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docs/manual: add details about top-level parallel build support 

[Peter: change version reference to Buildroot 2020.02]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
2020.02.x
Thomas Petazzoni 2019-11-05 17:46:44 +01:00 committed by Peter Korsgaard
parent 41ea767113
commit 532574b259
3 changed files with 54 additions and 4 deletions
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47
docs/manual/common-usage.txt
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@ -347,6 +347,53 @@ Refer to the help text of this script for more details:
utils/size-stats-compare -h
----------------
[[top-level-parallel-build]]
=== Top-level parallel build
.Note
This section deals with a very experimental feature, which is known to
break even in some non-unusual situations. Use at your own risk.
Buildroot has always been capable of using parallel build on a per
package basis: each package is built by Buildroot using +make -jN+ (or
the equivalent invocation for non-make-based build systems). The level
of parallelism is by default number of CPUs + 1, but it can be
adjusted using the +BR2_JLEVEL+ configuration option.
Until 2020.02, Buildroot was however building packages in a serial
fashion: each package was built one after the other, without
parallelization of the build between packages. As of 2020.02,
Buildroot has experimental support for *top-level parallel build*,
which allows some signicant build time savings by building packages
that have no dependency relationship in parallel. This feature is
however marked as experimental and is known not to work in some cases.
In order to use top-level parallel build, one must:
. Enable the option +BR2_PER_PACKAGE_DIRECTORIES+ in the Buildroot
configuration
. Use +make -jN+ when starting the Buildroot build
Internally, the +BR2_PER_PACKAGE_DIRECTORIES+ will enable a mechanism
called *per-package directories*, which will have the following
effects:
* Instead of a global _target_ directory and a global _host_ directory
common to all packages, per-package _target_ and _host_ directories
will be used, in +$(O)/per-package/<pkg>/target/+ and
+$(O)/per-package/<pkg>/host/+ respectively. Those folders will be
populated from the corresponding folders of the package dependencies
at the beginning of +<pkg>+ build. The compiler and all other tools
will therefore only be able to see and access files installed by
dependencies explicitly listed by +<pkg>+.
* At the end of the build, the global _target_ and _host_ directories
will be populated, located in +$(O)/target+ and +$(O)/host+
respectively. This means that during the build, those folders will
be empty and it's only at the very end of the build that they will
be populated.
include::eclipse-integration.txt[]
include::advanced.txt[]

3
docs/manual/faq-troubleshooting.txt
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@ -239,3 +239,6 @@ help reduce the build time:
* Buy new hardware. SSDs and lots of RAM are key to speeding up the
builds.
* Experiment with top-level parallel build, see
xref:top-level-parallel-build[].

8
docs/manual/quickstart.txt
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@ -60,10 +60,10 @@ To start the build process, simply run:
$ make
--------------------
You *should never* use +make -jN+ with Buildroot: top-level parallel
make is currently not supported. Instead, use the +BR2_JLEVEL+ option
to tell Buildroot to run the compilation of each individual package
with +make -jN+.
By default, Buildroot does not support top-level parallel build, so
running +make -jN+ is not necessary. There is however experimental
support for top-level parallel build, see
xref:top-level-parallel-build[].
The `make` command will generally perform the following steps: