Parallel Programming with Microsoft Visual C++®: Design Patterns for Decomposition and Coordination on Multicore Architectures by Colin Campbell & Ade Miller

Author:Colin Campbell & Ade Miller [Colin Campbell]
Language: eng
Format: epub
Tags: COMPUTERS / Programming / Microsoft Programming
ISBN: 9780735651746
Publisher: Microsoft Press
Published: 2011-03-22T16:00:00+00:00

Figure 7-1. Sample pipeline

Stages of the pipeline read from a dedicated input, or source, and write to a particular output, or target. For example, the “Correct Case” stage uses buffer 1 as its source and writes to buffer 2 as its target. All the stages of the pipeline can execute at the same time because the three messaging blocks buffer any shared inputs and outputs. If there are four available cores, the four stages can run in parallel.

Stages in the pipeline block (that is, wait) on inputs. An input wait is familiar from other programming contexts—if an enumeration or a stream doesn’t have a value, the consumer of that enumeration or stream waits until a value is available or an end–of-file condition occurs. Using buffers that hold more than one value at a time compensates for variability in the time it takes to process each value. Buffers allow stages of the pipeline to be asynchronous.

Note: When using the unbounded_buffer<T> class you should define a special value as your end-of-file token. This special value is sometimes called the sentinel value. When using sentinel values you must be careful that the end-of-file signal can never occur as one of the regular messages. This example uses the value given by the PhraseSource:: FinishedSentinel () static method to signal the end of the sequence of values.

Choosing a sentinel value can be harder than it seems at first. It’s often the case that all values of the type T have meaning as valid payloads of an unbounded_buffer<T> instance. For example, if your payload type is a string, you might be tempted to use the empty string as the sentinel value, but this would only be safe if you can guarantee that the empty string is never used as a normal value to be processed by the pipeline. In practice, the null pointer is often used as the sentinel value.

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