[jira] [Created] (YARN-11907) FairSharePolicy: compareDemand() causes queue starvation for empty queues with pending apps

["Ryu Kobayashi (Jira)" <ji...@apache.org>]

Ryu Kobayashi created YARN-11907:
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             Summary: FairSharePolicy: compareDemand() causes queue starvation for empty queues with pending apps
                 Key: YARN-11907
                 URL: https://issues.apache.org/jira/browse/YARN-11907
             Project: Hadoop YARN
          Issue Type: Bug
          Components: fairscheduler
            Reporter: Ryu Kobayashi


h3. Problem

FairSharePolicy.compareDemand() assigns lowest priority to queues with demand=0, preventing progression to Stage 2 (minShare comparison). This causes severe queue starvation when a queue becomes empty but has applications waiting for
ApplicationMaster allocation.
h3. Reproduction Scenario

A leaf queue with the following characteristics experiences starvation:
 * Queue becomes temporarily empty (all previous jobs complete)
 * New applications are submitted and enter ACCEPTED state
 * Queue has demand=0 (no running containers) but getNumRunnableApps() > 0 (apps waiting for AM)
 * Other queues continue receiving AM allocations
 * The empty queue receives no AM allocations for extended periods (hours)

Observed behavior:
 * Queue remains starved for 2+ hours
 * Multiple applications wait 100+ minutes for AM allocation
 * Cluster has available resources during this period
 * Other queues receive thousands of container allocations

h3. Root Cause{*}{*}

[FairSharePolicy.java|https://github.com/apache/hadoop/blob/rel/release-3.4.2/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/policies/FairSharePolicy.java#L119-L131]

Lines 119-131:
{code:java}
private int compareDemand(Schedulable s1, Schedulable s2) {
  int res = 0;
  long demand1 = s1.getDemand().getMemorySize();
  long demand2 = s2.getDemand().getMemorySize();
  if ((demand1 == 0) && (demand2 > 0)) {
    res = 1;  // Queue with demand=0 receives lowest priority
  } else if ((demand2 == 0) && (demand1 > 0)) {
    res = -1;
  }
  return res;
} {code}

When a queue has no running containers (demand=0) but has applications in ACCEPTED state waiting for AM allocation, the queue is assigned lowest priority in Stage 1 comparison and never progresses to Stage 2 (minShare comparison), making
minResources configuration ineffective.
h3. Why Configuration Cannot Solve This

Setting minResources in fair-scheduler.xml does not prevent this starvation due to two issues:

1. Stage 1 blocks Stage 2: compareDemand() returns non-zero for empty queues, preventing progression to compareMinShareUsage()
2. MinShare calculation caps at demand: Even if reaching Stage 2, lines 136-141 cap minShare:
long minShare1 = Math.min(s1.getMinShare().getMemorySize(),
    s1.getDemand().getMemorySize());  // When demand=0, minShare becomes 0



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