make -j optimization

[jc at trillian.mit.edu (John Chambers)]

David Kramer komments:

| In GNU make, you can specify a -j <n> option, to run <n> commands
| simultaneously.  This is good.
|
| I'm having an argument with a coworker over whether specifying a value for
| <n> greater than the number of the processors in the system actually does
| any good or not.  I don't see how it can.  In fact, specifying one fewer
| than the number of processors shoudl have almost the same performance as
| specifying the number of processors.

Well, if this were true, then on a  single-processor  machine,  there
should  be  no advantage in running several makes in parallel.  But I
know from lots of experience that this is far from true.   I  have  a
number of makefiles for C programs that have nearly every function in
a different file, so there are lots of .o files to make. So I've done
parallelism  the brute-force way, by firing up make subprocesses with
'&' on the end of the line.  This should be a lot less efficient than
a  make  that  can do the job automagically.  I've found that on most
machines, this can speed up a build by a factor of 3 or 4.

So I'd predict  that  with  N  processors,  compiling  4*N  files  in
parallel  should give an improvement, but past that point you may not
find that it's any faster.

Anyone have numbers for any particular machines?