11.5 Places (original) (raw)

11.5 Places🔗ℹ

Parallelism with Places in The Racket Guide introduces places.

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 on all platforms that support Racket CS, the default implementation of Racket. The 3m implementation also supports parallel execution of places by default on Windows, Linux x86/x86_64, and Mac OS x86/x86_64. To enable support for other platforms with 3m, use --enable-places withconfigure 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 in parallel in the CS implementation or independently in the 3m implementation, 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 throughplace 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 byshared-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 ashared 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’ssynchronization 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 aplace descriptor. Place channels can be equal?without being eq? after being sent messages through aplace channel.

Constraints on messages across a place channel—and therefore on the kinds of data that places share—enable greater parallelism thanfuture, 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:

The "place-worker.rkt" module (in a file that is separate from the above code) must export theplace-main function that each place executes, whereplace-main must accept a single place channelargument:

"place-worker.rkt"

Place channels are subject to garbage collection, like other Racket values, and a thread that is blocked reading from aplace 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, andcurrent-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.

11.5.1 Using Places🔗ℹ

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

Returns #t if v is a place descriptorvalue, #f otherwise. Every place descriptoris also a place channel.

Returns #t if v is place channel,#f otherwise.

Creates a place to run the procedure that is identified bymodule-path and start-name. The result is aplace 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-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.

If location is provided, it must be a place location, such as a distributed places node produced by create-place-node.

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 valuev. 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 andstart-name returns, then the place terminates with thecompletion value 0.

In the created place, the current-input-port parameter is set to an empty input port, while the values of thecurrent-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.

Most parameters in the created place have their original initial values, but the created place inherits the creating place’s values for the following parameters: current-directory,current-library-collection-paths,current-library-collection-links, and current-compiled-file-roots.

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

The dynamic-place binding is protected in the sense ofprotect-out, so access to this operation can be prevented by adjusting the code inspector (see Code Inspectors).

Changed in version 8.2.0.7 of package base: Changed created place to inherit the creating place’s current-directoryvalue.

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, anderr ports are connected to the current-input-port,current-output-port, and current-error-port ports, respectively, for theplace. Any of the ports can be #f, in which case afile-stream port (for an operating-system pipe) is created and returned by dynamic-place*. Theerr 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:

• 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.

The dynamic-place* binding is protected in the same way asdynamic-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.

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

Immediately terminates the place, setting the place’scompletion 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).

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.

Sends an immutable message v on channel pch and then waits for a message (perhaps a reply) on the same channel.

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:

• numbers, characters, booleans, keywords, and#;
• symbols, where the eq?ness of uninternedsymbols is preserved within a single message, but not across messages;
• strings and byte strings, where mutable strings and byte strings are automatically replaced by immutable variants;
• paths (for any platform);
• pairs, lists, boxes, vectors, and immutableprefab structures containing message-allowed values, where a mutable box is automatically replaced by an immutable box, a mutable vector is automatically replaced by an immutable vector and where impersonators of boxes, vectors andprefab structures are copied;
• hash tables where mutable hash tables are automatically replaced by immutable variants, and where a hash table impersonator is copied;
• place channels, where a place descriptor is automatically replaced by a plain place channel;
• file-stream ports and TCP ports, where the underlying representation (such as a file descriptor, socket, or handle) is duplicated in the sending place and attached to a fresh port in the receiving place;
• C pointers as created or accessed via ffi/unsafe; and
• values produced by shared-flvector,make-shared-flvector, shared-fxvector,make-shared-fxvector, shared-bytes, andmake-shared-bytes.

Changed in version 8.4.0.7 of package base: Include boxes in allowed messages.

A structure type property and associated predicate for implementations of place locations. The value ofprop:place-location must be a procedure of four arguments: the place location itself, a module path, a symbol for the start function exported by the module, and a place name (which can be#f for an anonymous place).

A place location can be passed as the #:at argument todynamic-place, which in turn simply calls theprop:place-location value of the place location.

A distributed places note created with create-place-nodeis an example of a place location.

11.5.2 Syntactic Support for Using Places🔗ℹ

The bindings in this section are not provided byracket/place/dynamic.

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

The generated submodule has the name place-body-nfor an integer n, and the submodule exports a mainfunction that takes a place channel for the new place. The submodule is not intended for use, however, except by the expansion of theplace form.

The place binding is protected in the same way asdynamic-place.

(place* maybe-port ... id 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*.

The place* binding is protected in the same way asdynamic-place.

(place/context id body ...+)

Like place, but body ... may have free lexical variables, which are automatically sent to the newly-created place. Note that these variables must have values accepted byplace-message-allowed?, otherwise an exn:fail:contract exception is raised.

Returns the number of parallel computation units (e.g., processors or cores) that are available on the current machine.

This is the same binding as available from racket/future.

11.5.3 Places Logging🔗ℹ

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:

(struct place-event (place-id action value time)
#:prefab)

The place-id field is an exact integer that identifies a place.

The time field is an inexact number that represents time in the same way as current-inexact-milliseconds.

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 valuefield 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 6.0.0.2 of package base: Added logging via 'placeand place-event.