3.3 Scheduling Iterations and Job Flow
3.3.1 Scheduling Iterations
In any given scheduling iteration, many activities take place. These are broken into the following major categories: Update State Information
Refresh Reservations
Schedule Reserved Jobs
Schedule Priority Jobs
Backfill Jobs
Update Statistics
Handle User Requests
3.3.1.1 Update State Information
Each iteration, the scheduler contacts the resource manager(s) and requests up to date information on compute resources, workload, and policy configuration. On most systems, these calls are to a centralized resource manager daemon which possesses all information.
3.3.1.2 Refresh Reservations
3.3.1.3 Schedule Reserved Jobs
3.3.1.4 Schedule Priority Jobs
In scheduling jobs, multiple steps occur.
3.3.1.5 Backfill Jobs
3.3.1.6 Update Statistics
3.3.1.7 Handle User Requests
User requests include any call requesting state information, configuration changes, or job or resource manipulation commands. These requests may come in the form of user client calls, peer daemon calls, or process signals.
3.3.1.8 Perform Next Scheduling Cycle
Maui operates on a polling/event driven basis.
When all scheduling activities are complete, Maui will process user requests
until a new resource manager event is received or an internal event is
generated. Resource manager events include activities such as a new
job submission or completion of an active job, addition of new node resources,
or changes in resource manager policies. Internal events include
admin 'schedule' requests, reservation
activation/deactivation, or the expiration of the RMPOLLINTERVAL
timer.
3.3.2 Detailed Job Flow
3.3.2.1 Determine Basic Job Feasibility
The first step in scheduling is determining which jobs are feasible. This step eliminates jobs which have job holds in place, invalid job states (i.e., Completed, Not Queued, Defered, etc), or unsatisfied preconditions. Preconditions may include stage-in files or completion of preliminary job steps.
3.3.2.2 Prioritize Jobs
With a list of feasible jobs created, the next step involves determining the relative priority of all jobs within that list. A priority for each job is calculated based on job attributes such as job owner, job size, length of time the job has been queued, and so forth.
3.3.2.3 Enforce Configured Throttling Policies
Any configured throttling policies are then applied constraining how many jobs, nodes, processors, etc are allowed on a per credential basis. Jobs which violate these policies are not considered for scheduling.
3.3.2.4 Determine Resource Availability
For each job, Maui attempts to locate the required compute resources needed by the job. In order for a match to be made, the node must possess all node attributes specified by the job and possess adequate available resources to meet the TasksPerNode job constraint (Default TasksPerNode is 1) Normally, Maui determine a node to have adequate resources if the resources are neither utilized by nor dedicated to another job using the calculation
R.Available = R.Configured - MAX(R.Dedicated,R.Utilized).
The RESOURCEAVAILABILITYPOLICY parameter can be modified to adjust this behavior.
3.3.2.5 Allocate Resources to Job
If adequate resources can be found for a job, the node allocation policy is then applied to select the best set of resources. These allocation policies allow selection criteria such as speed of node, type of reservations, or excess node resources to be figured into the allocation decision to improve the performance of the job and/or maximize the freedom of the scheduler in making future scheduling decisions.
3.3.2.6 Distribute Jobs Tasks Across Allocated Resources
With the resources selected, Maui then maps job tasks to the actual resources. This distribution of tasks is typically based on simple task distribution algorithms such as round-robin or max blocking, but can also incorporate parallel language library (i.e., MPI, PVM, etc) specific patterns used to minimize interprocess communication overhead.
3.3.2.7 Launch Job
With the resources selected and task distribution mapped, the scheduler then contacts the resource manager and informs it where and how to launch the job. The resource manager then initiates the actual job executable.