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 the 3m (main) and CS variants of Racket, and only by default for Windows, Linux x86/x86_64, and Mac OS 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.
Implementation and operating-system constraints may limit the scalability of places. For example, although places can perform garbage collections independently in the 3m variant, a garbage collection may need to manipulate a page table that is shared across all places, and that shared page table can be a bottleneck with enough places—
perhaps around 8 or 16.
A place is a parallel task that is effectively a separate instance of the Racket virtual machine, although all places run within a single operating-system process. 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—
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 channels are subject to garbage collection, like other Racket values, and a thread that is blocked reading from a place channel can be garbage collected if place channel’s writing end becomes unreachable. However, unlike normal channel blocking, if otherwise unreachable threads are mutually blocked on place channels that are reachable only from the same threads, the threads and place channels are all considered reachable, instead of unreachable.
When a place is created, its parameter values are generally set to the initial values of the parameters in the creating place, except that the current values of the following parameters are used: current-library-collection-paths, current-library-collection-links, and current-compiled-file-roots.
A newly created place is registered with the current custodian, so that the place is terminated when the custodian is shut down.
(dynamic-place module-path start-name [ #:at location #:named named]) → place? module-path : (or/c module-path? path?) start-name : symbol? location : (or/c #f place-location?) = #f named : any/c = #f
The module indicated by module-path must export a function with the name start-name. 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-name 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.
(dynamic-place* module-path start-name [ #:in in #:out out #:err err]) →
place? (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)
The caller of dynamic-place* is responsible for closing all returned ports; none are closed automatically.
The dynamic-place* procedure returns four values:
a place descriptor value representing the created place;
an output port piped to the place’s standard input, or #f if in was a port;
an input port piped from the place’s standard output, or #f if out was a port;
an input port piped from the place’s standard error, or #f if err was a port or 'stdout.
(place id body ...+)
The generated submodule has the name place-body-n for an integer n, and the submodule exports a main function that takes a place channel for the new place. The submodule is not intended for use, however, except by the expansion of the place form.
(place* maybe-port ... id body ...+)
| #:in in-expr | #:out out-expr | #:err err-expr
(place/context id body ...+)
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.
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:
paths (for any platform);
pairs, lists, vectors, and immutable prefab structures containing message-allowed values, where a mutable vector is automatically replaced by an immutable vector and where impersonators of vectors and prefab structures are copied;
Place events are reported to a logger named 'place. In addition to its string message, each event logged for a place has a data value that is an instance of a place-event prefab structure:
The place-id field is an exact integer that identifies a place.
The action field is a symbol:
'create: a place was created. This event is logged in the creating place, and the event’s value field has the ID for the created place.
'reap: a place that was previously created in the current place has exited (and that fact has been detected, possibly via place-wait). The event’s value field has the ID for the exited place.
'enter: a place has started, logged within the started place. The event’s value field has #f.
'exit: a place is exiting, logged within the exiting place. The event’s value field has #f.
'put: a place-channel message has been sent. The event’s value field is a positive exact integer that approximates the message’s size.
'get: a place-channel message has been received. The event’s value field is a positive exact integer that approximates the message’s size.
Changed in version 220.127.116.11 of package base: Added logging via 'place and place-event.