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ref-manual/variables.rst: expand BB_NUMBER_THREADS description

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Explain that reducing its value to reduce system load
is not sufficient if PARALLEL_MAKE is not reduced as well.

(From yocto-docs rev: f4986faa5576ec1c146e606220421a686c4a9b57)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Michael Opdenacker
2022-11-08 19:36:16 +01:00
committed by Richard Purdie
parent e39e68053f
commit 15a8edcead
1 changed files with 27 additions and 0 deletions
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@@ -530,6 +530,33 @@ system and gives an overview of their function and contents.
":ref:`dev-manual/common-tasks:speeding up a build`" ":ref:`dev-manual/common-tasks:speeding up a build`"
section in the Yocto Project Development Tasks Manual. section in the Yocto Project Development Tasks Manual.
On the other hand, if your goal is to limit the amount of system
resources consumed by BitBake tasks, setting :term:`BB_NUMBER_THREADS`
to a number lower than the number of CPU threads in your machine
won't be sufficient. That's because each package will still be built
and installed through a number of parallel jobs specified by the
:term:`PARALLEL_MAKE` variable, which is by default the number of CPU
threads in your system, and is not impacted by the
:term:`BB_NUMBER_THREADS` value.
So, if you set :term:`BB_NUMBER_THREADS` to "1" but don't set
:term:`PARALLEL_MAKE`, most of your system resources will be consumed
anyway.
Therefore, if you intend to reduce the load of your build system by
setting :term:`BB_NUMBER_THREADS` to a relatively low value compared
to the number of CPU threads on your system, you should also set
:term:`PARALLEL_MAKE` to a similarly low value.
An alternative to using :term:`BB_NUMBER_THREADS` to keep the usage
of build system resources under control is to use the smarter
:term:`BB_PRESSURE_MAX_CPU`, :term:`BB_PRESSURE_MAX_IO` or
:term:`BB_PRESSURE_MAX_MEMORY` controls. They will prevent BitBake
from starting new tasks as long as thresholds are exceeded. Anyway,
as with :term:`BB_NUMBER_THREADS`, such controls won't prevent the
tasks already being run from using all CPU threads on the system
if :term:`PARALLEL_MAKE` is not set to a low value.
:term:`BB_SERVER_TIMEOUT` :term:`BB_SERVER_TIMEOUT`
Specifies the time (in seconds) after which to unload the BitBake Specifies the time (in seconds) after which to unload the BitBake
server due to inactivity. Set :term:`BB_SERVER_TIMEOUT` to determine how server due to inactivity. Set :term:`BB_SERVER_TIMEOUT` to determine how