I ran the attached bb-matrix.sh on the following system:

CPU (1): Intel(R) Core(TM) i7 CPU         870  @ 2.93GHz
Cores: 4
Threads: 8
Memory: 8186560 kB
OS Disk: INTEL SSDSA2M040G2GC (SSD)
Build Disk: Hitachi HDT721050SLA360 (Spinning Media)

The script runs builds with all combinations of BB_NUMBER_THREADS and
PARALLEL_MAKE from 4 through 16.

Once BB_NUMBER_THREADS hit 10, the kernel OOM Killer started killing off
tasks and build time tripled. Those runs have been removed the dataset.

All of the runs with PARALLEL_MAKE=10 also failed, for a variety of
reasons. See bb-pm-errors.txt for details. For whatever reason, 10 seems
to be a bad number. Additional failures were seen at 09-11 and 10-14.
These have all been removed from the dat file.

From the remaining results, a clear downward trend in build time is
evident with increasing BB_NUMBER_THREADS through 8, while build time
mostly increases again with 9 (and dramatically so with 10, not shown).
Optimal build time is achieved with BB_NUMBER_THREADS=8.

Along the BB_NUMBER_THREADS=8 line, there is no clear trend with
increasing values of PARALLEL_MAKE. Local downward trends appear from
4-7 and from 11-14. Optimal build time occurs with PARALLEL_MAKE=14,
however, it only bests PARALLEL_MAKE=7 by 68 seconds.

While optimal build time is achieved with BB=8 and PM=14, a more
resource friendly setting of BB=8 and PM=6 yields nearly as good results.

To reproduce the plots and get an interactive view that you can rotate:

$ gnuplot --persist < bb-pm-matrix.plt

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel