[racket] Using future and touch

hashes are definitely not future-safe, unfortunately.

Robby


On Thu, Jul 25, 2013 at 6:50 PM, Joe Gilray <jgilray at gmail.com> wrote:

> Here is an update:
>
> I rewrote gcd and used a hash-table for the squares check.  The hash-table
> change sped up the sequential code about 10x!  Then I tried using future /
> touch and it seems to improve the performance a little, but there is quite
> a bit of blocking (on >= and hash-ref) still.
>
> New code below.
>
> -Joe
>
> ; function that searches for progressive numbers for a given range of b
> values
> (define (find-progressive-num2 b-start b-end b-incr lim ht)
>   (define (mgcd a b) (if (= b 0) a (mgcd b (modulo a b))))
>   (for/sum ([b (in-range b-start b-end b-incr)])
>     (let loopa ([a (add1 b)] [suma 0])
>       (cond
>         [(> (mgcd a b) 1) (loopa (add1 a) suma)]
>         [(>= (* a a a b) lim) suma]
>         [else
>          (let loopc ([c 1] [sumc 0])
>            (define n (+ (* a a a c c b) (* c b b)))
>            (cond
>              [(>= n lim) (loopa (add1 a) (+ suma sumc))]
>              [(hash-has-key? ht n) (loopc (add1 c) (+ sumc n))]
>              [else (loopc (add1 c) sumc)]))]))))
>
> ;(require future-visualizer)
> (define (euler141b)
>   (define lim 1000000000000)
>   (define ht (make-hash))
>   (for ([i (in-range 1 1000000)]) (hash-set! ht (sqr i) 1))
>   ; (visualize-futures
>   (let ([f1 (future (λ () (find-progressive-num2 1 1000 4 lim ht)))]
>         [f2 (future (λ () (find-progressive-num2 2 1000 4 lim ht)))]
>         [f3 (future (λ () (find-progressive-num2 3 1000 4 lim ht)))])
>     (+ (find-progressive-num2 4 1000 4 lim ht) (touch f1) (touch f2)
> (touch f3))))
>
>
> On Wed, Jul 24, 2013 at 9:46 PM, Robby Findler <
> robby at eecs.northwestern.edu> wrote:
>
>> You might try places. Writing your own gcd seems straightforward. I'm not
>> sure about integer-sqrt?, tho. Maybe you could make a table or something if
>> you know there are not that many numbers.
>>
>> Or maybe someone will adjust the runtime to make those future safe!
>>
>> Robby
>>
>>
>> On Wed, Jul 24, 2013 at 11:34 PM, Joe Gilray <jgilray at gmail.com> wrote:
>>
>>> So I should write my own (gcd   ) and (square?   ) functions?
>>>
>>> I can try that, but isn't there a simple way to use threads?
>>>
>>> Thanks,
>>> -Joe
>>>
>>>
>>>
>>>
>>> On Wed, Jul 24, 2013 at 7:47 PM, Robby Findler <
>>> robby at eecs.northwestern.edu> wrote:
>>>
>>>> When I run this, the future visualizer shows that gcd and
>>>> square-number? block the futures. square-number? is implemented in TR and
>>>> if you take it, you find that integer-sqrt also blocks the futures. I'm not
>>>> sure if those functions can be made to run safely in futures or not.
>>>>
>>>> Robby
>>>>
>>>>
>>>> On Wed, Jul 24, 2013 at 7:26 PM, Joe Gilray <jgilray at gmail.com> wrote:
>>>>
>>>>> I have a ProjectEuler problem that I wanted to speed up so I thought I
>>>>> would try to speed it up with future / touch.
>>>>>
>>>>> I tried the following:
>>>>>
>>>>> ; function that searches for progressive numbers for a given range of
>>>>> b values
>>>>> (define (find-progressive-num b-start b-end b-incr lim)
>>>>>   (for/sum ([b (in-range b-start b-end b-incr)])
>>>>>     (let loopa ([a (add1 b)] [suma 0])
>>>>>       (cond
>>>>>         [(> (gcd a b) 1) (loopa (add1 a) suma)]
>>>>>         [(>= (* a a a b) lim) suma]
>>>>>         [else
>>>>>          (let loopc ([c 1] [sumc 0])
>>>>>            (define n (+ (* a a a c c b) (* c b b)))
>>>>>            (cond
>>>>>              [(>= n lim) (loopa (add1 a) (+ suma sumc))]
>>>>>              [(square-number? n) (loopc (add1 c) (+ sumc n))]
>>>>>              [else (loopc (add1 c) sumc)]))]))))
>>>>>
>>>>> ; ProjectEuler problem #141
>>>>> ; n = q * d + r
>>>>> ; q/d = d/r = a/b (where a and b are relatively prime)
>>>>> ; so d = a*r/b and q = a^2 * r / b^2
>>>>> ; since a and b are coprime r must be divisible by b^2 or r = c*b^2
>>>>> ; substituting: d = a*c*b and q = a^2*c
>>>>> ; n = a^3 * c^2 * b + c * b^2
>>>>> (define (euler141)
>>>>>   (define lim 10000000000)
>>>>>   (let ([f1 (future (λ () (find-progressive-num 1 1000 2 lim)))])
>>>>>     (+ (find-progressive-num 2 1000 2 lim) (touch f1))))
>>>>>
>>>>> Unfortunately this runs no faster than the sequential version.  I
>>>>> tried using the future-visualizer but I couldn't understand what it was
>>>>> telling me (I guess some operation is blocking).  I also tried finer
>>>>> grained threads (one for each value of b), but that did no better.
>>>>>
>>>>> Can anyone give me some pointers to successfully using future / touch?
>>>>>
>>>>> Thanks,
>>>>> -Joe
>>>>>
>>>>> ____________________
>>>>>   Racket Users list:
>>>>>   http://lists.racket-lang.org/users
>>>>>
>>>>>
>>>>
>>>
>>
>
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