Concurrent and Parallel Haskell have very different purposes.
Concurrent Haskell is for applications which have an inherent structure of interacting, concurrent tasks (i.e. `threads'). Threads in such programs may be required. For example, if a concurrent thread has been spawned to handle a mouse click, it isn't optional—the user wants something done!
A Concurrent Haskell program implies multiple `threads' running within a single Unix process on a single processor.
You will find at least one paper about Concurrent Haskell hanging off of Simon Peyton Jones's Web page.
Parallel Haskell is about speed—spawning threads onto multiple processors so that your program will run faster. The `threads' are always advisory—if the runtime system thinks it can get the job done more quickly by sequential execution, then fine.
A Parallel Haskell program implies multiple processes running on multiple processors, under a PVM (Parallel Virtual Machine) framework. An MPI interface is under development but not fully functional, yet.
Parallel Haskell is still relatively new; it is more about “research fun” than about “speed.” That will change.
Check the GPH Page for more information on “GPH” (Haskell98 with extensions for parallel execution), the latest version of “GUM” (the runtime system to enable parallel executions) and papers on research issues. A list of publications about GPH and about GUM is also available from Simon's Web Page.
Some details about Parallel Haskell follow. For more information
about concurrent Haskell, see the module
Control.Concurrent in the library documentation.
GHC provides two functions for controlling parallel execution, through
interface Parallel where infixr 0 `par` infixr 1 `seq` par :: a -> b -> b seq :: a -> b -> b
(x `par` y) sparks the evaluation of
(to weak head normal form) and returns
y. Sparks are queued for
execution in FIFO order, but are not executed immediately. At the
next heap allocation, the currently executing thread will yield
control to the scheduler, and the scheduler will start a new thread
(until reaching the active thread limit) for each spark which has not
already been evaluated to WHNF.
(x `seq` y) evaluates
x to weak head normal
form and then returns
seq primitive can be used to
force evaluation of an expression beyond WHNF, or to impose a desired
execution sequence for the evaluation of an expression.
For example, consider the following parallel version of our old
import Parallel nfib :: Int -> Int nfib n | n <= 1 = 1 | otherwise = par n1 (seq n2 (n1 + n2 + 1)) where n1 = nfib (n-1) n2 = nfib (n-2)
For values of
n greater than 1, we use
par to spark a thread
nfib (n-1), and then we use
seq to force the
parent thread to evaluate
nfib (n-2) before going on to add
together these two subexpressions. In this divide-and-conquer
approach, we only spark a new thread for one branch of the computation
(leaving the parent to evaluate the other branch). Also, we must use
seq to ensure that the parent will evaluate
n1 in the expression
(n1 + n2 + 1). It is not sufficient to
reorder the expression as
(n2 + n1 + 1), because the compiler may
not generate code to evaluate the addends from left to right.
seq are wired into GHC, and unfold
into uses of the
seq# primitives, respectively. If
you'd like to see this with your very own eyes, just run GHC with the
-ddump-simpl option. (Anything for a good time…)
Runnable threads are scheduled in round-robin fashion. Context
switches are signalled by the generation of new sparks or by the
expiry of a virtual timer (the timer interval is configurable with the
-C[<num>] RTS option). However, a context switch doesn't
really happen until the current heap block is full. You can't get any
faster context switching than this.
When a context switch occurs, pending sparks which have not already
been reduced to weak head normal form are turned into new threads.
However, there is a limit to the number of active threads (runnable or
blocked) which are allowed at any given time. This limit can be
adjusted with the
RTS option (the default is 32). Once the
thread limit is reached, any remaining sparks are deferred until some
of the currently active threads are completed.