#lang scribble/doc

@title[#:tag "places"]{Places}

@; ----------------------------------------------------------------------

@(require scribble/manual scribble/urls scribble/struct "mz.rkt"
          (for-label racket
                     racket/base
                     racket/contract
                     racket/place
                     racket/future
                     racket/flonum
                     racket/fixnum))

@; ----------------------------------------------------------------------

@guideintro["effective-places"]{places}

@note-lib[racket/place]

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

@margin-note{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 @DFlag{enable-places} with
  @exec{configure} when building Racket. The @racket[place-enabled?]
  function reports whether places run in parallel.}

A @deftech{place} is a parallel task that is effectively a separate
instance of the Racket virtual machine. Places communicate through
@deftech{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
@racket[shared-flvector], @racket[make-shared-flvector],
@racket[shared-fxvector], @racket[make-shared-fxvector],
@racket[shared-bytes], and @racket[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
@deftech{shared memory space}. See @racket[place-message-allowed?].

A @tech{place channel} can be used as a @tech{synchronizable event}
(see @secref["sync"]) to receive a value through the channel.
A @tech{place channel} is @tech{ready for synchronization} when
a message is available on the channel, and the @tech{place channel}'s
@tech{synchronization result} is the message (which is removed on
synchronization). A place
can also receive messages with @racket[place-channel-get], and
messages can be sent with @racket[place-channel-put].

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

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

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

@racketblock[
(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 @filepath{place-worker.rkt} module must export the
@racket[place-main] function that each place executes, where
@racket[place-main] must accept a single @tech{place channel}
argument:

@racketmod[
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 @tech{garbage collection}, like other
Racket values, and a @tech{thread} that is blocked reading from a
@tech{place channel} can be garbage collected if @tech{place
channel}'s writing end becomes unreachable. @elemtag['(caveat
"place-channel-gc")]{However}, unlike normal @tech{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.


@defproc[(place-enabled?) boolean?]{

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


@defproc[(place? [v any/c]) boolean?]{
  Returns @racket[#t] if @racket[v] is a @deftech{place descriptor}
  value, @racket[#f] otherwise. Every @tech{place descriptor}
  is also a @tech{place channel}.
}

@defproc[(place-channel? [v any/c]) boolean?]{
  Returns @racket[#t] if @racket[v] is @tech{place channel}, 
  @racket[#f] otherwise.
}


@defproc[(dynamic-place [module-path (or/c module-path? path?)]
                        [start-name symbol?])
                        place?]{

 Creates a @tech{place} to run the procedure that is identified by
 @racket[module-path] and @racket[start-name]. The result is a
 @tech{place descriptor} value that represents the new parallel task;
 the place descriptor is returned immediately. The place descriptor
 value is also a @tech{place channel} that permits communication with
 the place.

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

 When the @tech{place} is created, the initial @tech{exit handler}
 terminates the place, using the argument to the exit handler as the
 place's @deftech{completion value}. Use @racket[(exit _v)] to
 immediately terminate a place with the completion value
 @racket[_v]. Since a completion value is limited to an exact integer
 between @racket[0] and @racket[255], any other value for @racket[v]
 is converted to @racket[0].

 If the function indicated by @racket[module-path] and
 @racket[start-proc] returns, then the place terminates with the
 @tech{completion value} @racket[0].

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

 The @racket[module-path] argument must not be a module path of the
 form @racket[(#,(racket quote) _sym)] unless the module is predefined (see
 @racket[module-predefined?]).}


@defproc[(dynamic-place* [module-path (or/c module-path? path?)]
                         [start-name symbol?]
                         [#:in in (or/c input-port? #f) #f]
                         [#:out out (or/c output-port? #f) (current-output-port)]
                         [#:err err (or/c output-port? #f) (current-error-port)])
                         (values place? (or/c output-port? #f) (or/c input-port? #f) (or/c input-port? #f))]{

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

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

The @racket[dynamic-place*] procedure returns four values:

@itemize[

 @item{a place descriptor value representing the created place;}

 @item{an output port piped to the place's standard input, or
 @racket[#f] if @racket[in] was a port;}

 @item{an input port piped from the place's standard output, or
 @racket[#f] if @racket[out] was a port;}

 @item{an input port piped from the place's standard error, or
 @racket[#f] if @racket[err] was a port or @racket['stdout].}

]

