[racket] Style and Performance question
On Mon, May 9, 2011 at 10:25 PM, Sam Tobin-Hochstadt <samth at ccs.neu.edu> wrote:
> On Mon, May 9, 2011 at 11:19 PM, Robby Findler
> <robby at eecs.northwestern.edu> wrote:
>> Even without typed racket, this:
>>
>> (define (my-sqrt y)
>> (let loop [(x (/ y 2.0))]
>
>> is much faster than this:
>>
>> (define (my-sqrt y)
>> (let loop [(x (/ y 2))]
>
> Yes, this is one of the optimizations TR does. :)
It is not a meaning preserving transformation to turn the 2 into a 2.0
in this function, I don't think.
Also, Matthias explicitly wrote "2.0" in the tr version. He didn't
show the other version, but my timing results suggest he wrote "2" in
the other one (and Patrick certainly did).
> As to why TR is so much faster on this program: this is exactly the
> sort of program that TR excels at -- generic arithmetic on floats,
> which we can turn into fast machine instructions. You won't find a
> better case for TR than this, except if you add complex numbers.
I don't think that this is accurate, Sam.
This is the difference I see and it is more in line with what I would
expect given how I think TR inserts unsafe calls, etc.
[robby at gaoping] ~/git/plt/src/build$ racket ~/tmp-tr.rkt
cpu time: 187 real time: 221 gc time: 6
[robby at gaoping] ~/git/plt/src/build$ racket ~/tmp.rkt
cpu time: 301 real time: 400 gc time: 103
[robby at gaoping] ~/git/plt/src/build$ cat ~/tmp.rkt
#lang racket/base
(define epsilon 1e-7)
(define (my-sqrt y)
(let loop [(x (/ y 2.0))]
(let [(error (- y (* x x)))]
(if (< (abs error) epsilon)
x
(loop (+ x (/ error 2 x)))))))
(define (time-and-compare size count)
(define n (build-list size (lambda (_) (random 10))))
(collect-garbage) (collect-garbage) (collect-garbage)
(define l (time (for ([x (in-range 0 count)])
(for-each my-sqrt n))))
(void))
(time-and-compare 100 2000)
[robby at gaoping] ~/git/plt/src/build$ cat ~/tmp-tr.rkt
#lang typed/racket
(define epsilon 1e-7)
(: my-sqrt : Natural -> Real)
(define (my-sqrt y)
(let loop [(x (/ y 2.0))]
(let [(error (- y (* x x)))]
(if (< (abs error) epsilon)
x
(loop (+ x (/ error 2 x)))))))
(: time-and-compare : Natural Natural -> Void)
(define (time-and-compare size count)
(define n (build-list size (lambda (_) (random 10))))
(collect-garbage) (collect-garbage) (collect-garbage)
(define l (time (for ([x (in-range 0 count)])
(for-each my-sqrt n))))
(void))
(time-and-compare 100 2000)