[Sbcl-devel] slow printing of floating point numbers in SBCL

[Evan M. <eva...@gm...>]

Hi,

I am doing a numerical simulation in common lisp (SBCL), and I need to
print a large amount of double-precision floating point numbers into an
ASCII file.

>From what I read, SBCL uses a state of the art for float printing, but
I noticed that it is still quite slow, about 4 seconds to print
100,000 times the number pi, whereas Chicken scheme (which is quite
fast but not the fastest implementation around) would take about 0.5
seconds with my small benchmark below.

I thought initially that it was a type problem so I tryed to cal
directly the underlying print function sb-impl::output-float, but the
results are similar.

Also, with both SBCL and Chicken I tried to change the output to
/dev/null or to print not pi but random numbers generated in the loop,
but the run times are roughly the same.

Are there some options that I overlooked that could make the code run
faster?

Thanks in advance,

Evan



evan@evan:~$ sbcl
This is SBCL 0.9.17, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
; loading system definition from
; /usr/local/lib/sbcl/sb-bsd-sockets/sb-bsd-sockets.asd into #<PACKAGE "ASDF0">
; registering #<SYSTEM SB-BSD-SOCKETS {B26BF89}> as SB-BSD-SOCKETS
; registering #<SYSTEM SB-BSD-SOCKETS-TESTS {A7BD271}> as SB-BSD-SOCKETS-TESTS
; loading system definition from /usr/local/lib/sbcl/sb-posix/sb-posix.asd into
; #<PACKAGE "ASDF0">
; registering #<SYSTEM SB-POSIX {A9A58E9}> as SB-POSIX
; registering #<SYSTEM SB-POSIX-TESTS {AAC5031}> as SB-POSIX-TESTS
* (defun float-sbcl-1 (&optional (n 100000))
  (declare (optimize (speed 3) (safety 0))
           (fixnum n))
  (with-open-file (out "/tmp/float-sbcl1.dat"
                       :direction :output
                       :if-exists :supersede
                       :if-does-not-exist :create)
    (let ((*print-pretty* nil))
      (loop :repeat n :do (print pi out)))))

FLOAT-SBCL-1
* (defun float-sbcl-2 (&optional (n 100000))
  (declare (optimize (speed 3) (safety 0))
           (fixnum n))
  (with-open-file (out "/tmp/float-sbcl2.dat"
                       :direction :output
                       :if-exists :supersede
                       :if-does-not-exist :create)
    (let ((*print-pretty* nil))
      (loop :repeat n :do (progn
                            (sb-impl::output-float pi out)
                            (write-char #\newline out))))))

FLOAT-SBCL-2
* (time (float-sbcl-1))

Evaluation took:
  4.467 seconds of real time
  4.256266 seconds of user run time
  0.048003 seconds of system run time
  [Run times include 0.132 seconds GC run time.]
  0 calls to %EVAL
  1 page fault and
  281,611,432 bytes consed.
NIL
* (time (float-sbcl-2))

Evaluation took:
  4.268 seconds of real time
  4.192262 seconds of user run time
  0.016001 seconds of system run time
  [Run times include 0.088 seconds GC run time.]
  0 calls to %EVAL
  0 page faults and
  281,600,392 bytes consed.
NIL
* (quit) 
evan@evan:~$ cat print-float.scm 

(define *times* 100000)
(define *pi* 3.141592653589793)
(define *file* (open-output-file "/tmp/float-chicken.dat"))

(let loop ((i 0))
     (when (< i *times*)
       (display *pi* *file*)
       (newline *file*)
       (loop (+ i 1))))

(close-output-port *file*)
evan@evan:~$ csc -V

      /    /      /             
 ___ (___    ___ (     ___  ___ 
|    |   )| |    |___)|___)|   )
|__  |  / | |__  | \  |__  |  / 

Version 2.6 - linux-unix-gnu-x86 - [ dload ptables applyhook ]
(c)2000-2007 Felix L. Winkelmann

Enter "chicken -help" for information on how to use it.
evan@YahooBB219198244001:~$ csc print-float.scm 
evan@YahooBB219198244001:~$ time ./print-float

real    0m0.546s
user    0m0.352s
sys     0m0.020s
evan@YahooBB219198244001:~$