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guide: small edits to concurrency chapter

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Also, split sections on futures and places into a "parallelism" chapter.
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Matthew Flatt 2013-11-02 12:32:02 -06:00
parent c692f13200
commit a5c655c716
7 changed files with 129 additions and 57 deletions
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154
pkgs/racket-pkgs/racket-doc/scribblings/guide/concurrency.scrbl
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@ -1,16 +1,29 @@
#lang scribble/doc
@(require scribble/manual scribble/eval "guide-utils.rkt")
@(require scribble/manual
scribble/eval
"guide-utils.rkt"
(for-label racket))
@(define concurrency-eval (make-base-eval))
@(define reference-doc '(lib "scribblings/reference/reference.scrbl"))
@title[#:tag "concurrency"]{Concurrency, Threads, and Synchronization}
@title[#:tag "concurrency"]{Concurrency and Synchronization}
@section{Thread basics}
Racket provides @deftech{concurrency} in the form of
@deftech{threads}, and it provides a general @racket[sync] function
that can be used to synchronize both threads and other implicit forms of
concurrency, such as @tech{ports}.
To execute a procedure concurrrently, use @racket[thread]. This example
creates 2 new threads from the main thread:
Threads run concurrently in the sense that one thread can preempt
another without its cooperation, but threads do not run in parallel in
the sense of using multiple hardware processors. See
@secref["parallelism"] for information on parallelism in Racket.
@section{Threads}
To execute a procedure concurrrently, use @racket[thread]. The
following example creates two new threads from the main thread:
@racketblock[
(displayln "This is the original thread")
@ -36,8 +49,8 @@ uses @racket[kill-thread] to terminate the worker thread:
clicked, so in DrRacket the @racket[thread-wait] is not necessary.}
If the main thread finishes or is killed, the application exits, even if
other threads were still running. A thread can use @racket[thread-wait] to
wait for another thread to finish. Here the main thread uses
other threads are still running. A thread can use @racket[thread-wait] to
wait for another thread to finish. Here, the main thread uses
@racket[thread-wait] to make sure the worker thread finishes before the main
thread exits:
@ -52,9 +65,9 @@ thread exits:
@section{Thread Mailboxes}
Each thread has a mailbox for receiving messages. @racket[thread-send]
Each thread has a mailbox for receiving messages. The @racket[thread-send] function
asynchronously sends a message to another thread's mailbox, while
@racket[thread-receive] will return the oldest message from the current
@racket[thread-receive] returns the oldest message from the current
thread's mailbox, blocking to wait for a message if necessary. In the
following example, the main thread sends data to the worker thread to be
processed, then sends a @racket['done] message when there is no more data and
@ -76,7 +89,7 @@ waits for the worker thread to finish.
(thread-wait worker-thread)
]
In the next example the main thread delegates work to multiple arithmetic
In the next example, the main thread delegates work to multiple arithmetic
threads, then waits to receive the results. The arithmetic threads process work
items then send the results to the main thread.
@ -87,7 +100,10 @@ items then send the results to the main thread.
(match (thread-receive)
[(list oper1 oper2 result-thread)
(thread-send result-thread
(format "~a + ~a = ~a" oper1 oper2 (operation oper1 oper2)))
(format "~a + ~a = ~a"
oper1
oper2
(operation oper1 oper2)))
(loop)])))))
(define addition-thread (make-arithmetic-thread +))
@ -97,9 +113,11 @@ items then send the results to the main thread.
(for ([item worklist])
(match item
[(list '+ o1 o2)
(thread-send addition-thread (list o1 o2 (current-thread)))]
(thread-send addition-thread
(list o1 o2 (current-thread)))]
[(list '- o1 o2)
(thread-send subtraction-thread (list o1 o2 (current-thread)))]))
(thread-send subtraction-thread
(list o1 o2 (current-thread)))]))
(for ([i (length worklist)])
(displayln (thread-receive)))
@ -114,10 +132,10 @@ single resource.
In the following example, multiple threads print to standard output
concurrently. Without synchronization, a line printed by one thread might
appear in the middle of a line printed by another thread. By using a semaphore
initialized with a count of 1, only 1 thread will print at a time.
@racket[semaphore-wait] blocks until the semaphore's internal counter is
non-zero, then decrements the counter and returns. @racket[semaphore-post]
increments the counter so that another thread can unblock then print.
initialized with a count of @racket[1], only one thread will print at a time.
The @racket[semaphore-wait] function blocks until the semaphore's internal counter is
non-zero, then decrements the counter and returns. The @racket[semaphore-post] function
increments the counter so that another thread can unblock and then print.
@racketblock[
(define output-semaphore (make-semaphore 1))
@ -132,8 +150,27 @@ increments the counter so that another thread can unblock then print.
(for-each thread-wait threads)
]
Semaphores are a low-level technique. Often a better solution is to restrict
resource access to a single thread. For instance, synchronizing access to
The pattern of waiting on a semaphore, working, and posting to the
semaphore can also be expressed using
@racket[call-with-semaphore],which has the advantage of posting to the
semaphore if control escapes (e.g., due to an exception):
@racketblock[
(define output-semaphore (make-semaphore 1))
(define (make-thread name)
(thread (lambda ()
(for [(i 10)]
(call-with-semaphore
output-semaphore
(lambda ()
(printf "thread ~a: ~a~n" name i)))))))
(define threads
(map make-thread '(A B C)))
(for-each thread-wait threads)
]
Semaphores are a low-level technique. Often, a better solution is to restrict
resource access to a single thread. For example, synchronizing access to
standard output might be better accomplished by having a dedicated thread for
printing output.
@ -145,10 +182,10 @@ channel, so channels should be used when multiple threads need to consume items
from a single work queue.
In the following example, the main thread adds items to a channel using
@racket[channel-put] while multiple worker threads consume those items using
@racket[channel-get]. Each call to either procedure may block until another
@racket[channel-put], while multiple worker threads consume those items using
@racket[channel-get]. Each call to either procedure blocks until another
thread calls the other procedure with the same channel. The workers process
the items and then pass their results to the result-thread via the result-channel.
the items and then pass their results to the result thread via the @racket[result-channel].
@racketblock[
(define result-channel (make-channel))
@ -183,17 +220,20 @@ the items and then pass their results to the result-thread via the result-channe
@section{Buffered Asynchronous Channels}
Buffered asynchronous channels are similar to the channels described above, but
