[Buildroot] [PATCH v5 1/2] packages: fix and improve support for top-level parallel make

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Dear Fabio Porcedda,

On Wed,  1 Jul 2015 10:10:46 +0200, Fabio Porcedda wrote:
> The boost and jack2 packages fail to build when PARALLEL_JOBS is empty
> so instead of using an empty PARALLEL_JOBS don't use it in the MAKE
> variable when top-level parallel make is being used.
> 
> To simplify the use of top-level parallel make, check the MAKEFLAGS
> variable to know automatically if the -j option is being used, also use
> the "=" operator instead of the ":=" operator because the MAKEFLAGS
> variable can be checked only in a "recursively expanded variable".
> The "override" keyword must be used in order to change the automatic
> variable "MAKE".
> 
> When the top-parallel make is being used the sub-make are called without
> specifying the "-j" option in order to let GNU make share the job slots
> specified in the top make. This is done because GNU make is able
> to share the job slots available between each instance of make so if you
> want to increase the number of jobs you just need to increase the <jobs>
> value in the top make -j<jobs> command.
> 
> If we specify the -j<jobs> option in each instance of make, it is less
> efficient, e.g. in a processor with 8 cores we specify -j9 in each instance:
> the number of processes goes up to 81 because each sub-make can execute
> 9 processes. The excessive number of processes is not a good thing
> because in my tests even -j16 is slower than -j9.
> Instead if we don't specify the -j<jobs> option in the sub-make, the top
> make share the job slots automatically between each instance, so the
> number of process in this examples goes up to 9 that is faster than
> using up to 81 processes.
> 
> e.g. when the -j3 option is specified only in the top make:
> 
> possible state n. 1:
> process 1 - <packagea>-build
> process 2 - <packagea>-build
> process 3 - <packagea>-build
> 
> possible state n. 2:
> process 1 - <packagea>-extract
> process 2 - <packageb>-configure
> process 3 - <packagec>-build
> 
> possible state n. 3:
> process 1 - <packagea>-build make -j1
> process 2 - <packageb>-build make -j1
> process 3 - <packagec>-build make -j1
> 
> Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
> Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
> ---
>  Makefile            | 4 ++--
>  package/Makefile.in | 3 ++-
>  2 files changed, 4 insertions(+), 3 deletions(-)

Applied, thanks.

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com