But why ...
(time-it (exact->inexact (expt 2 -1050)) test)
0 - 1
(time-it (exact->inexact (expt 2 -1000)) test)
0 - 0
... and ...
(time-it (expt 2 -1050) test_2)
26 - 28
(time-it (expt 2 -1000) test_2)
3 - 4
(time-it (expt 2 -1050) test)
26 - 28
(time-it (expt 2 -1000) test)
3 - 4
Careful with exact-inexact conversions?
=================
#lang racket
(provide (all-defined-out))
(define (test value)
;; Perform 100,000 floating point multiplies.
;; (Only side effect is time)
(let loop ([count 5000] [x value])
(when (count . > . 0)
(loop (sub1 count)
(* value 0.999999999999)))))
(define (test_2 value)
(let loop ([count 5000] [x value])
(when (count . > . 0)
(exact->inexact value)
(loop (sub1 count)
value))))
(define (time-it start-value fun)
;; Run (test start-value) 100 times and print
;; 5th and 95th percentile
(define unsorted-times
(for/list ([i (in-range 100)])
(define-values (results cputime realtime gctime)
(time-apply fun (list start-value)))
realtime))
(define times (sort unsorted-times <))
(define 5p (list-ref times 5))
(define 95p (list-ref times 95))
(printf "~a - ~a\n" 5p 95p))
On Wed, 01 Aug 2012 17:48:25 +0200, Michael Wilber <mwil...@uccs.edu>
wrote:
So here's something fun to look out for in your own programs.
On my slower 32-bit machine, very small numbers are much slower than
slightly less small numbers.
(time-it (expt 2 -1000))
126 - 235
(time-it (expt 2 -1050))
1187 - 2071
On my faster 64-bit machine, the performance difference is an order of
magnitude:
(time-it (expt 2 -1000))
55 - 57
(time-it (expt 2 -1050))
777 - 831
This also happens in C, Java, Python, Javascript, and presumably any
language that uses IEEE754 floating point numbers.
Careful with those small numbers!
;; fun-with-denormalized-floats.rkt ;;;;;;;;;;;;;;;;;;;;;;;;
#lang racket
(provide (all-defined-out))
(define (test value)
;; Perform 100,000 floating point multiplies.
;; (Only side effect is time)
(let loop ([count 100000] [x value])
(when (count . > . 0)
(loop (sub1 count)
(* value 0.999999999999)))))
(define (time-it start-value)
;; Run (test start-value) 100 times and print
;; 5th and 95th percentile
(define unsorted-times
(for/list ([i (in-range 100)])
(define-values (results cputime realtime gctime)
(time-apply test (list start-value)))
realtime))
(define times (sort unsorted-times <))
(define 5p (list-ref times 5))
(define 95p (list-ref times 95))
(printf "~a - ~a\n" 5p 95p))
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--
---------------------------------------------------------
Tobias Hammer
DLR / Institute of Robotics and Mechatronics
Tel.: 08153/28-1487
Mail: tobias.ham...@dlr.de
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