The hash table in Guile is rather standard (at least according to what was standard in the old ages :). (I *have* some vague memory that I might have implemented a simpler/faster table at some point, but that is not in the code base now.)
The structure is a vector of alists. It's of course important that the alists don't get too long, so there's some resizing going on. If you call (make-hash-table), the size of the table starts out at 31, so in your use case, there will be several resizing steps. What happens with speed if you do (make-hash-table 5000000) instead? Best regards, Mikael On Mon, Feb 21, 2022 at 2:55 PM Stefan Israelsson Tampe < stefan.ita...@gmail.com> wrote: > 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 > > > >
