What is PBS in Computer?

PBS, or Portable Batch System, is computer software used for job scheduling in high-performance computing (HPC) environments. It manages and allocates computing resources to execute batch jobs efficiently.

Understanding PBS

At its core, PBS acts as a workload manager. Think of it as a traffic controller for computing tasks. Instead of users directly running applications on specific computers, they submit "jobs" to PBS. PBS then decides when and where those jobs should run based on factors like:

• Resource availability: Are there enough processors, memory, or other resources available on a particular machine?
• Job priorities: Some jobs may be more important than others and should be run sooner.
• User quotas: Users might have limits on the amount of resources they can use.
• System policies: Rules set by the system administrator about resource usage.

Key Functions of PBS

• Job Submission: Users submit jobs to the PBS system, often specifying resource requirements (e.g., number of processors, memory).
• Job Queuing: Jobs are placed in a queue awaiting available resources.
• Resource Allocation: PBS allocates resources to jobs based on predefined criteria.
• Job Execution: Once resources are allocated, the job is executed on the designated compute node.
• Job Monitoring: PBS monitors the job's progress and provides status updates.
• Job Completion/Termination: Upon completion or failure, PBS records the job's status and releases the resources.

How PBS Works in a Cluster Environment

In a typical UNIX cluster, PBS consists of a central server and several compute nodes:

1. User Submits Job: A user submits a job to the PBS server using a command-line interface. This often involves creating a "job script" containing instructions and resource requests.
2. PBS Server Queues the Job: The PBS server places the job in a queue.
3. Resource Allocation: The server continuously monitors the cluster's resources and determines the best compute node to run the job.
4. Job Execution on a Compute Node: The server instructs the selected compute node to execute the job.
5. Monitoring and Control: The server monitors the job's execution and manages its resources.
6. Results and Completion: Once the job is completed, the results are returned to the user, and the resources are released.

Example of a PBS Job Script (Simple)

#!/bin/bash
#PBS -l nodes=1:ppn=4  # Request 1 node with 4 processors per node
#PBS -l walltime=00:10:00 # Request a walltime (max runtime) of 10 minutes
#PBS -N my_job         # Assign a name to the job

cd $PBS_O_WORKDIR  # Change directory to where the job was submitted

echo "Starting job on host: $(hostname)"
echo "Running on $(nproc) processors"

./my_program  # Execute the program

This script requests one node with four processors, a maximum runtime of 10 minutes, and names the job "my_job". It then changes the directory to the submission directory, prints some information, and executes the program "my_program".

Alternatives to PBS

While PBS (and its open-source variants like Torque) are widely used, other workload managers exist, including:

• Slurm (Simple Linux Utility for Resource Management): Another popular open-source job scheduler.
• LSF (Load Sharing Facility): A commercial workload manager.
• HTCondor: A specialized workload management system designed for high-throughput computing.

In summary, PBS is a crucial component in managing and scheduling computational tasks in cluster environments, ensuring efficient utilization of resources and enabling high-performance computing.