simple, "smart" child kiling scheme. Perl snippet copy pasted from 20120122 release of GNU parallel.

kill-children.pl

# Copyright (C) 2007,2008,2009,2010,2011,2012 Ole Tange and Free Software
# Foundation, Inc.
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, see <http://www.gnu.org/licenses/>
# or write to the Free Software Foundation, Inc., 51 Franklin St,
# Fifth Floor, Boston, MA 02110-1301 USA

sub kill {
  # kill the jobs
  my $self = shift;
  my @family_pids = $self - > family_pids();
  # Record this jobs as failed
  $self - > set_exitstatus(1);
  # Send two TERMs to give time to clean up
  for my $signal("TERM", "TERM", "KILL") {
    my $alive = 0;
    for my $pid(@family_pids) {
      if (kill 0, $pid) {
        # The job still running
        kill $signal, $pid;
        $alive = 1;
      }
    }
    # Wait 200 ms between TERMs - but only if any pids are alive
    if ($signal eq "TERM" and $alive) {
      ::usleep(200);
    }
  }
}