Proposed change to make -j

[Ruslan Ermilov]

On Fri, Nov 24, 2006 at 08:04:17AM +0100, Rink Springer wrote:
> 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...
> 
That would be an unportable extension which depends on the FreeBSD
sysctl (hw.ncpu).  What Jason suggested is just fine and UNIXish.
Also, on most systems around here where I did testing (UP and SMP),
I saw the imrpovement in wall clock time with -j going up to 2xNCPU,
but on other systems I only saw it with -j up to NCPU, so there's
no one true formula.  It's a product of many factors, including
properly written makefiles in a build set.


Cheers,
-- 
Ruslan Ermilov
ru at FreeBSD.org
FreeBSD committer
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