How Do I Guarantee Access to Both Cores of a CPU?

How Do I Guarantee Access
To Both Cores of a Single CPU?

01-Feb-2007

Introduction

When you run a job on Ada, the job scheduler will allocate processors to you based on the way you request them. This may result in 4 processors per node, or it may give you 1 processor per node scattered over 4 nodes. There are a variety of ways you can influence this selection. However, there may be circumstances in which you need to be guaranteed that you are allocated both cores of the same dual-core CPU. This means that you have requested two processors (cores), and you need both of them to be on the same dual-core CPU. This could be useful for certain types of jobs that share memory because both cores of a CPU share the memory controller.

How do I request both cores?

In order to be guaranteed both cores of a CPU, include in your PBS batch script the following line to select both cores (processors):

#PBS -l nodes=1:ppn=2
#PBS -W x=NACCESSPOLICY:SINGLEJOB

This will give you exclusive access to one node and will give you two processors on that node.

Then run your job from your PBS batch script as follows:


mpiexec $XD1LAUNCHER -j  ./myjob.exe

This will run the job using the xd1launcher program which takes advantage of the Cray XD1 execution environment and will instruct the system to run your job entirely within two cores of the same CPU. Use man xd1launcher for more information.