IntroductionΒΆ

Message passing is currently a popular approach for implementing concurrent data processing applications. In this model, you decompose a large processing task into separate steps that execute concurrently and communicate solely by passing messages or data items between one another. This concurrency model is an intuitive way to structure a large processing task to exploit parallelism in a shared memory environment without incurring the complexity and overhead costs associated with multi-threaded applications.

In order to use a message passing model, you need an efficient data structure for passing messages between the processing elements of your application. A common approach is to utilize queues for storing and retrieving messages. Varon-T is a C library that implements a disruptor queue (originally implemented in the Disruptor Java library), which is a particularly efficient FIFO queue implementation. Disruptor queues achieve their efficiency through a number of related techniques:

  • Objects are stored in a ring buffer, which uses a fixed amount of memory regardless of the number of data records processed.
  • Objects are stored directly inline in the slots of the ring buffer, and their life cycle is controlled by the disruptor queue, not the application. This eliminates any per-record memory allocation overhead.
  • The ring buffer’s storage is implemented as a regular C array, so the data instances are all adjacent to each other in memory. This allows us to take advantage of cache striding and locality.
  • In most cases, the producers and consumers of a queue are coordinated without needing any costly locks, or even atomic CAS operations. Instead, they only require relatively cheap memory barriers.
  • Multiple consumers are able to drain a single queue, even when there’s a temporal constraint on the consumers — i.e., where a “downstream” consumer must wait to process a particular record until an “upstream” consumer has finished with it. By sharing a single queue, you eliminate additional memory allocations and copies.

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