Re: parallel make help

[Ralf Wildenhues]

Hi Bob,

* Bob Rossi wrote on Thu, Apr 10, 2008 at 03:28:56PM CEST:
> I currently have my project setup so that each subdirectory has a
> Makefile.am. I've noticed that when I do 'make -j4' that it does 1
> directory at a time, then links a little library or makes a program.
> 
> If I moved everything to a single Makefile.am at the top level, would 
> that improve the speed of things so that it could continue to build 
> in parallel? 

Typically yes, the other Bob already addressed this.

> If it is in fact faster, like I'm imagining it might be, is there any
> way to make the "Makefile.am in each subdir" approach be just as fast?

I've thought of this, in fact just a couple of weeks ago.  It is
possible to formulate SUBDIRS rules in a way that allows parallel
execution of the sub makes (by something like turning each subdir
into a target).  However, doing so by default will certainly break
lots of existing setups.  That's why my current thinking is that
it may be possible to introduce a new feature, say,
  PARALLEL_SUBDIRS = ...

which would then show automake that these subdirectories really
can do work in parallel.  Maybe even get this functionality turned
on per-project with an automake option, dunno yet.

I haven't thought this through yet.  There are some quite subtle
issues that users need to be aware of for this to work correctly.

> I eventually want to spread my build out over N machines using autotools
> and dist-gcc.

Do you mean distcc?  That should work.

Cheers,
Ralf