[plt-scheme] Futures performance

On Wed, May 26, 2010 at 11:44 PM, Jon Rafkind <rafkind at cs.utah.edu> wrote:
> On 05/25/2010 08:53 PM, Groshev Dmitry wrote:
>>
>> What am I doing wrong? Here is the code and some tests:
>>
>> (require scheme/future)
>>
>> (define (fib n)
>>   (case n
>>     [(0) 0]
>>     [(1) 1]
>>     [else (let ((a (fib (- n 1)))
>>                 (b (fib (- n 2))))
>>             (+ a b))]))
>>
>> (define (fibp n)
>>   (case n
>>     [(0) 0]
>>     [(1) 1]
>>     [else (let ((a (future (lambda () (fib (- n 1)))))
>>                 (b (future (lambda () (fib (- n 2))))))
>>             (+ (touch a) (touch b)))]))
>>
>>
>>>
>>> (time (fib 30))
>>>
>>
>> cpu time: 546 real time: 539 gc time: 0
>> 832040
>>
>>>
>>> (time (fibp 30))
>>>
>>
>> cpu time: 8986 real time: 7814 gc time: 391
>> 832040
>>
>
> Is that actually the code you are running? If I take your code verbatim I
> get times like these
>
> ;; I set n to 40 so you can see a real difference
> cpu time: 6100 real time: 6448 gc time: 0
> 102334155
> cpu time: 6132 real time: 3995 gc time: 0
> 102334155
>
>
> But if I change the code so that `fibp' makes recursive calls to `fibp'
> instead of `fib' then I see `fibp' taking much longer.
> ;; n = 33
> cpu time: 496 real time: 498 gc time: 0
> 3524578
> cpu time: 17429 real time: 18720 gc time: 748
> 3524578
>

Last I heard, you can't create futures within a future (the runtime
blocks the future that tried).

If that's still the case, it explains why you see no speedup. I don't
know if that's enough to cause the slowdown you're seeing.

> Which is similar to your results. What sort of machine are you on? I'm on a
> dual-core amd 1.8ghz laptop.
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