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Grand Central Dispatch (GCD) is a revolutionary approach to multicore computing that is woven throughout the fabric of Mac OS X version 10.6 Snow Leopard. GCD combines an easy-to-use programming model with highly-efficient system services to radically simplify the code needed to make best use of multiple processors. The technologies in GCD improve the performance, efficiency, and responsiveness of Snow Leopard out of the box, and will deliver even greater benefits as more developers adopt them.

The central insight of GCD is shifting the responsibility for managing threads and their execution from applications to the operating system. As a result, programmers can write less code to deal with concurrent operations in their applications, and the system can perform more efficiently on single-processor machines, large multiprocessor servers, and everything in between. Without a pervasive approach such as GCD, even the best-written application cannot deliver the best possible performance, because it doesn’t have full insight into everything else happening in the system.

GCD is implemented as a set of extensions to the C language as well as a new API and runtime engine. While initially inspired by the challenge of multicore computing, these actually solve a more general problem: how to efficiently schedule multiple independent chunks of work. GCD does this using four primary abstractions:

  • block objects
  • dispatch queues
  • synchronization
  • event sources

This article provides an high-level overview of these abstractions and their API, as well as examples of how to use them.

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