new_contents: #!/usr/bin/python
  #
  # This program is intended to be run via sudo, and does the following
  # things:
  #
  # 1. Sets up a high-performance networking environment by tweaking
  #    various sysctl settings.
  # 2. Runs either an interactive shell or a command line program.
  # 3. Resets the environment back to what it was.
  
  import os, sys, tempfile
  
  sysctls = dict(
      Darwin={
          'kern.maxfiles': 22528,
          'kern.maxfilesperproc': 20480,
          'net.inet.ip.portrange.first': 1024,
          'net.inet.ip.portrange.hifirst': 1024,
          },
      Linux={
          'net.core.rmem_max': 16777216,
          'net.core.wmem_max': 16777216,
          'net.ipv4.ip_local_port_range': '1024 65535',
          'net.ipv4.tcp_fin_timeout': 15,
          'net.ipv4.tcp_max_syn_backlog': 16384,
          'net.ipv4.tcp_rmem': '4096 87380 16777216',
          'net.ipv4.tcp_tw_recycle': 1,
          'net.ipv4.tcp_tw_reuse': 1,
          'net.ipv4.tcp_wmem': '4096 65536 16777216',
          },
      )
  
  ulimits = dict(
      Darwin={
          '-n': 20480,
          },
      Linux={
          '-n': 131072,
          },
      )
  
  if os.access('/sbin/sysctl', os.X_OK):
      sysctl = '/sbin/sysctl'
  elif os.access('/usr/sbin/sysctl', os.X_OK):
      sysctl = '/usr/sbin/sysctl'
  else:
      print >> sys.stderr, 'where is sysctl!?'
      sys.exit(1)
      
  changed_sysctls = {}
  
  def change_sysctl(name, newval):
      oldval = os.popen('%s -n %s' % (sysctl, name), 'r').read().strip().replace('\t', ' ')
      if not oldval:
          print >> sys.stderr, 'could not get value of ' + name
          return
      if oldval == str(newval):
          return
      ret = os.system('%s -w %s=%r 2>/dev/null' % (sysctl, name, newval))
      if ret != 0:
          print >> sys.stderr, 'could not change %s from %s to %r' % (name, oldval, newval)
          return
      changed_sysctls[name] = oldval
  
  platform = os.uname()[0]
  
  for (n,v) in sysctls[platform].iteritems():
      change_sysctl(n,v)
  
  fd, path = tempfile.mkstemp('.sh')
  
  fp = os.fdopen(fd, 'w')
  for (n, v) in ulimits[platform].iteritems():
      print >> fp, 'ulimit %s %s' % (n, v)
      if len(sys.argv) > 1:
          print >> fp, 'exec ' + ' '.join(sys.argv[1:])
      else:
          print >> fp, 'exec %s -l' % os.environ.get('SHELL', '/bin/bash')
  fp.close()
  os.system('exec /bin/sh ' + path)
  os.unlink(path)
  
  for (n,v) in changed_sysctls.iteritems():
      change_sysctl(n,v)
  
subject: Add a benchmarking shell.