Introduction

What is a Resource Manager?

While TORQUE has a built-in scheduler, pbs_sched, it is typically used solely as a resource manager with a scheduler making requests to it. Resources managers provide the low-level functionality to start, hold, cancel, and monitor jobs. Without these capabilities, a scheduler alone can not control jobs.

What are Batch Systems?

While TORQUE is flexible enough to handle scheduling a conference room, it is primarily used in batch systems. Batch systems are a collection of computers and other resources (networks, storage systems, license servers, and so forth) that operate under the notion that the whole is greater than the sum of the parts. Some batch systems consist of just a handful of machines running single-processor jobs, minimally managed by the users themselves. Other systems have thousands and thousands of machines executing users' jobs simultaneously while tracking software licenses and access to hardware equipment and storage systems.

Pooling resources in a batch system typically reduces technical administration of resources while offering a uniform view to users. Once configured properly, batch systems abstract away many of the details involved with running and managing jobs, allowing higher resource utilization. For example, users typically only need to specify the minimal constraints of a job and do not need to know the individual machine names of each host on which they are running. With this uniform abstracted view, batch systems can execute thousands and thousands of jobs simultaneously.

Batch systems are comprised of four different components: (1) Master Node, (2) Submit/Interactive Nodes, (3) Compute Nodes, and (4) Resources.

Master Node - A batch system will have a master node where pbs_server runs. Depending on the needs of the systems, a master node may be dedicated to this task, or it may fulfill the roles of other components as well.

Submit/Interactive Nodes - Submit or interactive nodes provide an entry point to the system for users to manage their workload. For these nodes, users are able to submit and track their jobs. Additionally, some sites have one or more nodes reserved for interactive use, such as testing and troubleshooting environment problems. These nodes have client commands (such as qsub and qhold).

Compute Nodes - Compute nodes are the workhorses of the system. Their role is to execute submitted jobs. On each compute node, pbs_mom runs to start, kill, and manage submitted jobs. It communicates with pbs_server on the master node. Depending on the needs of the systems, a compute node may double as the master node (or more).

Resources - Some systems are organized for the express purpose of managing a collection of resources beyond compute nodes. Resources can include high-speed networks, storage systems, license managers, and so forth. Availability of these resources is limited and needs to be managed intelligently to promote fairness and increased utilization.

Basic Job Flow

The life cycle of a job can be divided into four stages: (1) creation, (2) submission, (3) execution, and (4) finalization.

Creation - Typically, a submit script is written to hold all of the parameters of a job. These parameters could include how long a job should run (walltime), what resources are necessary to run, and what to execute. The following is an example submit file:
```
#PBS -N localBlast
#PBS -S /bin/sh
#PBS -l nodes=1:ppn=2,walltime=240:00:00
#PBS -M user@my.organization.com
#PBS -m ea

source ~/.bashrc
cd $HOME/work/dir
sh myBlast.sh -i -v
```
This submit script specifies the name of the job (localBlast), what environment to use (/bin/sh), that it needs both processors on a single node (nodes=1:ppn=2), that it will run for at most 10 days, and that TORQUE should email user@my.organization.com when the job exits or aborts. Additionally, the user specifies where and what to execute.
Submission - A job is submitted with the qsub command. Once submitted, the policies set by the administration and technical staff of the site dictate the priority of the job and therefore, when it will start executing.

Execution - Jobs often spend most of their lifecycle executing. While a job is running, its status can be queried with qstat.

Finalization - When a job completes, by default, the stdout and stderr files are copied to the directory where the job was submitted.