make -jn not necessarily helpful

[Justin C. Sherrill]

On Tue, March 14, 2006 12:03 pm, Freddie Cash wrote:
> Try moving the make clean to the end, and rebooting between runs.
> That will eliminate any caching or disk buffering that may be
> happening.

That step I pasted repeated with different -j levels, so a 'make clean'
happened after and before every step.

I can't imagine something as large as a 'make buildworld' would be
significantly cached, and even if it was, then I'd see a sudden
performance improvement after the first run.

> I haven't timed the buildworld runs on my systems in a long time, but
> the last time I did, -j2 was faster than -j1 on a UP system.  All
> higher -j settings were slower.  And on a dual-Opteron sytem, -j4 and
> -j6 alternated between being the fastest.

If you have a system on which you can try it, I'd like to know the
results.  I suspect there may be a cutoff where this does not help, and I
want to define this better than the folklore it is now.