Original bug ID: 4935
Reporter: letouzey
Status: closed (set by @gasche on 2016-02-16T00:24:19Z)
Resolution: suspended
Priority: normal
Severity: feature
Version: 3.11.1
Category: -for ocamlbuild use https://github.com/ocaml/ocamlbuild/issues
Related to: #5138
Monitored by: @ygrek "Julien Signoles" @hcarty

Bug description

I'm trying to compile Coq via ocamlbuild (See also bug #4934). Currently, the use of the -j option brings almost no time gain on a multi-core machine (while make -j is close to 2x gain on dual-core and 3.5x gain on quad-core).

Here again, I may well be the culprit and not have written my myocamlbuild.ml in an appropriate manner. Any hint is welcome...

Best regards,
Pierre Letouzey

Instructions to reproduce:

0. you'll need camlp5 installed
1. get a copy of coq development archive
   svn checkout svn://scm.gforge.inria.fr/svn/coq/trunk/ [^]
2. ./configure -local -opt
3. Launch a first build (./build is a wrapper for ocamlbuild, with no -j) :
   ./build
4. Redo the same with -j 0 :
   rm -rf _build
   ocamlbuild -j 0 coq.otarget

Here, on a quad-core machine, first build says:
Finished, 2716 targets (0 cached) in 00:13:10.

While the second one says
Finished, 2716 targets (0 cached) in 00:12:37.

On the same machine, make -j perform the same build in about 4min.