all the SUBDIR entries in parallel, instead of serially. Apply this
option to a selected number of Makefiles, which can greatly speed up the
build on multi-core machines, when using make -j.
This can be extended to more Makefiles later on, whenever they are
verified to work correctly with parallel building.
I tested this on a 24-core machine, with make -j48 buildworld (N = 6):
before stddev after stddev
======= ====== ======= ======
real time 1741.1 16.5 959.8 2.7
user time 12468.7 16.4 14393.0 16.8
sys time 1825.0 54.8 2110.6 22.8
(user+sys)/real 8.2 17.1
E.g. the build was approximately 45% faster in real time. On machines
with less cores, or with lower -j settings, the speedup will not be as
impressive. But at least you can now almost max out a machine with
buildworld!
Submitted by: jilles
MFC after: 2 weeks
905ed8bc75