adding predicate ordering design proposal

contributors/design-proposals/predicates-ordering.md

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+# predicates ordering
+
+
+
+Status: proposal
+
+Author: yastij
+Approvers: 
+* gmarek
+* bsalamat
+* k82cn
+
+
+
+
+## Abstract
+
+This document describes how and why reordering predicates helps to achieve performance for the kubernetes scheduler.
+We will expose the motivations behind this proposal, The two steps/solution we see to tackle this problem and the timeline decided to implement these.
+
+
+## Motivation
+
+While working on a [Pull request](https://github.com/kubernetes/kubernetes/pull/50185) related to a proposal, we saw that the order of running predicates isn’t defined. 
+
+This makes the scheduler perform extra-computation that isn’t needed, As an example we [outlined](https://github.com/kubernetes/kubernetes/pull/50185) that the kubernetes scheduler runs predicates against nodes even if marked “unschedulable”.
+
+Reordering predicates allows us to avoid this problem, by computing the most restrictive predicates first. To do so, we propose two reordering types.
+
+
+
+## Static ordering
+
+This ordering will be the default ordering. If a policy config is provided with a subset of predicates, only those predicates will be invoked using the static ordering. 
+
+
+
+
+|Position                  | Predicate                        | comments (note, justification...)              |
+ ----------------- | ---------------------------- | ------------------
+| 1 | `CheckNodeConditionPredicate`  | we really don’t want to check predicates against unschedulable nodes. |
+| 2           | `PodFitsHost`            | we check the pod.spec.nodeName. |
+| 3           | `PodFitsHostPorts` | we check ports asked on the spec. |
+| 4 | `PodMatchNodeSelector`            | check node label after narrowing search. |
+| 5           | `PodFitsResources `            | this one comes here since it’s not restrictive enough as we do not try to match values but ranges. |
+| 6           | `NoDiskConflict` | Following the resource predicate, we check disk |
+| 7 | `PodToleratesNodeTaints '`            | check toleration here, as node might have toleration |
+| 8          | `PodToleratesNodeNoExecuteTaints`            | check toleration here, as node might have toleration |
+| 9           | `CheckNodeLabelPresence ` | labels are easy to check, so this one goes before |
+| 10 | `checkServiceAffinity `            | - |
+| 11           | `MaxPDVolumeCountPredicate `            | - |
+| 12           | `VolumeNodePredicate ` | - |
+| 13 | `VolumeZonePredicate `            | - |
+| 14           | `CheckNodeMemoryPressurePredicate`            | doesn’t happen often |
+| 15           | `CheckNodeDiskPressurePredicate` | doesn’t happen often |
+| 16 | `InterPodAffinityMatches`            | Most expensive predicate to compute |
+
+
+## End-user ordering
+
+Using scheduling policy file, the cluster admin can override the default static ordering. This gives administrator the maximum flexibility regarding scheduler behaviour and enables scheduler to adapt to cluster usage. 
+Please note that the order must be a positive integer, also, when providing equal ordering for many predicates, scheduler will determine the order and won't guarantee that the order will remain the same between them.
+Finally updating the Policy will require a scheduler restart.
+
+as an example the following is scheduler policy file using an end-user ordering:
+
+``` json
+{
+"kind" : "Policy",
+"apiVersion" : "v1",
+"predicates" : [
+	{"name" : "PodFitsHostPorts", "order": 2},
+	{"name" : "PodFitsResources", "order": 3},
+	{"name" : "NoDiskConflict", "order": 5},
+	{"name" : "PodToleratesNodeTaints", "order": 4},
+	{"name" : "MatchNodeSelector", "order": 6},
+	{"name" : "PodFitsHost", "order": 1}
+	],
+"priorities" : [
+	{"name" : "LeastRequestedPriority", "weight" : 1},
+	{"name" : "BalancedResourceAllocation", "weight" : 1},
+	{"name" : "ServiceSpreadingPriority", "weight" : 1},
+	{"name" : "EqualPriority", "weight" : 1}
+	],
+"hardPodAffinitySymmetricWeight" : 10
+}
+```
+
+
+## Timeline
+
+* static ordering: GA in 1.9
+* dynamic ordering: TBD based on customer feedback