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Parallel builds across makefiles

From: sgardell
Subject: Parallel builds across makefiles
Date: Thu, 14 Jul 2016 15:40:49 -0700 (PDT)

We are using a standard distro of gmake 4.1 on RH 6.4.

We have a fairly well structured makefile system that leverages the ACE make
framework. A linear build (-j1) is about 3 hours. Running a parallel build
that builds leaf nodes (libraries) in parallel is about 1 hour. So far so

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

I am reasonably certain that I am actually seeing parallel build behavior
since I had to fix a handful of additional dependency issues to get it to
build. Also reasonably certain that the build is not doing different things
since the count of compiler invocations matches. Looking at 'top' during the
"highly parallelized" runs shows the system essentially idle.

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