Glossary

Here we define Flux-specific and general HPC and workload management terms used in our documentation that may not be familiar to all readers.

enclosing instance: The Flux instance that a process naturally interacts with. It is the instance referred to by the FLUX_URI environment variable, or if that is not set, it is the system instance.
expedited: A job is said to be expedited if its urgency is set to the maximum value of 31. An expedited job's priority is always set to the maximum value.
FSD: A common string representation of time duration, defined by 23/Flux Standard Duration. Example: 2.5h.
guest: A Flux user that is not the instance owner. Guests are only allowed to run in a Flux instance configured for multi-user support, normally a system instance.
held: A job is said to be held if its urgency is set to zero. This prevents it from being considered for scheduling until the urgency is raised.
hostlist: A compact string representation of a list of hostnames, defined by 29/Hostlist Format. Example: fluke[0-127,130].
idset: A compact string representation of a set of non-negative integers, defined by 22/Idset String Representation. Example: 2,4,6,1-100.
IMP: The Independent Minister of Privilege. The simple setuid root component of Flux, from the flux-security project, that allows an instance owner to perform a limited set of tasks on behalf of a guest user in a multi-user Flux instance.
initial program: A user-defined program, such as a batch script, launched on the first node of a Flux instance. Its purpose is to launch and monitor a workload. Once it is complete, the instance exits.
instance owner: The user that started the Flux instance. The instance owner has control over all aspects of the Flux instance's operation.
job: The smallest unit of work that can be allocated resources and run by Flux. A job can be a Flux instance which in turn can run more jobs.
jobspec: The JSON or YAML object representing a Flux job request, defined by 14/Canonical Job Specification (the general specification) and 25/Job Specification Version 1 (the current version). It includes the abstract resource requirements of the job and instructions for job execution.
priority: The order in which the scheduler considers jobs. By default, priority is derived from the urgency and submit time, but a priority plugin can be used to override this calculation.
R: The JSON object used by Flux to represent a concrete resource set. See 20/Resource Set Specification Version 1.
resource inventory: The concrete set of resources managed by a given Flux instance.
scheduler: The Flux component that fulfills resource allocation requests from the resource inventory. Abstract resource requirements are extracted from the user-provided jobspec, and fulfilled with a resource set expressed as R. In addition to fitting concrete resources to abstract requests, the scheduler must balance goals such as fairness and resource utilization when it decides upon a schedule for fulfilling competing requests.
slot: The abstract resource requirements of one task.
step: In other workload managers, a job step is a unit of work within a job. Flux, which has a robust recursive definition of a job, does not use this term.
system instance: A multi-user Flux instance running as the primary resource manager on a cluster. The system instance typically runs as an unprivileged system user like flux, is started by systemd(1), and allows guest users to run jobs.
taskmap: A compact mapping between job task ranks and node IDs, defined by 34/Flux Task Map.
TBON: Tree based overlay network. Flux brokers are interconnected with one.
urgency: A job attribute that the user sets to indicate how urgent the work is. The range is 0 to 31, with a default value of 16. Urgency is defined by 30/Job Urgency.