code: def per_machine_target_cores(cores, num_jobs):
      
      if cores >= 32 and num_jobs == 1:
          cores = cores - 2
      elif cores >= 16 and num_jobs in [1, 2]:
          cores = cores - 1
      return cores
docs: Select target cores on larger machines to leave room for batch script and controller.
  
      On resource constrained environments, we want to pack all bcbio submissions onto a specific
      number of machines. This gives up some cores to enable sharing cores with the controller
      and batch script on larger machines.