[Scons-dev] Default number of jobs

[David Stone]

SCons currently defaults to only 1 job. On multi-processor systems,
this is sub-optimal. I recommend that we change the default behavior
to be equal to the number of processors on their system. This can be
done now by adding the following lines:

import multiprocessing
SetOption('num_jobs', multiprocessing.cpu_count())

There is some debate about the exact amount of jobs to run, but most
people agree that you want N * number of processors, with the debate
being about what value of N to use. The most commonly cited values are
1, 1.5, and 2. I was in the 1.5 camp, but after doing some profiling,
I found that 1 gives the best performance. On a 4 core Intel i5
machine with 4 GiB of memory, these are my results:
http://doublewise.net/misc/scons-build-jobs-table.png

zyp in the SCons chat ran similar tests on an Intel i7 machine with 32
GiB of memory, 4 cores and 8 hardware threads (so the number returned
by multiprocessing.cpu_count() is 8), and these are their results:
http://paste.jvnv.net/view/izIQC

Both of our tests show pretty significant decreases in time up to the
number of cores. In my case, cores == hardware threads, but it looks
like when they are different, you still get small increases in speed
as you increase to the number of hardware threads. Adding jobs above
the number of cores gives a steady increase in time (and at some point
the time taken would grow significantly as the computer runs out of
memory). This was not found to be a significant risk for any realistic
distribution of cores and memory.

In summary, virtually every user will benefit from having the number
of jobs default to being exactly their number of hardware threads
(which is what is returned by multiprocessing.cpu_count()), and there
is almost no risk of losing performance to this change.