Re: Proposed change to make -j

On Thu, Nov 23, 2006 at 04:30:02PM -0800, Jason Evans wrote:
John Birrell wrote:
Currently 'make -j' reports an error if the number of jobs
isn't specified.

I'd like to change make(1) to treat -j (without a number) as
meaning "set the number of jobs to the number of processors".

On sun4v, each processor isn't too powerful and system performance
is only decent when you use all the processors - 32 in my case.

I've been working on a parallel 'make release' process which
would benefit from having -j set by default. At the moment I
set MAKEFLAGS=j32 in my environment and this achieves the desired
result, but -j would be more general.

For gmake, a bare -j says to not limit the number of simultaneous jobs.
I don't think that it's a good idea to use a flag name that already
has a different (and potentially useful) meaning for gmake.

I have some concern that no matter what number of jobs you decide to run
simultaneously when your proposed flag is specified (N, N+1, 2N, etc.,
where N is the number of CPUs), that choice will not be universally
useful, since the best choice of argument to -j depends on what 'make'
is building. Might it be better to use sysctl(8) in your command line?

make -j `sysctl -n hw.ncpu`

What about 'make -j0'? This current gives an error message...

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