Difference between revisions of "Concurrency"

Revision as of 12:02, 9 May 2024

Note: you may want to read Parallelism vs. Concurrency, as the terms have historically been conflated.

This page contains notes and information about how to write concurrent programs in Haskell.

For practicality, the content is GHC-centric at the moment, although this may change as Haskell evolves.

Overview

GHC provides multi-scale support for concurrent programming, from very fine-grained, small tasks to coarse-grained explicit threads and locks, along with other models of concurrent programming including actors, CSP-style concurrency, and Intel Concurrent Collections. Synchronization between tasks is possible via messages, regular Haskell variables, MVar-based shared state or transactional memory.

Getting started

Manage simultaneous I/O actions (eg. multiple connections on a web server)
Start with Concurrent Haskell (forkIO, MVar)

Learn more about concurrency, then try using network protocol libraries like HTTP or zeromq.
Work with clusters or do distributed programming
Look out for ongoing research into distributed 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.

Community

Ask questions on Haskell Cafe
See what concurrent-haskell researchers and developers are working on
Follow @concurrenthaskell on Twitter
StackOverflow on Haskell: concurrency

Tools

ThreadScope - concurrent programs not working as expected? Use the ThreadScope debugger and watch the fireworks.
Various libraries, including those for concurrency.

Documentation

Glossary
Learning to use concurrency in Haskell
Current research
[... Concurrent Programming in Haskell] (online book)

Alternative approaches

CHP: CSP-style concurrency for Haskell.

@@ Line 1: / Line 1: @@
+''Note: you may want to read [[Parallelism vs. Concurrency]], as the terms have historically been conflated.''
-[[Category:GHC|Concurrency]]
-[[Category:Parallel]]
-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.
+This page contains notes and information about how to write concurrent programs in Haskell.
 For practicality, the content is GHC-centric at the moment, although this may change as Haskell evolves.
@@ Line 8: / Line 7: @@
 == 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.
+GHC provides multi-scale support for concurrent programming, from very fine-grained, small tasks to coarse-grained explicit threads and locks, along with other models of concurrent programming including actors, CSP-style concurrency, and Intel Concurrent Collections. Synchronization between tasks is possible via messages, regular Haskell variables, <code>MVar</code>-based shared state or transactional memory.
 == Getting started ==
+# Manage simultaneous I/O actions (eg. multiple connections on a web server)
-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.
+#: Start with Concurrent Haskell (<code>forkIO</code>, <code>MVar</code>)
+#: [[Parallel/Reading|Learn more about concurrency]], then try using [[Applications_and_libraries/Network#Libraries|network protocol libraries]] like HTTP or zeromq.
-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.
+# Work with clusters or do distributed programming
-From there, try the [[Parallel/Reading|reading list for parallelism in Haskell]].
+#: Look out for [[Parallel/Research|ongoing research]] into distributed Haskell.
 == Digging deeper ==
@@ Line 29: / Line 28: @@
 You get access to concurrency operations by importing the library [http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent.html Control.Concurrent].
+== Community ==
-{{GHC/Multicore}}
+* Ask questions on [[Mailing lists|Haskell Cafe]]
-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]].
+* See what [https://groups.google.com/group/concurrent-haskell concurrent-haskell] researchers and developers are working on
+* Follow [http://twitter.com/#!/concurrenthaskell @concurrenthaskell] on Twitter [[image:Twitter-mini.png]]
+* StackOverflow on Haskell: [http://stackoverflow.com/questions/tagged/haskell%2bconcurrency concurrency]
+== Tools ==
+* [[ThreadScope]] - concurrent programs not working as expected? Use the ThreadScope debugger and watch the fireworks.
+* Various [[Applications and libraries/Concurrency and parallelism|libraries]], including those for concurrency.
+== Documentation ==
+* [[Parallel/Glossary|Glossary]]
+* [[Parallel/Reading|Learning to use concurrency in Haskell]]
+* [[Parallel/Research|Current research]]
+* [http://chimera.labs.oreilly.com/books/1230000000929/index.html [...] Concurrent Programming in Haskell] (online book)
 == Alternative approaches ==
@@ Line 39: / Line 53: @@
 == See also ==
+* [[:Category:Concurrency|Concurrency]] category
-* [[Parallel]] portal
-* Parallelism and concurrency [[Parallel/Research|research]]
+* Concurrency [[Parallel/Research|research]]
+[[Category:GHC]]
+[[Category:Concurrency]]