[racket] Question about hash table efficiency

If I am right, then it would make sense to use hash-update when the hash is big enough that a log-time traversal takes longer than a general procedure call's set-up and tear-down.
-Ian 
----- Original Message -----
From: James Bergstra <james.bergstra at gmail.com>
To: J. Ian Johnson <ianj at ccs.neu.edu>
Cc: users at racket-lang.org
Sent: Thu, 14 Feb 2013 13:47:51 -0500 (EST)
Subject: Re: [racket] Question about hash table efficiency

Thanks, I suppose I just had my hopes up. I thought the more compact
form might involve fewer expression evaluations and require just one
hash lookup instead of two, and therefore might run something like
twice as fast, rather than slightly slower.

On Thu, Feb 14, 2013 at 1:36 PM, J. Ian Johnson <ianj at ccs.neu.edu> wrote:
> My guess would be the higher-orderness on hash-update causes the (minor) slowdown.
> -Ian
> ----- Original Message -----
> From: James Bergstra <james.bergstra at gmail.com>
> To: users at racket-lang.org
> Sent: Thu, 14 Feb 2013 13:16:47 -0500 (EST)
> Subject: [racket] Question about hash table efficiency
>
> Hi list, just discovered racket and having fun learning about scheme
> again. Haven't used it since undergrad. Thanks!
>
> I was wondering about the efficiency of hash-update vs. hash-set and
> hash-ref. I thought the former would be faster, otherwise why have it?
> A little benchmark script reveals the same surprising result as my
> actual application though:
>
> """
> #lang racket
>
> ;; warm-up
> (time (void
>  (for/fold ([table (make-immutable-hasheq '())])
>   ([ii (in-range 100000)])
>   (hash-update table (modulo ii 100) add1 ii))))
>
> ;; update
> (newline)
> (time (void
>  (for/fold ([table (make-immutable-hasheq '())])
>   ([ii (in-range 100000)])
>   (hash-update table (modulo ii 100) add1 ii))))
>
> ;; set/ref
> (newline)
> (time (void
>  (for/fold ([table (make-immutable-hasheq '())])
>   ([ii (in-range 100000)])
>   (hash-set table (modulo ii 100) (add1 (hash-ref table (modulo ii 100) ii))))))
>
> """
>
> This prints out:
>
> cpu time: 112 real time: 114 gc time: 32
>
> cpu time: 80 real time: 82 gc time: 0
>
> cpu time: 76 real time: 75 gc time: 4
>
>
> My original application used mutable hash tables and the same effect
> was observed (if not more dramatically). Any thoughts on why the
> update form is slower? I'm running this in Racket v5.1.1.
>
> - James
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