Re: [Caml-list] Accelerating compilation

[Xavier Leroy <Xavier.Leroy@inria.fr>]

On 06/09/13 15:56, Romain Bardou wrote:

> 1) Separate typing and code generation, in ocamlc and in ocamlopt
> For instance, provide an option -typing-only which would mean "only
> produce the .cmi but do not produce the .cmo or the .cmx". 

I strongly suspect that bytecode generation takes much less time than
parsing and typechecking, so you wouldn't gain much compared to simply
compiling to bytecode.

Some obvious recommendations:

- Make sure the .opt compilers (ocamlc.opt, ocamlopt.opt) are
  installed.  They are significantly faster than their non-.opt
  counterparts, and some Linux distros package the .opt compilers
  separately and don't install them by default.

- Put your project on a fast disk, ideally a SSD.  ocamlc can be I/O
  bound sometimes.  The only times where I observe somewhat slow
  compilations is when I work in an NFS partition mounted from our
  super-reliable-but-rather-slow NAS.

> 2) Be able to disable Ocamlbuild's digest mechanism and use dates and
> file sizes instead
> 
> If I am not mistaken, this is one of the main reasons why Ocamlbuild is
> slower that make.

It's one possible reason among others, e.g. that ocamlbuild runs
ocamldep very often.  Again, some profiling would help understanding
the bottleneck(s).  But this is a reasonable suggestion.

> 3) Parallel compilation in Ocamlbuild
> Of course it would help but it is not easy to implement so I'm just
> putting it there to be exhaustive.

As others said, ocamlbuild has some support for parallelism, but
it's much less effective than "make -j".  One reason could be that
ocamlbuild discovers dependencies as it goes, so it's doing online
scheduling, in contrast with make's offline scheduling.  But within
the online scheduling approach, there is certainly room for
improvement.  Suggestions and patches welcome.

- Xavier Leroy