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Parallelism

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In Haskell we provide two ways to achieve parallelism:
 
In Haskell we provide two ways to achieve parallelism:
* Concurrency, which can be used for parallelising IO.
 
 
* Pure parallelism, which can be used to speed up pure (non-IO) parts of the program.
 
* Pure parallelism, which can be used to speed up pure (non-IO) parts of the program.
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* Concurrency, which can be used for parallelising IO.
  +
  +
Pure Parallelism (Control.Parallel): Speeding up a pure computation using multiple processors. Pure parallelism has these advantages:
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* Guaranteed deterministic (same result every time)
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* no [[race conditions]] or [[deadlocks]]
   
 
[[Concurrency]] (Control.Concurrent): Multiple threads of control that execute "at the same time".
 
[[Concurrency]] (Control.Concurrent): Multiple threads of control that execute "at the same time".
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* Threads may execute on multiple processors simultaneously
 
* Threads may execute on multiple processors simultaneously
 
* Dangers: [[race conditions]] and [[deadlocks]]
 
* Dangers: [[race conditions]] and [[deadlocks]]
 
Pure Parallelism (Control.Parallel): Speeding up a pure computation using multiple processors. Pure parallelism has these advantages:
 
* Guaranteed deterministic (same result every time)
 
* no [[race conditions]] or [[deadlocks]]
 
   
 
Rule of thumb: use Pure Parallelism if you can, Concurrency otherwise.
 
Rule of thumb: use Pure Parallelism if you can, Concurrency otherwise.
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* The [[Parallel|parallelism and concurrency portal]]
 
* The [[Parallel|parallelism and concurrency portal]]
* Parallel [[Paralell/Reading|reading list]]
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* Parallel [[Parallel/Reading|reading list]]
 
* [[Parallel/Research|Ongoing research in Parallel Haskell]]
 
* [[Parallel/Research|Ongoing research in Parallel Haskell]]

Revision as of 14:25, 20 April 2011

Parallelism is about speeding up a program by using multiple processors.

In Haskell we provide two ways to achieve parallelism:

  • Pure parallelism, which can be used to speed up pure (non-IO) parts of the program.
  • Concurrency, which can be used for parallelising IO.

Pure Parallelism (Control.Parallel): Speeding up a pure computation using multiple processors. Pure parallelism has these advantages:

Concurrency (Control.Concurrent): Multiple threads of control that execute "at the same time".

  • Threads are in the IO monad
  • IO operations from multiple threads are interleaved non-deterministically
  • communication between threads must be explicitly programmed
  • Threads may execute on multiple processors simultaneously
  • Dangers: race conditions and deadlocks

Rule of thumb: use Pure Parallelism if you can, Concurrency otherwise.

Contents

1 Starting points

  • Control.Parallel. The first thing to start with parallel programming in Haskell is the use of par/pseq from the parallel library. Try the Real World Haskell chapter on parallelism and concurrency. The parallelism-specific parts are in the second half of the chapter.
  • If you need more control, try Strategies or perhaps the Par monad

2 Multicore GHC

Since 2004, GHC supports running programs in parallel on an SMP or multi-core machine. How to do it:

  • Compile your program using the -threaded switch.
  • Run the program with +RTS -N2 to use 2 threads, for example (RTS stands for runtime system; see the GHC users' guide). You should use a -N value equal to the number of CPU cores on your machine (not including Hyper-threading cores). As of GHC v6.12, you can leave off the number of cores and all available cores will be used (you still need to pass -N however, like so: +RTS -N).
  • Concurrent threads (forkIO) will run in parallel, and you can also use the par combinator and Strategies from the Control.Parallel.Strategies module to create parallelism.
  • Use +RTS -sstderr for timing stats.
  • To debug parallel program performance, use ThreadScope.

3 Alternative approaches

  • Nested data parallelism: a parallel programming model based on bulk data parallelism, in the form of the DPH and Repa libraries for transparently parallel arrays.
  • Intel Concurrent Collections for Haskell: a graph-oriented parallel programming model.

4 See also