make -j with j <= 4 seems to only load a single CPU core

[Jesper Juhl]

This is quite odd and I've no idea where to start looking for the
cause, but let me describe what I'm seeing and maybe someone can point
me in the right direction.

I'm running SMP 2.6.x kernels on a Athlon 64 X2 4400+

When I build new kernels I use 'make -j' to get both CPU cores busy
and minimize build time.

I've observed that when I use 'make -j 2', 'make -j 3' or 'make -j 4'
only ~half of my CPU resources get used during the build and when I
look at the output it looks exactely like output from plain 'make' for
something like 95% of the  build - that is, files get build
sequentially, not in parallel.
However, if I run 'make -j 5' or higher, then both cores get lots of
work to do and utilization of both cores stay close to 100% for almost
the entire build, and the output during the build shows lots of files
from different directories intermixed, so it's clearly building stuf
in parallel.

I find this quite strange since anything from 'make -j 2' and up
should be able to keep both cores resonably busy, but there seems to
be a huge difference between j <= 4 and j > 4.

Another datapoint: This is most pronounced when I am also running the
make process nice'd. Which I usually do in order to be able to use the
machine for other tasks while the build is in progress.
If I don't run the build nice'd then it starts to be resonably
parallel at j >= 3, but still doesn't *really* load both cores before
j >= 5.


Just to be completely clear - a few examples:

This is what I usually run to load both cores well :

nice make -j 5 2>&1 | tee build.log

this however gives me a more or less serial build with only half the
system resources used :

nice make -j 4 2>&1 | tee build.log

Without nice this loads the box somewhat OK (but not completely):

make -j 3 2>&1 | tee build.log

and this pretty much gives me a serial build:

make -j 2 2>&1 | tee build.log


Any good explanations for this behaviour ?


--
Jesper Juhl <jesper.juhl@xxxxxxxxx>
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