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10.5 Places

+Parallelism with Places in The Racket Guide introduces places.

The bindings documented in this section are provided by the racket/place and racket libraries, but not racket/base.

Places enable the development of parallel programs that take advantage of machines with multiple processors, cores, or hardware threads.

Currently, parallel support for places is enabled only for Racket 3m (which is the main variant of Racket), and only by default for Windows, Linux x86/x86_64, and Mac OS X x86/x86_64. To enable support for other platforms, use --enable-places with configure when building Racket. The place-enabled? function reports whether places run in parallel.

A place is a parallel task that is effectively a separate instance of the Racket virtual machine. Places communicate through place channels, which are endpoints for a two-way buffered communication.

To a first approximation, place channels support only immutable, transparent values as messages. In addition, place channels themselves can be sent across channels to establish new (possibly more direct) lines of communication in addition to any existing lines. Finally, mutable values produced by shared-flvector, make-shared-flvector, shared-fxvector, make-shared-fxvector, shared-bytes, and make-shared-bytes can be sent across place channels; mutation of such values is visible to all places that share the value, because they are allowed in a shared memory space. See place-message-allowed?.

A place channel can be used as a synchronizable event (see Events) to receive a value through the channel. A place channel is ready for synchronization when a message is available on the channel, and the place channel’s synchronization result is the message (which is removed on synchronization). A place can also receive messages with place-channel-get, and messages can be sent with place-channel-put.

Two place channels are equal? if they are endpoints for the same underlying channels while both or neither is a place descriptor. Place channels can be equal? without being eq? after being sent messages through a place channel.

Constraints on messages across a place channel—and therefore on the kinds of data that places share—enable greater parallelism than future, even including separate garbage collection of separate places. At the same time, the setup and communication costs for places can be higher than for futures.

For example, the following expression launches two places, echoes a message to each, and then waits for the places to terminate:

(let ([pls (for/list ([i (in-range 2)])
              (dynamic-place "place-worker.rkt" 'place-main))])
   (for ([i (in-range 2)]
         [p pls])
      (place-channel-put p i)
      (printf "~a\n" (place-channel-get p)))
   (map place-wait pls))

The "place-worker.rkt" module must export the place-main function that each place executes, where place-main must accept a single place channel argument:

#lang racket
(provide place-main)
(define (place-main pch)
  (place-channel-put pch (format "Hello from place ~a"
                                  (place-channel-get pch))))


(place-enabled?)  boolean?

Returns #t if Racket is configured so that dynamic-place and place create places that can run in parallel, #f if dynamic-place and place are simulated using thread.


(place? v)  boolean?

  v : any/c
Returns #t if v is a place descriptor value, #f otherwise. Every place descriptor is also a place channel.


(place-channel? v)  boolean?

  v : any/c
Returns #t if v is place channel, #f otherwise.


(dynamic-place module-path start-name)  place?

  module-path : (or/c module-path? path?)
  start-name : symbol?
Creates a place to run the procedure that is identified by module-path and start-name. The result is a place descriptor value that represents the new parallel task; the place descriptor is returned immediately. The place descriptor value is also a place channel that permits communication with the place.

The module indicated by module-path must export a function with the name start-proc. The function must accept a single argument, which is a place channel that corresponds to the other end of communication for the place descriptor returned by place.

When the place is created, the initial exit handler terminates the place, using the argument to the exit handler as the place’s completion value. Use (exit v) to immediately terminate a place with the completion value v. Since a completion value is limited to an exact integer between 0 and 255, any other value for v is converted to 0.

If the function indicated by module-path and start-proc returns, then the place terminates with the completion value 0.

In the created place, the current-input-port parameter is set to an empty input port, while the values of the current-output-port and current-error-port parameters are connected to the current ports in the creating place. If the output ports in the creating place are file-stream ports, then the connected ports in the created place share the underlying streams, otherwise a thread in the creating place pumps bytes from the created place’s ports to the current ports in the creating place.

The module-path argument must not be a module path of the form (quote sym) unless the module is predefined (see module-predefined?).


(dynamic-place* module-path    
  [#:in in    
  #:out out    
  #:err err])  
(or/c output-port? #f)
(or/c input-port? #f)
(or/c input-port? #f)
  module-path : (or/c module-path? path?)
  start-name : symbol?
  in : (or/c input-port? #f) = #f
  out : (or/c output-port? #f) = (current-output-port)
  err : (or/c output-port? #f) = (current-error-port)
Like dynamic-place, but accepts specific ports to the new place’s ports, and returns a created port when #f is supplied for a port. The in, out, and err ports are connected to the current-input-port, current-output-port, and current-error-port ports, respectively, for the place. Any of the ports can be #f, in which case a file-stream port (for an operating-system pipe) is created and returned by dynamic-place*. The err argument can be 'stdout, in which case the same file-stream port or that is supplied as standard output is also used for standard error. For each port or 'stdout that is provided, no pipe is created and the corresponding returned value is #f.

The caller of dynamic-place* is responsible for closing all returned ports; none are closed automatically.

The dynamic-place* procedure returns four values:


(place id body ...+)

Creates a place that evaluates body expressions with id bound to a place channel. The bodys close only over id plus the top-level bindings of the enclosing module, because the bodys are lifted to a function that is exported by the module. The result of place is a place descriptor, like the result of dynamic-place.


(place* maybe-port ...
        body ...+)
maybe-port = 
  | #:in in-expr
  | #:out out-expr
  | #:err err-expr
Like place, but supports optional #:in, #:out, and #:err expressions (at most one of each) to specify ports in the same way and with the same defaults as dynamic-place*. The result of a place* form is also the same as for dynamic-place*.


(place-wait p)  exact-integer?

  p : place?
Returns the completion value of the place indicated by p, blocking until the place has terminated.

If any pumping threads were created to connect a non-file-stream port to the ports in the place for p (see dynamic-place), place-wait returns only when the pumping threads have completed.


(place-dead-evt p)  evt?

  p : place?
Returns a synchronizable event (see Events) that is ready for synchronization if and only if p has terminated. The synchronization result of a place-dead event is the place-dead event itself.

If any pumping threads were created to connect a non-file-stream port to the ports in the place for p (see dynamic-place), the event returned by place-dead-evt may become ready even if a pumping thread is still running.


(place-kill p)  void?

  p : place?
Immediately terminates the place, setting the place’s completion value to 1 if the place does not have a completion value already.


(place-break p [kind])  void?

  p : place?
  kind : (or/c #f 'hang-up 'terminate) = #f
Sends the main thread of place p a break; see Breaks.

Returns two place channels. Data sent through the first channel can be received through the second channel, and data sent through the second channel can be received from the first.

Typically, one place channel is used by the current place to send messages to a destination place; the other place channel is sent to the destination place (via an existing place channel).


(place-channel-put pch v)  void

  pch : place-channel?
  v : place-message-allowed?
Sends a message v on channel pch. Since place channels are asynchronous, place-channel-put calls are non-blocking.

See place-message-allowed? form information on automatic coercions in v, such as converting a mutable string to an immutable string.

Returns a message received on channel pch, blocking until a message is available.


(place-channel-put/get pch v)  any/c

  pch : place-channel?
  v : any/c
Sends an immutable message v on channel pch and then waits for a message (perhaps a reply) on the same channel.


(place-message-allowed? v)  boolean?

  v : any/c
Returns #t if v is allowed as a message on a place channel, #f otherwise.

If (place-enabled?) returns #f, then the result is always #t and no conversions are performed on v as a message. Otherwise, the following kinds of data are allowed as messages: