09-May-2008

Introduction

It is possible to request exclusive access to a node so that you will not share it with anyone else regardless of how many processors you need on that node. This might be important for memory or disk intensive jobs where interference from other jobs would impact performance.

Requesting Exclusive Access

There are several methods that can be used to gain exclusive access:

This combination of options will give the user one node, and only one processor (ppn), but will have exclusive access to the entire node. The remaining processors will be unused. Note that the showq command will show your job in the queue as having been assigned all of the processors, even though only one will be used.

This combination of options will give the user one node and all processors (thus exclusive access) but will specify that mpiexec should use only one processor.

• A special case consideration might be that you need to ensure that you will have exclusive access to all nodes assigned to your job whether or not you need all of the processors. For example:
  
  

  #PBS -l nodes=5:ppn=n #where n is the max processors on a node mpiexec -n 8 myprogram #run your program with mpiexec

  
  This example will assign n processors per node to your job (over 5 nodes) but you will only use 8 while n-8 processors will go unused rather than being shared with other users. This will guarantee exclusive access to all of the nodes even though your job might not need all of the processors on those nodes.
  
  
• Another special case consideration might be that you want exclusive access to a node for multiple jobs, not a single job. In order to accomplish this, use the following options in your PBS batch script:
  
  

  #PBS -l nodes=1:ppn=1 #PBS -W x=NACCESSPOLICY:SINGLEUSER

  
  This combination of options will give the user one node, and only one processor (ppn), but will have exclusive access to the entire node. The remaining processors are still available for additional jobs submitted by the same user.