Re: Vector and List Performance

[Pascal J. Bourguignon]

Nordlöw <per.nordlow@gmail.com> writes:

> I'm trying to figure the performance different between lists and
> vectors. Here is my mockup:
>
>   (defun bench (&rest forms)
>     "Convenience wrapper for benchmark-run-compiled."
>     (/ (nth 0 (benchmark-run 1024 forms)) 1024))
>
> (let ((length 1000000))
>   (cons
>    (bench (aref (make-vector length 0) (/ length 2)))
>    (bench (nth (/ length 2) (make-list length 0)))
>    ))
>
> Strangely I can't seem to find any significant different in
> performance when accessing the middle element in a long vector and
> long list. Shouldn't the random access performance be the big
> difference between vectors and lists? 

Tyere is.


> What have I missed?

You have missed that functions receive their argument already evaluated.
The argument to each of the calls to bench is 0, in both case.

(require 'cl) ; as always
(loop 
    with length = 1000000
    with index  = (1- length)
    for seq in (list (make-vector length 0) (make-list length 0))
    do (insert (format "%8S %s" (type-of seq)
                       (time (loop repeat 100 do (elt seq index))))))

C-x C-e inserts:

  vector Time: 2.030000e+02 ms
    cons Time: 7.819890e+05 ms



Note however that adding or removing elements from the head, or even
in the middle of a list is much faster than doing the same to a
vector.


-- 
__Pascal Bourguignon__