the put operation of asynchronous channels does not block unless the given
the ``put'' operation of asynchronous channels does not block---unless the given
channel was created with a buffer limit and the limit has been reached. The
asynchronous put operation is therefore somewhat similar to
asynchronous-put operation is therefore somewhat similar to
@racket[thread-send], but unlike thread mailboxes, asynchronous channels allow
multiple threads to consume items from a single channel. In the following
multiple threads to consume items from a single channel.
In the following
example, the main thread adds items to the work channel, which holds a maximum
of 3 items at a time. The worker threads process items from this channel and
of three items at a time. The worker threads process items from this channel and
then send results to the print thread.
@racketblock[
(require racket/async-channel)
(define print-thread
(thread (lambda ()
(let loop ()
@ -202,6 +242,7 @@ then send results to the print thread.
(define (safer-printf . items)
(thread-send print-thread
(apply format items)))
(define work-channel (make-async-channel 3))
(define (make-worker-thread thread-id)
(thread
@ -210,6 +251,7 @@ then send results to the print thread.
(define item (async-channel-get work-channel))
(safer-printf "Thread ~a processing item: ~a" thread-id item)
(loop)))))
(for-each make-worker-thread '(1 2 3))
(for ([item '(a b c d e f g h i j k l m)])
(async-channel-put work-channel item))
@ -220,16 +262,17 @@ processed. If the main thread were to exit without such synchronization, it is
possible that the worker threads will not finish processing some items or the
print thread will not print all items.
@section{Synchronizable events and sync}
@section{Synchronizable Events and @racket[sync]}
There are other ways to synchronize threads. The @racket[sync] function allows
threads to coordinate via @tech[#:doc reference-doc]{synchronizable events}s.
Many types double as events, allowing a uniform way to synchronize threads
using different types. Examples include channels, ports, and alarms.
threads to coordinate via @tech[#:doc reference-doc]{synchronizable events}.
Many values double as events, allowing a uniform way to synchronize threads
using different types. Examples of events include channels, ports, threads,
and alarms.
In the next example, a channel and an alarm are used as synchronizable events.
The workers sync on both in order to process the channel items up until the
alarm is activated. The channel items are processed and then results sent back
The workers @racket[sync] on both so that they can process channel items until the
alarm is activated. The channel items are processed, and then results are sent back
to the main thread.
@racketblock[
@ -264,17 +307,19 @@ to the main thread.
The next example shows a function for use in a simple TCP echo server. The
function uses @racket[sync/timeout] to synchronize on input from the given port
or a message in the thread's mailbox. @racket[sync/timeout]'s first argument
specifies the maximum number of seconds it should wait on the given events.
@racket[read-line-evt] returns an event that is ready when a line of input is
available in the given input port. @racket[thread-receive-evt] is ready when
or a message in the thread's mailbox. The first argument to @racket[sync/timeout]
specifies the maximum number of seconds it should wait on the given events. The
@racket[read-line-evt] function returns an event that is ready when a line of input is
available in the given input port. The result of @racket[thread-receive-evt] is ready when
@racket[thread-receive] would not block. In a real application, the messages
received in the thread mailbox could be used for control messages, etc.
@racketblock[
(define (serve in-port out-port)
(let loop []
(define evt (sync/timeout 2 (read-line-evt in-port 'any) (thread-receive-evt)))
(define evt (sync/timeout 2
(read-line-evt in-port 'any)
(thread-receive-evt)))
(cond
[(not evt)
(displayln "Timed out, exiting")
@ -285,18 +330,19 @@ received in the thread mailbox could be used for control messages, etc.
(flush-output out-port)
(loop)]
[else
(printf "Received a message in mailbox: ~a~n" (thread-receive))
(printf "Received a message in mailbox: ~a~n"
(thread-receive))
(loop)])))
]
The previous example could be used in a program like the following. This code
starts a server thread and a client thread which communicate over TCP. The
client prints 3 lines to the server, which echoes them back. The client's
copy-port call will block until EOF is received. The server will timeout after
2 seconds, closing the ports which then allows copy-port to finish and the
client will exit. The main thread uses @racket[thread-wait] to wait for the
client-thread to exit. Without @racket[thread-wait], the main thread might
exit before the other threads are finished.
The @racket[serve] function is used in the following example, which
starts a server thread and a client thread that communicate over TCP. The
client prints three lines to the server, which echoes them back. The client's
@racket[copy-port] call blocks until EOF is received. The server times out after
two seconds, closing the ports, which allows @racket[copy-port] to finish and the
client to exit. The main thread uses @racket[thread-wait] to wait for the
client thread to exit (since, without @racket[thread-wait], the main thread might
exit before the other threads are finished).
@racketblock[
(define port-num 4321)
@ -321,13 +367,13 @@ exit before the other threads are finished.
(thread-wait client-thread)
]
Sometimes you want to attach result behavior directly to the event passed to
@racket[sync]. In the following example, the worker thread synchronizes on 3
Sometimes, you want to attach result behavior directly to the event passed to
@racket[sync]. In the following example, the worker thread synchronizes on three
channels, but each channel must be handled differently. Using
@racket[handle-evt] associates a callback with the given event. When
@racket[sync] selects the given event, it calls the callback to generate the
synchronization result, rather than using the event's normal synchronization
result. Because the event is handled in the callback, there is no need to
result. Since the event is handled in the callback, there is no need to
dispatch on the return value of @racket[sync].
@racketblock[
@ -362,8 +408,9 @@ dispatch on the return value of @racket[sync].
(channel-put append-channel '("a" "b"))
]
@racket[handle-evt] invokes the callback in tail position, so it is safer to
use recursion, as in the following example.
The result of @racket[handle-evt] invokes its callback in tail position
with respect to @racket[sync], so it is safe to
use recursion as in the following example.
@racketblock[
(define control-channel (make-channel))
@ -393,3 +440,8 @@ use recursion, as in the following example.
(thread-wait worker)
]
The @racket[wrap-evt] function is like @racket[handle-evt], except
that its handler is not called in tail position with respect to
@racket[sync]. At the same time, @racket[wrap-evt] disables break
exceptions during its handler's invocation.

