Two years ago, when a colleague had root access to our
lab's SUN 4500s (8 processors each, each running at 500mhz),
we decided that all those spare processor cycles were
going to waste.  With his knowledge of UNIX calls and
my affinity for perl, we came up with a SETI runner which
fired up new instances when the system was idle, and
killed instances when the system load went up.
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my $uptime       = "uptime";  # or whatever it's called...

my $setiRoot     = '/usr/bin/seti';
my $setiBinary   = 'setiathome';
my $setiFlags    = '';
my $setiLog      = 'seti.log';

my $napTime      = 2;

my $maxInstances = 8;    # maximum number of SETI instances
my $minInstances = 0;    # minimum number of SETI instances
my $lowThreshold = 8.00; # start SETI
my $hiThreshold  = 9.00; # stop SETI

my @setiPids;            # pid array

while(1)
{
   sleep( $napTime );
   
   my $uptime = `$uptime`;
   
   $uptime = $1 if $uptime =~ /load average: (\d+\.\d\d)/;

   print STDERR "load average: $uptime\n";
   
   #
   # if the uptime is below a certain load,
   # then fire up a new SETI instance.
   #
   &startSETI
      if ( ($uptime < $lowThreshold) && (@seti < $maxInstances) );
   
   #
   # if the uptime is above a certain load,
   # then kill a SETI instance.
   #
   &stopSETI
      if ( ($uptime > $hiThreshold) && (@seti > $minInstances) );
}

#
# Organize our list of PIDs
#
sub startSETI
{
   print STDERR "Firing up a SETI...\n";
   print STDERR "We have ", scalar( @setiPids ), " PIDs:\n";
   print STDERR join( " ", @setiPids ), "\n";
   # list all processes
   my $number = scalar( @setiPids );
      
   chdir( "$setiRoot/seti$number" );
   
   print STDERR "Working directory: $setiRoot/seti$number\n";
   
   push( @setiPids, `$setiBinary $setiFlags > $setiLog` );
}

#
# Quick and dirty killer
#
sub stopSETI
{
   print STDERR "Shutting down a SETI...\n";
   
   kill 9, pop @setiPids;
}