Re: Parallel builds across makefiles

[Paul Smith]

On Thu, 2016-07-14 at 15:40 -0700, sgardell wrote:
> But when I try to build across makefiles in parallel then I see two
> different things:
>    1) If I turn on any sort of output sync it gets dramatically slower.
> Sometimes slower than our -j1 time!

I recommend trying with the latest release, GNU make 4.2.1 to see if
that helps.

If not then some debugging is needed as I can't reproduce this here.
It's true that, depending on the sync output mode you've selected (which
you didn't specify), you may not see individual updates as you
originally did (there may be long pauses in the output followed by a
massive dump of output) but the overall build time shouldn't be
significantly impacted.

>    2) Even with output sync turned completely off there is no meaningful
> improvement over a parallel build of just the leaves.

I'm not sure I understand this comment, but this isn't too surprising to
me.  The only improvement of complete parallelism across the entire tree
over parallelism of only the leaves would be at the edges, where right
as one leaf makefile is finishing up it could hand off some extra jobs
to another leaf makefile which wasn't done yet.  That may, depending on
the way the makefiles work and the amount of parallelism you have
enabled, not be very noticeable.