Testing With Graphics

Testing with Graphics: X-forwarding

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:

• 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 ~]$
  

• If debugjob is not suitable for your case due to the limitations either on time or resources (see above #Testing), then you have to 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.