=====Practical 2: OpenMP & Using CHPC=====

**Aim:** to launch a parallel job on the CHPC cluster //Lengau//.

====Slides====

The slides for the OpenMP lecture on Linux will be posted.

====Logging in====

Download and install putty-ssh and connect to ''lengau.chpc.ac.za'' using the username (**studentNN**) and password provided.

====User Guide====

Documentation for the CHPC systems starts with the [[quick:start|CHPC Quick Start Guide]].  Read it, but note the differences below which are specific to the ''student''//NN// accounts you are using.

===Student Accounts===

  * Your user name is: ''studentNN''
  * Your project is: ''Wchpc''
  * Your queue is: ''R522145''
  * Your PBS group is: ''training''

====Interactive Session====

A user can not simply ssh onto a compute node because you don't know which ones are not in use by a currently running job.  But the PBSPro scheduler does know.  So we ask the scheduler for an //interactive// job which gives us a login shell on a compute node:

  qsub -I -P Wchpc -q R522145 -W group_list=training

**Note:**
  * ''-I'' selects an //interactive// job
  * you must specify the project ''Wchpc''
  * the queue is ''R522145''
  * you need to specify the ''training'' group.

**Advanced options:**
  * ''-l select=1:ncpus=24:mpiprocs=24:nodetype=haswell_reg''
  * interactive jobs only get one node: ''select=1''
  * you can request all the cores on the node: ''ncpus=24''
  * you can run MPI code: indicate how many ranks you want with ''mpiprocs=''

If you find your interactive session timing out too soon then add ''-l walltime=4:0:0'' to the above command line to request the maximum 4 hours.

=====OpenMP Part I=====

To carry out the exercises in the first OpenMP practical you must run the examples on a compute node and **not** the login node.

After logging on to lengau, the first thing you will do is request an interactive session on a compute node:

  qsub -I -P Wchpc
  
Your prompt will change to something like:

  [student00@cnode0004 ~]$

Remember, this is a login shell so you can change the environment as before, for example, change the prompt with

  export PS1='\h:\w\$ '

and now see

  cnode0004:~$ 

====Practical 2 Worksheet====

Download and follow the {{:workshops:tut.pdf|worksheet}}.

Copy the //OpenMP_Tut.tar.gz// file from the student00 home directory to your home directory.

  cp ~/../student00/OpenMP_Tut.tar.gz ~/lustre
  
  cd ~/lustre

Then untar (unpack) it.

  tar -zxvf OpenMP_Tut.tar.gz

And then go into the //OpenMP/// tut directory and start the practical.  Have fun!

=====OpenMP Part II=====  

====Job Script====

Most of the prac can be done in an interactive session.