make -jn not necessarily helpful

[Richard Nyberg]

At Mon, 13 Mar 2006 23:52:21 -0500 (EST),
Justin C. Sherrill wrote:
> 
> Out of curiousity, I thought I'd try running make buildworld with the -j
> option in a few different configurations to see what difference it made. 
> I know it's supposed to speed up the process by a certain amount because
> of the parallel processing, but there's no direct quantifier.
> 
> I put together a shell script that had this several times over
> 
> make clean
> echo `date` >> /home/justin/benchmark.txt
> echo 'make -j2 buildworld' >> /home/justin/benchmark.txt
> make -j2 buildworld
> echo `date` >> /home/justin/benchmark.txt
> echo " " >> /home/justin/benchmark.txt
> 
> I did this with no -j, -j1, -j2, and -j3
> 
> Looking at the result: 1 hour 15 minutes 40-something seconds, every time,
> no matter the -j setting.  This is on a 1.6G Celeron, with a PATA drive
> and ~350M ram.  Would I see a difference if the CPU was faster and the
> drive was slower?
> 
> If someone had some free CPU cycles to waste and could time a buildworld
> as above, I'd be interested in the results.  I'd like to see under what
> conditions the -j flag becomes useful or not useful, or if perhaps I'm
> just missing something.

Using -j has always made my buildworlds slower (on UP machines).

        -Richard