Testing With Graphics

If you need to use graphics while testing your code, e.g. when using a debugger such as DDT or DDD, you have the following options.

Use the debugjob command

You can use the debugjob command, which automatically provides X-forwarding support.

$ ssh  niagara.scinet.utoronto.ca -X

USER@nia-login07:~$ debugjob
debugjob: Requesting 1 nodes for 60 minutes
xalloc: Granted job allocation 189857
xalloc: Waiting for resource configuration
xalloc: Nodes nia0030 are ready for job

[USER@nia1265 ~]$

Use the regular queue

If debugjob is not suitable for your case due to the limitations either on time or resources (see Testing, then you must follow these steps.

You will need two terminals in order to achieve this:

1. In the 1st terminal
   • ssh to niagara.scinet.utoronto.ca and issue your salloc command
   • wait until your resources are allocated and you are assigned the nodes
   • take note of the node where you are logged to, ie. the head node, let's say niaWXYZ

   $ ssh  niagara.scinet.utoronto.ca
   USER@nia-login07:~$ salloc --nodes 5 --time=2:00:00
   
   .salloc: Granted job allocation 141862
   .salloc: Waiting for resource configuration
   .salloc: Nodes nia1265 are ready for job
   
   [USER@nia1265 ~]$
   

2. On the second terminal:
   • ssh into niagara.scinet.utoronto.ca now using the -X flag in the ssh command
   • after that ssh -X niaWXYZ, ie. you will ssh carrying on the '-X' flag into the head node of the job
   • in the niaWXYZ you should be able to use graphics and should be redirected by x-forwarding to your local terminal

   ssh niagara.scinet.utoronto.ca -X
   USER@nia-login07:~$ ssh -X nia1265
   [USER@nia1265 ~]$ xclock   ## just an example to test the graphics, a clock should pop up, close it to exit
   [USER@nia1265 ~]$ module load ddt  ## load corresponding modules, eg. for DDT
   [USER@nia1265 ~]$ ddt  ## launch DDT, the GUI should appear in your screen
   

Observations:

• If you are using ssh from a Windows machine, you need to have an X-server, a good option is to use MobaXterm, that already brings an X-server included.
• If you are in Mac OS, substitute -X by -Y
• Instead of using two terminals, you could just use screen to request the resources and then detach the session and ssh into the head node directly.