Re: parallel compilation of subdirs

[John Graham-Cumming]

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David Larson wrote:
>> I have the following Makefile, which will go to each subdir and call
>> "make". But "make" in each subdir can not be executed in parallel
>> (when I "make" in the parent dir with -j option). I'm wondering if
>> there is any way to enable parallel compilation.
> 
> That is one of the problems with recursive make. See:
> 
> Recursive Make Considered Harmful.
> http://aegis.sourceforge.net/auug97.pdf

That is an interesting paper even if it's getting a little long in the
tooth.   You can also read more about parallelization of GNU Make in the
following article I wrote for CM Basics:

http://www.cmcrossroads.com/articles/ask-mr.-make/-the-pitfalls-and-benefits-of-gnu-make-parallelization.html

And if you are trying to do away with recursive make I put together
another article on that subject here:

http://www.cmcrossroads.com/articles/ask-mr.-make/painless-non%11recursive-make.html

Having said all that the original question posed by Yu is actually
answered in the GNU Make manual itself.  It is possible to do recursive,
parallel GNU Make because of the cleverness of GNU Make's job server
system (which you can read all about here:
http://make.paulandlesley.org/jobserver.html).

The relevant section of the GNU Make manual is:
http://www.gnu.org/software/make/manual/make.html#Phony-Targets.  It's a
pity that this important topic is covered under Phony Targets, but it
_is_ there.

Yu's original Makefile looked like this:

SUBDIRS = $(filter-out Makefile%, $(wildcard *))

.PHONY: all

all:
       @for dir in $(filter-out backup, $(SUBDIRS)); do \
       $(MAKE) -C $$dir; \
       done

The parallel way to do this is

SUBDIRS = $(filter-out backup Makefile%, $(wildcard *))

.PHONY: all $(SUBDIRS)

all: $(SUBDIRS)

$(SUBDIRS):
        $(MAKE) -C $@

John.


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