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Designing Parallel Algorithms: Part 2


Editor's Note: This multi-part are on parallel algorithm design is based on the book Designing and Building Parallel Programs by Ian Foster. Designing and Building Parallel Programs promotes a view of parallel programming as an engineering discipline, in which programs are developed in a methodical fashion and both cost and performance are considered in a design. Part 1 focuses on the topics of Methodical Design and Partitioning. Subsequent installments will focus on Communication, Agglomeration, and Mapping, before finally examining case studies of parallel algorithms in action. A special thanks to Ian Foster.


Designing Parallel Algorithms: Part 1

Designing Parallel Algorithms: Part 2

Designing Parallel Algorithms: Part 3


Communication

The tasks generated by a partition are intended to execute concurrently but cannot, in general, execute independently. The computation to be performed in one task will typically require data associated with another task. Data must then be transferred between tasks so as to allow computation to proceed. This information flow is specified in the communication phase of a design.

In our programming model, we conceptualize a need for communication between two tasks as a channel linking the tasks, on which one task can send messages and from which the other can receive. Hence, the communication associated with an algorithm can be specified in two phases:

  • First, we define a channel structure that links, either directly or indirectly, tasks that require data (consumers) with tasks that possess those data (producers).
  • Second, we specify the messages that are to be sent and received on these channels.

Depending on our eventual implementation technology, we may not actually create these channels when coding the algorithm. For example, in a data-parallel language, we simply specify data-parallel operations and data distributions. Nevertheless, thinking in terms of tasks and channels helps us to think quantitatively about locality issues and communication costs.

The definition of a channel involves an intellectual cost and the sending of a message involves a physical cost. Hence, we avoid introducing unnecessary channels and communication operations. In addition, we seek to optimize performance by distributing communication operations over many tasks and by organizing communication operations in a way that permits concurrent execution.

In domain decomposition problems, communication requirements can be difficult to determine. Recall that this strategy produces tasks by first partitioning data structures into disjoint subsets and then associating with each datum those operations that operate solely on that datum. This part of the design is usually simple. However, some operations that require data from several tasks usually remain. Communication is then required to manage the data transfer necessary for these tasks to proceed. Organizing this communication in an efficient manner can be challenging. Even simple decompositions can have complex communication structures.

In contrast, communication requirements in parallel algorithms obtained by functional decomposition are often straightforward: they correspond to the data flow between tasks. For example, in a climate model broken down by functional decomposition into atmosphere model, ocean model, and so on, the communication requirements will correspond to the interfaces between the component submodels: the atmosphere model will produce values that are used by the ocean model, and so on.

In the following discussion, we use a variety of examples to show how communication requirements are identified and how channel structures and communication operations are introduced to satisfy these requirements. For clarity in exposition, we categorize communication patterns along four loosely orthogonal axes: local/global, structured/unstructured, static/dynamic, and synchronous/asynchronous.

  • In local communication, each task communicates with a small set of other tasks (its "neighbors''); in contrast, global communication requires each task to communicate with many tasks.
  • In structured communication, a task and its neighbors form a regular structure, such as a tree or grid; in contrast, unstructured communication networks may be arbitrary graphs.
  • In static communication, the identity of communication partners does not change over time; in contrast, the identity of communication partners in dynamic communication structures may be determined by data computed at runtime and may be highly variable.
  • In synchronous communication, producers and consumers execute in a coordinated fashion, with producer/consumer pairs cooperating in data transfer operations; in contrast, asynchronous communication may require that a consumer obtain data without the cooperation of the producer.


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