Login nodes

Role of Login Nodes in an HPC Cluster

Login nodes are the main entry point into an HPC cluster for users. They are sometimes called front-end nodes, access nodes, or gateway nodes. You typically:

• Connect (e.g., via SSH) to a login node
• Prepare your working environment
• Submit jobs to the scheduler
• Monitor and manage jobs

All this happens without directly using the compute nodes.

Key points about login nodes:

• Shared by many users simultaneously
• Optimized for interactivity and light tasks, not heavy computation
• Directly visible from outside the cluster (often via the institutional network or VPN)
• Connected to home and project filesystems

Typical Tasks on Login Nodes

Login nodes are designed for interactive, relatively low-intensity work. Common, appropriate tasks include:

• Session management
• Logging in and out
• Managing multiple sessions (e.g., via tmux or screen)
• Using SSH key-based authentication
• File and directory work
• Creating and organizing directories for projects
• Copying data to and from the cluster (e.g., scp, rsync)
• Unpacking archives (tar, unzip)
• Inspecting files with less, head, tail
• Code editing and light development
• Using terminal editors (vim, nano, emacs)
• Simple refactoring and editing of scripts
• Browsing and editing configuration files (e.g., job scripts)
• Compiling and building software (within reason)
• Running cmake, make, or similar commands for building your codes
• Linking against cluster-provided libraries and modules
• Performing test builds and small test runs
• Job preparation and management
• Writing and editing job submission scripts
• Loading/unloading environment modules required by your job
• Submitting jobs to the scheduler
• Checking job status and examining output/error logs
• Light interactive testing
• Running very small test cases to check:
• Does the program start?
• Do input files load correctly?
• Are paths and modules correctly set?
• Timing short operations that complete quickly

When in doubt, if something runs for more than a few minutes or uses many cores, it probably does not belong on a login node.

What You Should NOT Do on Login Nodes

Because login nodes are shared and meant to stay responsive, heavy workloads are inappropriate and often explicitly forbidden by policy.

Tasks to avoid on login nodes:

• Long-running computations
• Running production simulations or large analyses
• Any process that runs for more than a brief test period
• Loops that run over many parameter combinations
• Parallel and GPU jobs
• MPI runs, e.g. mpirun -np 64 ./mycode
• OpenMP runs using many threads (e.g., OMP_NUM_THREADS=32)
• GPU-consuming tasks (if GPUs are even available on the login node)
• Resource-hungry workflows
• Large memory usage (e.g., loading huge datasets into Python/R/Matlab)
• Heavy I/O (e.g., scanning or rewriting many large files)
• Massive compile jobs with high parallelism (e.g., make -j64 on a shared node)
• Background jobs that hog resources
• Leaving big data processing scripts running for hours in the background
• Launching multiple simultaneous resource-intensive tasks

Running heavy workloads on login nodes can:

• Slow down or block other users
• Trigger automatic job termination by cluster monitoring
• Violate usage policies and result in account restrictions

How Login Nodes Relate to Other Cluster Components

Login nodes sit between you and the internal cluster:

• External access
• You typically reach login nodes from your laptop or workstation via SSH:
• ssh your_username@login.cluster.example.org
• Scheduler and compute nodes
• From the login node, you interact with the job scheduler.
• The scheduler then allocates resources on compute nodes, not on the login node.
• You may start interactive jobs that give you a shell on a compute node, but they are still requested via the login node.
• Shared filesystems
• Login nodes typically mount the same home and project filesystems as compute nodes.
• This allows you to:
• Prepare files on the login node
• Access those same files in your jobs on compute nodes
• Paths (e.g., $HOME, project directories) are generally consistent across login and compute nodes.

Interactive Workflows from a Login Node

Although you don’t run heavy computations on login nodes, you often start interactive workflows from them that move the heavy work elsewhere.

Typical patterns:

• Interactive compute sessions
• From a login node, you request an interactive session via the scheduler.
• The scheduler allocates compute resources and gives you a shell on a compute node.
• You can then run interactive tests or debugging there without overloading the login node.
• Job scripting workflow
• Edit job script on login node.
• Submit job via scheduler command.
• Wait for job to run on compute nodes.
• Return to login node to check outputs and logs.
• Module/environment setup
• Experiment with loading modules and setting environment variables on the login node.
• Once correct, put the same commands into your job scripts.

Resource Limits and Monitoring on Login Nodes

Login nodes usually enforce stricter limits than compute nodes to protect responsiveness:

• Typical limits
• CPU time per process
• Maximum running time for interactive processes
• Memory per process or per user
• Number of processes per user
• No access (or limited access) to specialized hardware (GPUs, large memory)
• How to check your usage
• Process listing:
• ps, top, or htop (if available)
• Disk usage:
• df -h to see filesystem usage
• du -sh to see directory sizes
• Some sites provide custom commands or web portals that show:
• Who is logged into a login node
• How heavily it is loaded

If you see that a login node is heavily loaded (high CPU load or many processes), consider:

• Minimizing your own resource use
• Logging out of idle sessions
• Using alternative login nodes if multiple are provided

Working with Multiple Login Nodes

Many clusters provide more than one login node:

• Load distribution
• Users spread across multiple login nodes so no single one is overwhelmed.
• The site may recommend or enforce which node you should use (e.g., via a load-balancing alias).
• Consistency
• Login nodes often share:
• The same software environment
• The same filesystems
• But small differences can exist (OS version, modules, local scratch paths).
• Stateless or semi-stateless design
• Login nodes typically should not store important data on local disks.
• Always use networked home/project spaces for persistent files.
• If a login node goes down, you should be able to log into another without data loss.

Security and Access Considerations

Because login nodes are externally accessible, they are tightly controlled:

• Authentication
• SSH with passwords, SSH keys, or two-factor authentication
• Sometimes VPN required before SSH is allowed
• Limited login methods defined by the site
• Authorization and policies
• Only authorized users can log in.
• Usage policies usually define:
• Acceptable workloads on login nodes
• Time and resource limits
• Prohibited activities (e.g., crypto-mining, personal backups)
• Network access from login nodes
• Outbound connectivity may be:
• Open to the internet
• Restricted to certain hosts/services
• Some clusters require you to run network-heavy operations (e.g., large downloads) from specific data transfer nodes instead of login nodes.

Practical Tips for Using Login Nodes Effectively

• Be lightweight
• Keep interactive commands short and modest in resource use.
• Avoid multi-hour runs; turn them into scheduled jobs.
• Use screen/tmux wisely
• Tools like screen or tmux let you keep sessions alive, but:
• Don’t leave heavy processes running inside them.
• Clean up old, unused sessions.
• Separate tasks
• Use login nodes for:
• Editing
• Job submission
• Log inspection
• Use compute nodes (via jobs or interactive allocations) for:
• CPU- or memory-intensive work
• Production runs
• Follow site-specific documentation
• Each cluster may have:
• Different limits on login node usage
• Dedicated data transfer nodes
• Recommended patterns for compilation
• Always read and follow your site’s usage guidelines.

By using login nodes as lightweight, interactive access points and offloading heavy computation to compute nodes via the scheduler, you help keep the cluster responsive and fair for all users.