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https://issues.apache.org/jira/browse/YUNIKORN-3460?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Hedger Lai updated YUNIKORN-3460:
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Summary: [Shim] Minimize victim pods in k8shim preemption by introducing
reprieve verification (was: Minimize victim pods in k8shim preemption by
introducing reprieve verification)
> [Shim] Minimize victim pods in k8shim preemption by introducing reprieve
> verification
> -------------------------------------------------------------------------------------
>
> Key: YUNIKORN-3460
> URL: https://issues.apache.org/jira/browse/YUNIKORN-3460
> Project: Apache YuniKorn
> Issue Type: Improvement
> Components: shim - kubernetes
> Reporter: Hedger Lai
> Assignee: Hedger Lai
> Priority: Major
>
> h3. Problem
> In k8shim, {{PreemptionFilter()}} removes candidate victim pods sequentially
> starting from {{startIndex}}. Once filter plugins pass at index {{i}}, all
> victims from index {{0}} to {{i}} are returned to Core and preempted. This
> leads to over-preemption because earlier candidate pods that were not the
> actual cause of predicate failures (e.g., {{PodAntiAffinity}}) are
> unnecessarily evicted alongside the actual violator.
> h3. Root Cause
> Kubernetes {{kube-scheduler}} implements a two-phase preemption algorithm in
> {{selectVictimsOnNode}}: a forward search phase followed by a reprieve phase
> ({{reprievePod}}) that attempts to add back victims one by one to ensure only
> the minimal required set is evicted.
> In YuniKorn, {{predicateManagerImpl.PreemptionFilter()}} only implements the
> forward search phase:
> {code:go}
> for i := startIndex; i < len(victims); i++ {
> p.removePodFromNodeNoFail(preemptingNode, victims[i])
> status, _ := p.runFilterPlugins(ctx, *p.allocationFilters, cycleState,
> pod, preemptingNode)
> if status.IsSuccess() {
> return i
> }
> }
> {code}
> Furthermore, {{si.PreemptionPredicatesResponse}} only returns a scalar
> {{Index int32}}, forcing Core to assume all victims in {{[0, Index]}} must be
> preempted.
> h3. Fix
> 1. Extend {{si.PreemptionPredicatesResponse}} to optionally return {{repeated
> string victimAllocationKeys}} (retaining {{Index}} for backward
> compatibility).
> 2. In k8shim {{PreemptionFilter()}}, after predicates pass at index {{i}},
> introduce a reprieve pass to add back pods one by one. Pods whose restoration
> still allows predicates to pass will be retained.
> 3. Return the minimal set of victim allocation keys to Core.
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