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diff --git a/docs/users_guide/parallel.xml b/docs/users_guide/parallel.xml
index 8b5abdb..92bad19 100644
--- a/docs/users_guide/parallel.xml
+++ b/docs/users_guide/parallel.xml
@@ -33,12 +33,12 @@
url="http://research.microsoft.com/copyright/accept.asp?path=/users/simonpj/papers/concurrent-haskell.ps.gz">
Concurrent Haskell paper is still an excellent
resource, as is Tackling
+ url="http://research.microsoft.com/%7Esimonpj/papers/marktoberdorf/">Tackling
the awkward squad.
To the programmer, Concurrent Haskell introduces no new language constructs;
rather, it appears simply as a library,
+ url="&libraryBaseLocation;/Control-Concurrent.html">
Control.Concurrent. The functions exported by this
library include:
@@ -61,9 +61,9 @@ the FFI with concurrency.
papers are an excellent introduction to what STM is, and how to use
it.
- The main library you need to use STM is
- Control.Concurrent.STM. The main features supported are these:
+ The main library you need to use is the
+ stm library. The main features supported are these:
Atomic blocks.
Transactional variables.
@@ -109,15 +109,15 @@ All these features are described in the papers mentioned earlier.
One way to do so is forking threads using Concurrent Haskell (), but the simplest mechanism for extracting parallelism from pure code is
to use the par combinator, which is closely related to (and often used
- with) seq. Both of these are available from Control.Parallel:
+ with) seq. Both of these are available from the parallel library:
infixr 0 `par`
-infixr 1 `seq`
+infixr 1 `pseq`
-par :: a -> b -> b
-seq :: a -> b -> b
+par :: a -> b -> b
+pseq :: a -> b -> b
The expression (x `par` y)
sparks the evaluation of x
@@ -136,24 +136,35 @@ import Control.Parallel
nfib :: Int -> Int
nfib n | n <= 1 = 1
- | otherwise = par n1 (seq n2 (n1 + n2 + 1))
+ | otherwise = par n1 (pseq 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 to evaluate nfib (n-1),
- and then we use seq to force the
+ and then we use pseq 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
+ pseq to ensure that the parent will evaluate
n2 before 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.
+
+ Note that we use pseq rather
+ than seq. The two are almost equivalent, but
+ differ in their runtime behaviour in a subtle
+ way: seq can evaluate its arguments in either
+ order, but pseq is required to evaluate its
+ first argument before its second, which makes it more suitable
+ for controlling the evaluation order in conjunction
+ with par.
+
+
When using par, the general rule of thumb is that
the sparked computation should be required at a later time, but not too
soon. Also, the sparked computation should not be too small, otherwise
@@ -161,19 +172,31 @@ nfib n | n <= 1 = 1
amount of parallelism gained. Getting these factors right is tricky in
practice.
+ It is possible to glean a little information about how
+ well par is working from the runtime
+ statistics; see .
+
More sophisticated combinators for expressing parallelism are
- available from the Control.Parallel.Strategies module.
+ available from the Control.Parallel.Strategies
+ module in the parallel package.
This module builds functionality around par,
expressing more elaborate patterns of parallel computation, such as
parallel map.
+Data Parallel Haskell
+ GHC includes experimental support for Data Parallel Haskell (DPH). This code
+ is highly unstable and is only provided as a technology preview. More
+ information can be found on the corresponding DPH
+ wiki page.
+
+