Please explain make -j to my little brain

[Ashley Moran]

Hi

I've read the following snippet out of the handbook hundreds of times and 
still don't understand it.  I even asked one of the developers I work with 
and he was baffled too.

> It is now possible to specify a -j option to make which will cause it to
> spawn several simultaneous processes. This is most useful on multi-CPU
> machines. However, since much of the compiling process is IO bound rather
> than CPU bound it is also useful on single CPU machines.

What I want to know is, if compiling is IO bound, and you increase the number 
of simultaneous processes compiling your world, where do the extra processes 
get data from if the IO bandwidth is all used.

Have I misunderstood the term IO bound?  Please help, I feel like a right 
tool.

Just as a side line... does anybody know the best -j value to build world on a 
4-core box?

Ashley

-- 
"If you do it the stupid way, you will have to do it again"
  - Gregory Chudnovsky