Racket – parallelismo – 2

dd0Continuo copiando qui [doc]/guide/parallelism.html e finisco il capitolo.

Parallelismo con place

The racket/place library provides support for performance improvement through parallelism with the place form. The place form creates a place, which is effectively a new Racket instance that can run in parallel to other places, including the initial place. The full power of the Racket language is available at each place, but places can communicate only through message passing—using the place-channel-put and place-channel-get functions on a limited set of values—which helps ensure the safety and independence of parallel computations.

As a starting example, the racket program below uses a place to determine whether any number in the list has a double that is also in the list:

p11

The identifier ch after place is bound to a place channel. The remaining body expressions within the place form are evaluated in a new place, and the body expressions use ch to communicate with the place that spawned the new place.

In the body of the place form above, the new place receives a list of numbers over ch and binds the list to l. It then calls any-double? on the list and binds the result to l-double?. The final body expression sends the l-double? result back to the original place over ch.

Note: When using places inside DrRacket, the module containg place code must be saved to a file before it will execute.

In DrRacket, after saving and running the above program, evaluate (main) in the interactions window to create the new place. Alternatively, save the program as “double.rkt” and run from a command line with

p12

where the -t flag tells racket to load the double.rkt module, the -m flag calls the exported main function, and -tm combines the two flags.

The place form has two subtle features. First, it lifts the place body to an anonymous, module-level function. This lifting means that any binding referenced by the place body must be available in the module’s top level. Second, the place form dynamic-requires the enclosing module in a newly created place. As part of the dynamic-require, the current module body is evaluated in the new place. The consequence of this second feature is that place should not appear immediately in a module or in a function that is called in a module’s top level; otherwise, invoking the module will invoke the same module in a new place, and so on, triggering a cascade of place creations that will soon exhaust memory.

#lang racket
 
(provide main)
 
; Don't do this!
(define p (place ch (place-channel-get ch)))
 
(define (indirect-place-invocation)
  (define p2 (place ch (place-channel-get ch))))
 
; Don't do this, either!
(indirect-place-invocation)

Places distribuite

The racket/place/distributed library provides support for distributed programming.

The example bellow demonstrates how to launch a remote racket node instance, launch remote places on the new remote node instance, and start an event loop that monitors the remote node instance.

The example code can also be found in “racket/distributed/examples/named/master.rkt“. Kwasy, si trova (almeno per me) qui:

p13

#lang racket/base       ;; master.rkt

(require racket/place/distributed
         racket/class
         racket/place
         racket/runtime-path
         "bank.rkt"
         "tuple.rkt")
(define-runtime-path bank-path "bank.rkt")
(define-runtime-path tuple-path "tuple.rkt")
 
(provide main)
 
(define (main)
  (define remote-node (spawn-remote-racket-node 
                        "localhost" 
                        #:listen-port 6344))
  (define tuple-place (supervise-place-at 
                        remote-node 
                        #:named 'tuple-server 
                        tuple-path 
                        'make-tuple-server))
  (define bank-place  (supervise-place-at 
                        remote-node bank-path 
                        'make-bank))
 
  (message-router
    remote-node
    (after-seconds 4
      (displayln (bank-new-account bank-place 'user0))
      (displayln (bank-add bank-place 'user0 10))
      (displayln (bank-removeM bank-place 'user0 5)))
 
    (after-seconds 2
      (define c (connect-to-named-place remote-node 
                                        'tuple-server))
      (define d (connect-to-named-place remote-node 
                                        'tuple-server))
      (tuple-server-hello c)
      (tuple-server-hello d)
      (displayln (tuple-server-set c "user0" 100))
      (displayln (tuple-server-set d "user2" 200))
      (displayln (tuple-server-get c "user0"))
      (displayln (tuple-server-get d "user2"))
      (displayln (tuple-server-get d "user0"))
      (displayln (tuple-server-get c "user2"))
      )
    (after-seconds 8
      (node-send-exit remote-node))
    (after-seconds 10
      (exit 0))))

The spawn-remote-racket-node primitive connects to “localhost” and starts a racloud node there that listens on port 6344 for further instructions. The handle to the new racloud node is assigned to the remote-node variable. Localhost is used so that the example can be run using only a single machine. However localhost can be replaced by any host with ssh publickey access and racket. The supervise-named-dynamic-place-at creates a new place on the remote-node. The new place will be identified in the future by its name symbol 'tuple-server. A place descriptor is expected to be returned by invoking dynamic-place with the tuple-path module path and the 'make-tuple-server symbol.

The code for the tuple-server place exists in the file “tuple.rkt“. The “tuple.rkt” file contains the use of define-named-remote-server form, which defines a RPC server suitiable for invocation by supervise-named-dynamic-place-at.

#lang racket/base       ;; tuple.rkt
(require racket/match
         racket/place/define-remote-server)
 
(define-named-remote-server tuple-server
  (define-state h (make-hash))
  (define-rpc (set k v)
    (hash-set! h k v)
    v)
  (define-rpc (get k)
    (hash-ref h k #f))
  (define-cast (hello)
    (printf "Hello from define-cast\n")
    (flush-output)))

The define-named-remote-server form takes an identifier and a list of custom expressions as its arguments. From the identifier a place-thunk function is created by prepending the make- prefix. In this case make-tuple-server. The make-tuple-server identifier is the place-function-name given to the supervise-named-dynamic-place-at form above. The define-state custom form translates into a simple define form, which is closed over by the define-rpc form.

The define-rpc form is expanded into two parts. The first part is the client stubs that call the rpc functions. The client function name is formed by concatenating the define-named-remote-server identifier, tuple-server, with the RPC function name set to form tuple-server-set. The RPC client functions take a destination argument which is a remote-connection% descriptor and then the RPC function arguments. The RPC client function sends the RPC function name, set, and the RPC arguments to the destination by calling an internal function named-place-channel-put. The RPC client then calls named-place-channel-get to wait for the RPC response.

The second expansion part of define-rpc is the server implementation of the RPC call. The server is implemented by a match expression inside the make-tuple-server function. The match clause for tuple-server-set matches on messages beginning with the 'set symbol. The server executes the RPC call with the communicated arguments and sends the result back to the RPC client.

The define-cast form is similar to the define-rpc form except there is no reply message from the server to client

(module tuple racket/base
  (require racket/place
           racket/match)
  (define/provide
   (tuple-server-set dest k v)
   (named-place-channel-put dest (list 'set k v))
   (named-place-channel-get dest))
  (define/provide
   (tuple-server-get dest k)
   (named-place-channel-put dest (list 'get k))
   (named-place-channel-get dest))
  (define/provide
   (tuple-server-hello dest)
   (named-place-channel-put dest (list 'hello)))
  (define/provide
   (make-tuple-server ch)
    (let ()
      (define h (make-hash))
      (let loop ()
        (define msg (place-channel-get ch))
        (define (log-to-parent-real
                  msg
                  #:severity (severity 'info))
          (place-channel-put
            ch
            (log-message severity msg)))
        (syntax-parameterize
         ((log-to-parent (make-rename-transformer
                           #'log-to-parent-real)))
         (match
          msg
          ((list (list 'set k v) src)
           (define result (let () (hash-set! h k v) v))
           (place-channel-put src result)
           (loop))
          ((list (list 'get k) src)
           (define result (let () (hash-ref h k #f)))
           (place-channel-put src result)
           (loop))
          ((list (list 'hello) src)
           (define result
             (let ()
               (printf "Hello from define-cast\n")
               (flush-output)))
           (loop))))
        loop))))

😳 Non provato, troppo panicante :mrgreen:

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