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@ -10,7 +10,7 @@
@title[#:tag "effective-futures"]{Parallelism with Futures}
The @racketmodname[racket/future] library provides support for
performance improvement through parallelism with the @racket[future]
performance improvement through parallelism with @deftech{futures} and the @racket[future]
and @racket[touch] functions. The level of parallelism available from
those constructs, however, is limited by several factors, and the
current implementation is best suited to numerical tasks.

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@ -55,6 +55,8 @@ precise details to @|Racket| and other reference manuals.
@include-section["performance.scrbl"]
@include-section["parallelism.scrbl"]
@include-section["running.scrbl"]
@include-section["other.scrbl"]

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@ -0,0 +1,20 @@
#lang scribble/doc
@(require scribble/manual "guide-utils.rkt"
(for-label racket/flonum
racket/unsafe/ops
racket/performance-hint))
@title[#:tag "parallelism"]{Parallelism}
Racket provides two forms of @deftech{parallelism}: @tech{futures} and
@tech{places}. On a platform that provides multiple processors,
parallelism can improve the run-time performance of a program.
See also @secref["performance"] for information on sequential
performance in Racket. Racket also provides threads for
@tech{concurrency}, but threads do not provide parallelism; see
@secref["concurrency"] for more information.
@include-section["futures.scrbl"]
@include-section["places.scrbl"]
@include-section["distributed.scrbl"]

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@ -520,8 +520,3 @@ occurrence of the variable @racket[_fishes]. That constitutes
a reference to the list, ensuring that the list is not itself
garbage collected, and thus the red fish is not either.
@; ----------------------------------------------------------------------
@include-section["futures.scrbl"]
@include-section["places.scrbl"]
@include-section["distributed.scrbl"]

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@ -3,6 +3,8 @@
@title[#:tag "threads"]{Threads}
@guideintro["concurrency"]{threads}
See @secref["thread-model"] for basic information on the Racket
thread model. See also @secref["futures"].

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racket/collects/acks/acks.rkt
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@ -53,6 +53,7 @@
"Mike T. McHenry, "
"Philippe Meunier, "
"Scott Owens, "
"David T. Pierson, "
"Jon Rafkind, "
"Jamie Raymond, "
"Grant Rettke, "