10.5 Places

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.

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

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 can also receive messages with place-channel-get, and messages can be sent with place-channel-put.

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 lanches 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))))

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.

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).

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

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:

• numbers, characters, booleans, and #<void>;

• symbols that are interned;

• strings and byte strings, where mutable strings and byte strings are automatically replaced by immutable variants;

• pairs, lists, vectors, and immutable prefab structures containing message-allowed values, where a mutable vector is automatically replaced by an immutable vector;

• place channels, where a place descriptor is automatically replaced by a plain place channel; and

• values produced by shared-flvector, make-shared-flvector, shared-fxvector, make-shared-fxvector, shared-bytes, and make-shared-bytes.