Original bug ID: 596
Reporter: administrator
Status: closed
Resolution: fixed
Priority: normal
Severity: minor
Category: ~DO NOT USE (was: OCaml general)

Bug description

behaviour of output_value

Full_Name: Hugo Herbelin
Version: 3.02
OS: Linux
Submission from: oto.inria.fr (192.93.2.2)

Dear implementors,

Using the Coq proof assistant, we observed a noticeably erratic
behaviour of output_value on values of compacted size > 100 KB.

On the examples we studied, the time needed for marshalling varies
from 16 KB/s to more than 1 MB/s for values with a priori no regular
structure (measure done on a G3 500 Mhz).

I was able to reproduce such a bad behaviour of output_value on the
following small example. Actually, from roughly n=880000, there is a
boom in the time needed by to_channel.

let n = int_of_string (Sys.argv.(1));;
let v = Array.create n None;;
for i=0 to n-1 do v.(i) <- Some i done;;
let ch = open_out_bin "toto";;
Marshal.to_channel ch v [];;
close_out ch;;

I looked at extern.c and applied a quick hack, resizing the hash
table as soon as a third is occupied instead of an half. This clearly
improves the performance on the above toy example (27 times faster for
n=900000) but I'm not sure this is the good answer to the
problem. Especially, on our real examples (the files of the Coq FTA
user contribution -- see coq.inria.fr), the worst case remains as low
as 25 KB/s (precisely 40s to output a 1 MB file, to be compared with
the 1.2 MB/s (i.e. 0.5s) obtained to output another 650 KB file).

Thanks in advance for your help.

Hugo