Hatables are slow

[Stefan Israelsson Tampe <stefan.itampe@gmail.com>]

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A datastructure I fancy is hash tables. But I found out that hashtables in
guile are really slow, How? First of all we make a hash table

(define h (make-hash-table))

Then add values
(for-each (lambda (i) (hash-set! h i i)) (iota 20000000))

Then the following operation cost say 5s
(hash-fold (lambda (k v s) (+ k v s)) 0 h)

It is possible with the foreign interface to speedt this up to 2s using
guiles internal interface. But this is slow for such a simple application.
Now let's change focus. Assume the in stead an assoc,

(define l (map (lambda (i) (cons i i)) (iota 20000000)))

Then
ime (let lp ((l ll) (s 0)) (if (pair? l) (lp (cdr l) (+ s (caar l))) s))
$5 = 199999990000000
;; 0.114530s real time, 0.114391s run time.  0.000000s spent in GC.

That's 20X faster. What have happened?, Well hashmaps has terrible memory
layout for scanning. So essentially keeping a list of the created values
consed on a list not only get you an ordered hashmap, you also have 20X
increase in speed, you sacrifice memory, say about 25-50% extra. The
problem actually more that when you remove elements updating the ordered
list is very expensive. In python-on-guile I have solved this by moving to
a doubly linked list when people start's to delete single elements. For
small hashmap things are different.

I suggest that guile should have a proper faster standard hashmap
implemention of such kind in scheme.

Stefan

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