Library prosa.analysis.facts.preemption.job.limited

Require Export prosa.model.schedule.limited_preemptive.
Require Export prosa.analysis.definitions.job_properties.
Require Export prosa.analysis.facts.behavior.all.
Require Export prosa.analysis.facts.model.sequential.
Require Export prosa.model.preemption.limited_preemptive.

Platform for Models with Limited Preemptions

In this section, we prove that instantiation of predicate job_preemptable to the model with limited preemptions indeed defines a valid preemption model.
Section ModelWithLimitedPreemptions.

Consider any type of tasks ...
  Context {Task : TaskType}.
  Context `{TaskCost Task}.

... and any type of jobs associated with these tasks.
  Context {Job : JobType}.
  Context `{JobTask Job Task}.
  Context `{JobArrival Job}.
  Context `{JobCost Job}.

In addition, assume the existence of a function that maps a job to the sequence of its preemption points...
  Context `{JobPreemptionPoints Job}.

..., i.e., we assume limited-preemptive jobs.
  #[local] Existing Instance limited_preemptive_job_model.

Consider any arrival sequence.
  Variable arr_seq : arrival_sequence Job.

Next, consider any limited-preemptive schedule of this arrival sequence ...
  Context {PState : ProcessorState Job}.
  Variable sched : schedule PState.
  Hypothesis H_schedule_respects_preemption_model:
    schedule_respects_preemption_model arr_seq sched.

...where jobs do not execute after their completion.
  Hypothesis H_completed_jobs_dont_execute: completed_jobs_dont_execute sched.

Next, we assume that preemption points are defined by the job-level model with limited preemptions.
  Hypothesis H_valid_limited_preemptions_job_model:
    valid_limited_preemptions_job_model arr_seq.

First, we prove a few auxiliary lemmas.
  Section AuxiliaryLemmas.

Consider a job j.
    Variable j : Job.
    Hypothesis H_j_arrives : arrives_in arr_seq j.

Recall that 0 is a preemption point.
    Remark zero_in_preemption_points: 0 \in job_preemptive_points j.
    Proof. by apply H_valid_limited_preemptions_job_model. Qed.

Using the fact that job_preemptive_points is a non-decreasing sequence, we prove that the first element of job_preemptive_points is 0.
    Lemma zero_is_first_element: first0 (job_preemptive_points j) = 0.
    Proof.
      have F := zero_in_preemption_points.
      destruct H_valid_limited_preemptions_job_model as [_ [_ C]]; specialize (C j H_j_arrives).
        by apply nondec_seq_zero_first.
    Qed.

We prove that the list of preemption points is not empty.
    Lemma list_of_preemption_point_is_not_empty:
      0 < size (job_preemptive_points j).
    Proof.
      move: H_valid_limited_preemptions_job_model ⇒ [BEG [END _]].
      apply/negPn/negP; rewrite -eqn0Ngt; intros CONTR; rewrite size_eq0 in CONTR.
      specialize (BEG j H_j_arrives).
      by move: CONTR BEG ⇒ /eqP; rewrite /beginning_of_execution_in_preemption_points ⇒ →.
    Qed.

Next, we prove that the cost of a job is a preemption point.
    Lemma job_cost_in_nonpreemptive_points: job_cost j \in job_preemptive_points j.
    Proof.
      move: H_valid_limited_preemptions_job_model ⇒ [BEG [END _]].
      move: (END _ H_j_arrives) ⇒ EQ.
      rewrite -EQ; clear EQ.
      rewrite /last0 -nth_last.
      apply/(nthP 0).
      ∃ ((size (job_preemptive_points j)).-1) ⇒ [|//].
      rewrite -(leq_add2r 1) !addn1 prednK //.
      exact: list_of_preemption_point_is_not_empty.
    Qed.

As a corollary, we prove that the sequence of non-preemptive points of a job with positive cost contains at least 2 points.
    Corollary number_of_preemption_points_at_least_two:
      job_cost_positive j →
      2 ≤ size (job_preemptive_points j).
    Proof.
      intros POS.
      move: H_valid_limited_preemptions_job_model ⇒ [BEG [END _]].
      have EQ: 2 = size [::0; job_cost j] ⇒ [//|].
      rewrite EQ; clear EQ.
      apply subseq_leq_size.
      - rewrite !cons_uniq.
        { apply/andP; split.
          + rewrite in_cons negb_or; apply/andP; split⇒ [|//].
            rewrite neq_ltn; apply/orP; left; eauto 2.
          + by apply/andP. }
      - intros t EQ; move: EQ; rewrite !in_cons.
        move ⇒ /orP [/eqP EQ| /orP [/eqP EQ|EQ]] //.
        + by rewrite EQ; apply zero_in_preemption_points.
        + by rewrite EQ; apply job_cost_in_nonpreemptive_points.
    Qed.

