On Wed, Feb 23, 2011 at 02:29:29AM -0600, Jonathan Nieder wrote:

    If the project grows too much, all those "make" calls start to
    take time.  It is possible for parallel "make" invocations to
    trample on each other's work if they are not well coordinated.

Aren't there cases where no amount of coordination will help? E.g.,
let's say I have two subdirs, lib/ and cmds/. From the top-level I do
"make -j". This invokes two recursive sub-makes, one per directory. The
make in cmds/ sees that we haven't built stuff in lib/ yet. So it
invokes a recursive make in lib/. Now we have two parallel makes running
in lib/, stomping on each other. The problem is that no single make was
allowed to see the whole dependency tree.

I know GNU make does have some magic for communicating between recursive
makes. Does it handle this situation?

You can fix this with a rule like "only invoke recursive makes on
directories _below_ you, never above or to the side". But then you can't
run "make" from inside cmds/.

You could have a dependency in the top-level that says recursing into
cmds/ depends on having finished recursing into libs/. But that's not
strictly correct. You want to be working on part of what's in cmds/
(building .o files) in parallel with what's going on in lib/, and then
wait on lib/ only for the linking portion.

Those are all things that are trivial to handle in a single Makefile.

 B. top-level Makefile slurps in Makefiles from subdirs.  Other
    Makefiles would

I like this better, but...

     - keep careful track of what directory "make" was run from; [*]
[...]
    [*] is a little hazy and sounds hackish.

Yeah, you have to be careful with paths. I think a more sane way would
be a single top-level Makefile that either contains everything, or
sources tidbits from subdirectories. But in either case, assumes it's
running from the top-level.

A dummy Makefile in each subdir that cd's to the toplevel and runs a
specific target. So from the top-level, "make" would build everything,
"make lib" would build stuff in the lib directory, and the Makefile in
lib/ would just do "cd .. && make lib".

It's not perfect, but it's simple and predictable.

-Peff