[racket] Web-cells, anonymous functions, and garbage collection

From: Galler (lzgaller at optonline.net)
Date: Wed Feb 13 22:34:09 EST 2013

Good evening,

I've been investigating lang web-server, performance and garbage 

I have a question regarding anonymous functions and web cells

Examples below require:

#lang web-server
(require racket/serialize

Constants appear serializable by their value being explicitly written 

For example,

(serialize 'this-is-a-song-about-japan)

;-> '((3) 0 () 0 () () this-is-a-song-about-japan)

Whereas anonymous functions appear serializable by the mechanism of 
being "lifted" (i.e. reified) to a module level binding

For example

(serialize (λ (x) x))

;-> '((3) 1 (('anonymous-module . "lifted.1386")) 0 () () (0))

Moving the discussion to web-cells

(define wc-constant (make-web-cell #f))

(define wc-anonymous (make-web-cell #f))

(web-cell-shadow wc-constant 'this-is-a-song-about-Japan)

(web-cell-shadow wc-anonymous (λ (x) x))

  (printf " wc-constant: ~A\n wc-anonymous: ~A\n"
           (serialize wc-1)
           (serialize wc-2))


wc-constant: ((3) 1 (((lib web-server/lang/web-cells.rkt) . 
deserialize-info:primitive-wc-v0)) 0 () () (0 lifted.1534-0))

wc-anonymous: ((3) 1 (((lib web-server/lang/web-cells.rkt) . 
deserialize-info:primitive-wc-v0)) 0 () () (0 lifted.1570-1))

While now both values are 'lifted' (i.e. reified), a close examination 
of the URL-string sent back to a client after send/suspend/dispatch

reveals that, with respect to the constant, lifted.1534 appears to be 
explicitly represented, elsewhere in the string as an associative pair 
i.e. (lifted.1534 this-is-a-song-about-Japan)

whereas for the anonymous function, its not clear that its reference, 
lifted.1570 is explicitly represented

Which leads me to wonder about whether the reference to the anonymous 
function, lifted.1570, is being maintained on the server,

And if so, how is it being garbage collected, as its impossible to ever 
determine that it is unreachable.

I will also point out that prior to a year ago, anonymous functions 
failed the serializability predicate. (serializable? anon-func) -> #f. I 
can pull out the correspondence/background related to that if desirable.

I apologize if I'm just misreading the data.

Here's the complete example,with a stateless web server. Note that when 
you inspect the url-string**for the datum this-is-a-song-about-Japan, 
its there, but when looking for the datum more-distinctive-text, its not 
present in the url-string.

**right click on the link, then copy/paste, or click the 'click-me' link 
and copy/paste the URL from the address bar)

Thanks very much.

Thanks again.

#lang web-server

(require  web-server/servlet-env)

(define wc-constant (make-web-cell #f))

(define wc-anonymous (make-web-cell #f))

(web-cell-shadow wc-constant 'this-is-a-song-about-Japan)

(web-cell-shadow wc-anonymous (λ (x) 'more-distinctive-text))

(define (start request)
   (letrec ((response-generator (λ (make-url)
                               (response/xexpr `(html (head)
                                                      (body (a ((href 
,(format "~A" (make-url receive-request)))) "click me"))))))
            (receive-request (λ (request)
                               (response/xexpr `(html (head)
                                                      (body "Thank you. 
We are done."))))))
     (send/suspend/dispatch response-generator)))

(serve/servlet start
                #:stateless? #t
                #:launch-browser? #t
                #:connection-close? #t
                #:quit? #f
                #:listen-ip #f
                #:port 8000
                #:servlet-path "/")

Thanks much.


Posted on the users mailing list.