# Library prosa.model.priority.definitions

From mathcomp Require Export seq.

Require Export prosa.model.task.concept.

Require Export prosa.util.rel.

Require Export prosa.util.list.

Require Export prosa.model.task.concept.

Require Export prosa.util.rel.

Require Export prosa.util.list.

# The FP, JLFP, and JLDP Priority Classes

... a JLFP policy as a relation among jobs, and ...

... a JLDP policy as a relation among jobs that may vary over time.

NB: The preceding definitions currently make it difficult to express
priority policies in which the priority of a job at a given time varies
depending on the preceding schedule prefix (e.g., least-laxity
first). That is, there is room for an even more general notion of a
schedule-dependent JLDP policy, where the priority relation among jobs
may vary depending both on time and the schedule prefix prior to a
given time. This is left to future work.
In the following section, we define key properties of common priority
policies that proofs often depend on.

## Properties of Priority Policies

Consider any type of tasks ...

... and any type of jobs associated with these tasks, ...

.. and assume that jobs have a cost and an arrival time.

In the following section, we define properties of JLDP policies, and by
extension also properties of FP and JLFP policies.

Consider any JLDP policy.

We define what it means for a JLDP policy to be reflexive, transitive,
and total.
A JLDP policy is reflexive if the relation among "jobs" is reflexive at
"every point in time."

A JLDP policy is transitive if the relation among "jobs" is transitive at
"every point in time."

A JLDP policy is total if the relation among "jobs" is total at
"every point in time."

Next, we define properties of JLFP policies.

Consider any JLFP policy.

We define what it means for a JLFP policy to be reflexive, transitive,
and total.
A JLFP policy is reflexive if the relation among "jobs" is reflexive.

A JLFP policy is transitive if the relation among "jobs" is transitive.

A JLFP policy is total if the relation among "jobs" is total.

Recall that jobs of a sequential task are necessarily executed in the
order that they arrive.
An arbitrary JLFP policy, however, can violate the sequential tasks
hypothesis. For example, consider two jobs j1, j2 of the same task
such that job_arrival j1 < job_arrival j2. It is possible that a JLFP
priority policy π will assign a higher priority to the second job π
j2 j1 = true. But such a situation would contradict the natural
execution order of sequential tasks. It is therefore sometimes
necessary to restrict the space of JLFP policies to those that assign
priorities in a way that is consistent with sequential tasks.
To this end, we say that a policy respects sequential tasks if, for any
two jobs j1, j2 of the same task, job_arrival j1 ≤ job_arrival j2
implies π j1 j2 = true.

Definition policy_respects_sequential_tasks :=

∀ j1 j2,

job_task j1 == job_task j2 →

job_arrival j1 ≤ job_arrival j2 →

hep_job j1 j2.

End JLFP.

∀ j1 j2,

job_task j1 == job_task j2 →

job_arrival j1 ≤ job_arrival j2 →

hep_job j1 j2.

End JLFP.

Finally, we define properties of FP policies.

Consider any FP policy.

We define what it means for an FP policy to be reflexive, transitive,
and total.
An FP policy is reflexive if the relation among "tasks" is reflexive.

An FP policy is transitive if the relation among "tasks" is transitive.

An FP policy is total if the relation among "tasks" is total.

To express the common assumption that task priorities are unique, we
define whether the given FP policy is antisymmetric over a task set
ts.

Definition antisymmetric_over_taskset (ts : seq Task) :=

antisymmetric_over_list hep_task ts.

End FP.

End Priorities.

antisymmetric_over_list hep_task ts.

End FP.

End Priorities.

## Derived Priority Relations

Consider any type of tasks ...

... and any type of jobs associated with these tasks.

Consider a JLFP-policy that indicates a higher-or-equal priority relation.

First, we introduce a relation that defines whether job j1 has
a higher-than-or-equal priority than job j2 and j1 is not
equal to j2.

Next, we introduce a relation that defines whether a job j1
has a higher-or-equal-priority than job j2 and the task of
j1 is not equal to task of j2.

Definition another_task_hep_job j1 j2 :=

hep_job j1 j2 && (job_task j1 != job_task j2).

End JLFPDerivedPriorityRelations.

hep_job j1 j2 && (job_task j1 != job_task j2).

End JLFPDerivedPriorityRelations.

In the following section, we derive two more auxiliary priority relations
for FP policies to incorporate the notions of higher and equal priority of tasks.

Consider any type of tasks and an FP policy that indicates a higher-or-equal
priority relation on the tasks.

First, we introduce a relation that defines whether task tsk1 has higher
priority than task tsk2 ,i.e., tsk1 has higher-or-equal priority than
tsk2 but tsk2 does not have higher-or-equal priority than tsk1.

Next, we introduce a relation that defines whether task tsk1 has equal
priority as task tsk2 ,i.e., tsk1 has higher-or-equal priority than tsk2
and tsk2 has higher-or-equal priority than tsk1.