Difference between revisions of "Concurrency"
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[[Category:Parallel]] |
[[Category:Parallel]] |
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| − | == Concurrent Programming in GHC == |
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| + | For practicality, the content is GHC-centric at the moment, although this may change as Haskell evolves. |
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| ⚫ | |||
| + | == Overview == |
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| ⚫ | GHC provides multi-scale support for parallel programming, from very fine-grained, small "sparks", to coarse-grained explicit threads and locks, along with other models of concurrent and parallel programming, including actors, CSP-style concurrency, nested data parallelism and Intel Concurrent Collections. Synchronization between tasks is possible via messages, regular Haskell variables, MVar shared state or transactional memory. |
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| ⚫ | GHC provides multi-scale support for parallel and concurrent programming, from very fine-grained, small "sparks", to coarse-grained explicit threads and locks, along with other models of concurrent and parallel programming, including actors, CSP-style concurrency, nested data parallelism and Intel Concurrent Collections. Synchronization between tasks is possible via messages, regular Haskell variables, MVar shared state or transactional memory. |
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| − | |||
| − | The concurrent programming models in GHC can be divided into the following forms: |
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| + | == Getting started == |
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| − | * Lightweight Haskell threads, explicit synchronization with STM or MVars. See the paper "Tackling the Awkward Squad" below. |
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| − | The most important (as of 2010) to get to know are the basic "concurrent Haskell" model of threads using forkIO and MVars, the use of transactional memory via STM. |
+ | The most important (as of 2010) to get to know are the basic "concurrent Haskell" model of threads using forkIO and MVars, the use of transactional memory via STM. |
| + | See "[https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.13.9123&rep=rep1&type=pdf Tackling the Awkward Squad]" to get started. |
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| − | === Starting points === |
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| ⚫ | |||
| − | * '''Basic concurrency: forkIO and MVars'''. |
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| ⚫ | |||
| ⚫ | : STM was added to GHC 6.4, and is described in the paper [ |
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| + | == Digging deeper == |
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| ⚫ | * '''Foreign function interface'''. If you are calling foreign functions in a concurrent program, you need to know about ''bound threads''. They are described in a Haskell workshop paper, [ |
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| ⚫ | |||
| ⚫ | |||
| ⚫ | : STM was added to GHC 6.4, and is described in the paper [https://citeseerx.ist.psu.edu/viewdoc/download Composable memory transactions]. The paper [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.64.1678&rep=rep1&type=pdf Lock-free data structures using STM in Haskell] gives further examples of concurrent programming using STM. |
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| ⚫ | * '''Foreign function interface'''. If you are calling foreign functions in a concurrent program, you need to know about ''bound threads''. They are described in a Haskell workshop paper, [https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.80.4811&rep=rep1&type=pdf Extending the Haskell Foreign Function Interface with Concurrency]. The GHC Commentary [http://darcs.haskell.org/ghc/docs/comm/rts-libs/multi-thread.html Supporting multi-threaded interoperation] contains more detailed explanation of cooperation between FFI calls and multi-threaded runtime. |
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| − | * The GHC manual gives a few useful flags that control scheduling (not usually necessary) [http://www.haskell.org/ghc/docs/latest/html/users_guide/sec-using-parallel.html#parallel-rts-opts RTS options]. |
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| ⚫ | |||
| − | === Multicore GHC === |
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{{GHC/Multicore}} |
{{GHC/Multicore}} |
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| + | Support for low-level parallelism features of modern processors is slowly coming along. As of version 7.8, GHC includes the ability to [https://ghc.haskell.org/trac/ghc/wiki/SIMD emit SIMD instructions], and also has a rudimentary ability to use [[AtomicMemoryOps|atomic memory operations]]. |
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| − | === Related work === |
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| + | |||
| + | == Alternative approaches == |
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| + | |||
| ⚫ | |||
| + | |||
| + | == See also == |
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| + | * [[Parallel]] portal |
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| − | * The Sun project to improve http://ghcsparc.blogspot.com/ GHC performance on Sparc] |
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| + | * Parallelism and concurrency [[Parallel/Research|research]] |
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| − | * A [http://www.well-typed.com/blog/38 Microsoft project to improve industrial applications of GHC parallelism]. |
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| − | * [http://www.haskell.org/~simonmar/bib/bib.html Simon Marlow's publications on parallelism and GHC] |
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| − | * [http://www.macs.hw.ac.uk/~dsg/gdh/ Glasgow Distributed Haskell] |
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| − | * http://www-i2.informatik.rwth-aachen.de/~stolz/dhs/ |
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| − | * http://www.informatik.uni-kiel.de/~fhu/PUBLICATIONS/1999/ifl.html |
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| − | * [http://www.mathematik.uni-marburg.de/~eden Eden] |
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Revision as of 06:28, 1 September 2022
This page contains notes and information about how to write concurrent programs in Haskell. If you're more interested in performance than non-determinism, learn about parallelism first.
For practicality, the content is GHC-centric at the moment, although this may change as Haskell evolves.
Overview
GHC provides multi-scale support for parallel and concurrent programming, from very fine-grained, small "sparks", to coarse-grained explicit threads and locks, along with other models of concurrent and parallel programming, including actors, CSP-style concurrency, nested data parallelism and Intel Concurrent Collections. Synchronization between tasks is possible via messages, regular Haskell variables, MVar shared state or transactional memory.
Getting started
The most important (as of 2010) to get to know are the basic "concurrent Haskell" model of threads using forkIO and MVars, the use of transactional memory via STM.
See "Tackling the Awkward Squad" to get started.
From there, try the reading list for parallelism in Haskell.
Digging deeper
- Software Transactional Memory (STM) is a newer way to coordinate concurrent threads. There's a separate Wiki page devoted to STM.
- STM was added to GHC 6.4, and is described in the paper Composable memory transactions. The paper Lock-free data structures using STM in Haskell gives further examples of concurrent programming using STM.
- Foreign function interface. If you are calling foreign functions in a concurrent program, you need to know about bound threads. They are described in a Haskell workshop paper, Extending the Haskell Foreign Function Interface with Concurrency. The GHC Commentary Supporting multi-threaded interoperation contains more detailed explanation of cooperation between FFI calls and multi-threaded runtime.
GHC concurrency specifics
You get access to concurrency operations by importing the library Control.Concurrent.
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.
Support for low-level parallelism features of modern processors is slowly coming along. As of version 7.8, GHC includes the ability to emit SIMD instructions, and also has a rudimentary ability to use atomic memory operations.
Alternative approaches
- CHP: CSP-style concurrency for Haskell.