On Fri, 21 Feb 2014 16:14:00 +0200, Nikos Chantziaras
<realnc@gmail.com> wrote:
> I recently needed to unmerge Netbeans and KDE from a machine. So
> removed the top-level packages of those, and then ran:
> 
>    emerge -a --depclean --jobs 20
> 
> However, "--jobs" is being ignored. So I'm sitting there, watching 
> hundreds of packages being unmerged one by one, taking a long time :-/
> 
> Is this normal? Shouldn't the --jobs option result in emerge
> unmerging multiple packages at the same time?
> 

From emerge(1)[1]:
       -j [JOBS], --jobs[=JOBS]
              Specifies  the  number of packages to build
              simultaneously. (...)

In the past, unmerging was almost entirely IO bound, and running
several in parallel would have (at best) no significant speedup. I
suspect the (fairly recent) preserve-libs feature and detection of
unmodified configuration files introduced some cpu-intensive parts,
which might be parallelizeable to some extent if you sacrifice some of
the recoverability of an aborted unmerge.

If you can benchmark low IO utilization during unmerges, I suggest you
open a feature request bug on <bugs.gentoo.org> requesting parallel
unmerge. :-)

In the mean time, you might be able to speed the process up somewhat by
setting FEATURES="-merge-sync" in the environment when you do unmerges.
(see make.conf(5)[2])

1: https://dev.gentoo.org/~zmedico/portage/doc/man/emerge.1.html
2: https://dev.gentoo.org/~zmedico/portage/doc/man/make.conf.5.html

-- 
eroen