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A Torque cluster consists of one head node and many compute nodes. The head node runs the pbs_server daemon and the compute nodes run the pbs_mom daemon. Client commands for submitting and managing jobs can be installed on any host (including hosts not running pbs_server or pbs_mom).
The head node also runs a scheduler daemon. The scheduler interacts with pbs_server to make local policy decisions for resource usage and allocate nodes to jobs. A simple FIFO scheduler, and code to construct more advanced schedulers, is provided in the Torque source distribution. Most Torque users choose to use a packaged, advanced scheduler such as Maui or Moab.
Users submit jobs to pbs_server using the qsub command. When pbs_server receives a new job, it informs the scheduler. When the scheduler finds nodes for the job, it sends instructions to run the job with the node list to pbs_server. Then, pbs_server sends the new job to the first node in the node list and instructs it to launch the job. This node is designated the execution host and is called Mother Superior. Other nodes in a job are called sister MOMs.
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