4.2 Priority Overview

Moab's prioritization mechanism allows component and subcomponent weights to be associated with many aspects of a job to enable fine-grained control over this aspect of scheduling. To allow this level of control, Moab uses a simple priority-weighting hierarchy where the contribution of each priority subcomponent is calculated as follows:


Each priority component contains one or more subcomponents as described in the section titled Job Priority Factors. For example, the Resource component consists of Node, Processor, Memory, Swap, Disk, Walltime, and PE subcomponents. While there are numerous priority components and many more subcomponents, a site need only focus on and configure the subset of components related to their particular priority needs. In actual usage, few sites use more than a small fraction (usually 5 or fewer) of the available priority subcomponents. This results in fairly straightforward priority configurations and tuning. By mixing and matching priority weights, sites may generally obtain the desired job-start behavior. At any time, you can issue the mdiag -p command to determine the impact of the current priority-weight settings on idle jobs. Likewise, the command showstats -f can assist the administrator in evaluating priority effectiveness on historical system usage metrics such as queue time or expansion factor.

As mentioned above, a job's priority is the weighted sum of its activated subcomponents. By default, the value of all component and subcomponent weights is set to 1 and 0 respectively. The one exception is the "QUEUETIME" subcomponent weight that is set to 1. This results in a total job priority equal to the period of time the job has been queued, causing Moab to act as a simple FIFO. Once the summed component weight is determined, this value is then bounded resulting in a priority ranging between 0 and MAX_PRIO_VAL which is currently defined as 1000000000 (one billion). In no case will a job obtain a priority in excess of MAX_PRIO_VAL through its priority subcomponent values.

Negative priority jobs may be allowed if desired; see ENABLENEGJOBPRIORITY and REJECTNEGPRIOJOBS for more information.

Using the mjobctl -p command, site administrators may adjust the base calculated job priority by either assigning a relative priority adjustment or an absolute system priority. A relative priority adjustment causes the base priority to be increased or decreased by a specified value. Setting an absolute system priority, SPRIO, causes the job to receive a priority equal to MAX_PRIO_VAL + SPRIO, and thus guaranteed to be of higher value than any naturally occurring job priority.

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