}

@defform[(place id body ...+)]{
  Creates a place that evaluates @racket[body]
  expressions with @racket[id] bound to a place channel.  The
  @racket[body]s close only over @racket[id] plus the top-level
  bindings of the enclosing module, because the
  @racket[body]s are lifted to a function that is exported by
  the module. The result of @racket[place] is a place descriptor,
  like the result of @racket[dynamic-place].
}

@defform/subs[(place* maybe-port ...
                      id 
                      body ...+)
              ([maybe-port code:blank
                           (code:line #:in in-expr)
                           (code:line #:out out-expr)
                           (code:line #:err err-expr)])]{
 Like @racket[place], but supports optional @racket[#:in], @racket[#:out],
 and @racket[#:err] expressions (at most one of each) to specify ports in the same way and
 with the same defaults as @racket[dynamic-place*]. The result of
 a @racket[place*] form is also the same as for  @racket[dynamic-place*].
 }


@defproc[(place-wait [p place?]) exact-integer?]{
  Returns the @tech{completion value} of the place indicated by @racket[p],
  blocking until the place has terminated.

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


@defproc[(place-dead-evt [p place?]) evt?]{

Returns a @tech{synchronizable event} (see @secref["sync"]) that is
@tech{ready for synchronization} if and only if @racket[p] has terminated.
@ResultItself{place-dead event}.

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


@defproc[(place-kill [p place?]) void?]{
  Immediately terminates the place, setting the place's
  @tech{completion value} to @racket[1] if the place does not have a
  completion value already.}


@defproc[(place-break [p place?]
                      [kind (or/c #f 'hang-up 'terminate) #f])
         void?]{
  Sends the main thread of place @racket[p] a break; see @secref["breakhandler"].
}


@defproc[(place-channel) (values place-channel? place-channel?)]{

  Returns two @tech{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 @tech{place} to
  send messages to a destination @tech{place}; the other place channel
  is sent to the destination @tech{place} (via an existing @tech{place
  channel}).
}

@defproc[(place-channel-put [pch place-channel?] [v place-message-allowed?]) void]{
  Sends a message @racket[v] on channel @racket[pch]. Since place channels 
  are asynchronous, @racket[place-channel-put] calls are non-blocking.

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

}

@defproc[(place-channel-get [pch place-channel?]) place-message-allowed?]{
  Returns a message received on channel @racket[pch], blocking until a 
 message is available.
}

@defproc[(place-channel-put/get [pch place-channel?] [v any/c]) any/c]{
  Sends an immutable message @racket[v] on channel @racket[pch] and then 
  waits for a message (perhaps a reply) on the same channel.
}

@defproc[(place-message-allowed? [v any/c]) boolean?]{

Returns @racket[#t] if @racket[v] is allowed as a message on a place channel,
@racket[#f] otherwise.

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

@itemlist[

 @item{@tech{numbers}, @tech{characters}, @tech{booleans}, and
       @|void-const|;}

 @item{@tech{symbols}, where the @racket[eq?]ness of @tech{uninterned}
       symbols is preserved within a single message, but not across
       messages;}
 
 @item{@tech{strings} and @tech{byte strings}, where mutable strings
       and byte strings are automatically replaced by immutable
       variants;}

 @item{@tech{paths} (for any platform);}

 @item{@tech{pairs}, @tech{lists}, @tech{vectors}, and immutable
       @tech{prefab} structures containing message-allowed values,
       where a mutable vector is automatically replaced by an
       immutable vector;}

 @item{@tech{hash tables} where mutable hash tables are automatically
       replaced by immutable variants;}

 @item{@tech{place channels}, where a @tech{place descriptor} is
       automatically replaced by a plain place channel;}

 @item{@tech{file-stream ports} and @tech{TCP ports}, where the
       underlying representation (such as a file descriptor, socket,
       or handle) is duplicated and attached to a fresh port in the
       receiving place;}

 @item{@tech[#:doc '(lib "scribblings/foreign/foreign.scrbl")]{C
       pointers} as created or accessed via @racketmodname[ffi/unsafe]; and}

 @item{values produced by @racket[shared-flvector],
       @racket[make-shared-flvector], @racket[shared-fxvector],
       @racket[make-shared-fxvector], @racket[shared-bytes], and
       @racket[make-shared-bytes].}

]}

@;------------------------------------------------------------------------
