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X Window (X11)
While graphical desktop environment is usually not available to HPC cluster users, one can still run applications with Graphical User Interface, such as ARM's (former Allinea's) or Intel's parallel debuggers and performance analysis tools on a cluster. This is possible thanks to the X Window System, also known as X11, or simply X.
In order for this to work, an X Server needs to run on a remote computer (your laptop or desktop) where you would like to have the GUI for a specific application displayed. The application runs on a cluster node and sends drawing instructions to the X Server on your computer. When you communicate with the GUI displayed on your computer, such as clicking on a menu item, information is sent over the network to the cluster node, and new drawing instructions are sent back to your computer. Thus there is a lot of network traffic involved in running such applications, and the response times may be quite high.
Many applications have command line versions that may be less convenient to use but do not require a lot of network communications and can run over slow network connections. Some GUI applications (e.g. ARM Forge tools) have remote GUI clients that run on a remote computer instead of running on a cluster node, eliminating the need for heavy network communications but still providing graphical interface.
GUI applications also usually require a lot of memory. Since there may be many users simulteniously logged into a cluster, to discourage users from running intensive tasks on the headnode, and thus negatively affecting other users, resources on the headnode are limited. Because of that some GUI applications will fail when attempted to run on a headnode. They still can be run on a compute node which has no such limits. We will talk later about running computations on compute nodes.
If you need to run a GUI application on a cluster node, you need to install X Server on your computer and ssh with -Y option:
ssh -Y ISUNetID@cluster.its.iastate.edu
(replace ISUNetID with your ISU NetID, and cluster with the cluster name).
X11 is part of Linux OS. MacOS no longer includes X11.app, but users can install free open source XQuartz application on their Mac computers. There are multiple third-party X server impementations for Windows systems.
For more details on how to configure X Forwarding for Mac and Windows, refer to https://researchit.las.iastate.edu/x-forwarding-mac-and-windows .
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