make buildworld run twice as fast

[Kris Kennaway]

On Thu, Dec 01, 2005 at 01:19:41PM -0800, Jose Borquez wrote:
> I recently rebuilt my FreeBSD system from sources and it took around 2 
> hours on an old PIII 800MHz pc to run "make buildworld".  If I include 
> the -j option to run multiple processes as once such as "make -j2 
> buildworld" does that mean it would finish in half the time?

No, that would only be true if you were using less than 50% of your
resources (e.g. CPU) - for example, if your system has 2 CPUs.
Otherwise, on a single CPU machine you are already using close to 100%
of CPU, but you may still achieve a few percent speed-up since
e.g. one compiler process can be running while another process is
blocked waiting for disk I/O.  However, this is balanced against
increased kernel overhead and memory use.

Kris
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