move prefilter out of predicates to improve performance

[elinx]

• when porting kube-scheduler plugins, the current implementation put PreFilter together with Filter into PredicateFn, the PredicateFn process each pod-node pair like the following pseudo code:

  for pod in pods:
    for nodes in nodes:
      predicateFn(pod, node) # prefilters & filters

• it's functioning well while PreFilter only needs the pod object(no node required), we don't need to put it in the innermost loop. So a promotion of PreFilter to the outer loop is made in this pr for performance's sake, as a plugin like inter-pod-afftinity is very time-consuming to prepare the pod interconnections in PreFilter. After this pr:

  for pod in pods:
    prePredicateFn(pod) # prefilters
    for nodes in nodes:
      predicateFn(pod, node) # filters

• Implementation notes:
  • Is it right to put stat in OnSessionOpen as previously put into the PredicateFn?
    yes, it's correct, just as the type of stat suggested CycleState its lifecycle should be in the whole scheduling cycle, and the kube-scheduler creates it in the ScheduleOnce function

    	  func (sched *Scheduler) scheduleOne(ctx context.Context) {
    	  	// ...
    	  	state := framework.NewCycleState()
    	    	scheduleResult, err := 
    	    		sched.Algorithm.Schedule(schedulingCycleCtx, prof, state, pod)
    	      // ...
    	  }

  • is it thread-safe to separate write(in PreFilter) and read (in Filter)?
    yes, the underlining data structure is a thread-safe map for CycleState to access data

resolves #2583

[Thor-wl]

Thanks for the optimization. It looks OK for me generally.

[volcano-sh-bot]

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