techniques, and Models of Computer Programmin ...
also note that their early experiments with implicit parallelism showed that implicit parallelism made debugging difficult and object models unnecessarily awkward.
A larger issue is that every program has some parallel and some serial logic. Binary I/O, for example, requires support for such serial operations as
Write() and
Seek(). If implicit parallelism is desired, this creates a new requirement for constructs and keywords to support code that cannot be threaded or distributed.
Notes
Parallel computing
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