Next we prove that "anti-density" property (from preemption.util file) holds for job_preemption_point j.
    Lemma antidensity_of_preemption_points:
      ∀ (ρ : work),
        ρ ≤ job_cost j →
        ~~ (ρ \in job_preemptive_points j) →
        first0 (job_preemptive_points j) ≤ ρ < last0 (job_preemptive_points j).
    Proof.
      intros ρ LE NotIN.
      move: H_valid_limited_preemptions_job_model ⇒ [BEG [END NDEC]].
      apply/andP; split.
      - by rewrite zero_is_first_element.
      - rewrite END//.
        rewrite ltn_neqAle; apply/andP; split⇒ [|//].
        apply/negP; intros CONTR; move: CONTR ⇒ /eqP CONTR.
        rewrite CONTR in NotIN.
        move: NotIN ⇒ /negP NIN; apply: NIN.
        by apply job_cost_in_nonpreemptive_points.
    Qed.

We also prove that any work that doesn't belong to preemption points of job j is placed strictly between two neighboring preemption points.
    Lemma work_belongs_to_some_nonpreemptive_segment:
      ∀ (ρ : work),
        ρ ≤ job_cost j →
        ~~ (ρ \in job_preemptive_points j) →
        ∃ n,
          n.+1 < size (job_preemptive_points j) ∧
          nth 0 (job_preemptive_points j) n < ρ < nth 0 (job_preemptive_points j) n.+1.
    Proof.
      intros ρ LE NotIN.
      case: (posnP (job_cost j)) ⇒ [ZERO|POS].
      { exfalso; move: NotIN ⇒ /negP NIN; apply: NIN.
        move: LE. rewrite ZERO leqn0; move ⇒ /eqP →.
          by apply zero_in_preemption_points. }
      move: (belonging_to_segment_of_seq_is_total
               (job_preemptive_points j) ρ (number_of_preemption_points_at_least_two POS)
               (antidensity_of_preemption_points _ LE NotIN)) ⇒ [n [SIZE2 /andP [N1 N2]]].
      ∃ n; split⇒ [//|].
      apply/andP; split⇒ [|//].
      move: N1; rewrite leq_eqVlt; move ⇒ /orP [/eqP EQ | //].
      exfalso.
      move: NotIN ⇒ /negP CONTR; apply: CONTR.
      rewrite -EQ; clear EQ.
      by rewrite mem_nth.
    Qed.

Recall that the module prosa.model.preemption.parameters also defines a notion of preemption points, namely job_preemption_points, which cannot have duplicated preemption points. Therefore, we need additional lemmas to relate job_preemption_points and job_preemptive_points.
First we show that the length of the last non-preemptive segment in job_preemption_points is equal to the length of the last non-empty non-preemptive segment of job_preemptive_points.
    Lemma job_parameters_last_np_to_job_limited:
        last0 (distances (job_preemption_points j)) =
        last0 (filter (fun x ⇒ 0 < x) (distances (job_preemptive_points j))).
    Proof.
      destruct H_valid_limited_preemptions_job_model as [A1 [A2 A3]].
      unfold job_preemption_points, job_preemptable, limited_preemptive_job_model.
      intros; rewrite distances_iota_filtered; eauto.
      rewrite -A2 //.
        by intros; apply last_is_max_in_nondecreasing_seq; eauto 2.
    Qed.

Next we show that the length of the max non-preemptive segment of job_preemption_points is equal to the length of the max non-preemptive segment of job_preemptive_points.
    Lemma job_parameters_max_np_to_job_limited:
      max0 (distances (job_preemption_points j)) =
      max0 (distances (job_preemptive_points j)).
    Proof.
      destruct H_valid_limited_preemptions_job_model as [A1 [A2 A3]].
      unfold job_preemption_points, job_preemptable, limited_preemptive_job_model.
      intros; rewrite distances_iota_filtered; eauto 2.
      - by rewrite max0_rem0 //.
      - rewrite -A2 //.
        by intros; apply last_is_max_in_nondecreasing_seq; eauto 2.
    Qed.

  End AuxiliaryLemmas.

We prove that the fixed_preemption_point_model function defines a valid preemption model.
  Lemma valid_fixed_preemption_points_model_lemma:
    valid_preemption_model arr_seq sched.
  Proof.
    intros j ARR; repeat split.
    { by apply zero_in_preemption_points. }
    { by apply job_cost_in_nonpreemptive_points. }
    { by move ⇒ t NPP; apply H_schedule_respects_preemption_model. }
    { intros t NSCHED SCHED.
      have ->: service sched j t.+1 = service sched j t
        by rewrite -service_last_plus_before not_scheduled_implies_no_service // addn0.
      apply: contraT ⇒ NP.
      have: scheduled_at sched j t by apply: H_schedule_respects_preemption_model.
      by move: NSCHED ⇒ /negP. }
  Qed.

End ModelWithLimitedPreemptions.
Global Hint Resolve valid_fixed_preemption_points_model_lemma : basic_rt_facts.