[PATCH] "automated" make -j value

[Kurt J. Lidl]

On Wed, Dec 13, 2006 at 08:19:52PM -0800, David O'Brien wrote:
> With multi-socket systems becoming more prevalent, and the continued
> increase in cores per processors, I thought it would be nice for
> 'make -j' to gain some automation.
> 
> Attached is a patch that makes "-j-" be the same as
> "-j `sysctl -n kern.smp.cpus`" and "-j=" be twice that.
> 
> comments?  (redirected back to list)

I think you can do it better than this, by just support
setting the concurrancy level via reading a environmental
variable.  Like say "NPROC", which is what CrayOS used, and
also appeared in the BSD/OS pmake varient several years ago.

Then you can just do this:
	export NPROC=`/sbin/sysctl hw.ncpu | awk '{print $3*2}'`
or this:
	export NPROC=`/sbin/sysctl hw.ncpu | awk '{print $3}'`
in your shells .rc files.

(Obviously, change the sysctl node as appropriate for your OS.)

I found this really useful when compiling a large tree of
sources, where some of the Makefiles didn't have their dependencies
written correctly, sucht that a parallel make wouldn't work
properly.  It's easy to turn off, just by unsetting the
environmental variable.  It's also easy to iterate over a set
of values to figure out which one will compile a tree the
fastest.  (FYI -- setting 3*hw.ncpu was optimal for BSD/OS.)

If you hack on make to put in automagic around -j, you should
add the environmental variable support too.  It's actually more
useful in a lot of cases.  (Mostly cause you don't have to
touch any Makefile to turn it on, it just works...)

-Kurt