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Isolation Scheduler Hosts Sorting | Add secondary sort (No of free slots) and tertiary sort (Hostname) #8709

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Isolation Scheduler Hosts Sorting | Add secondary sort (No of free slots) and tertiary sort (Hostname) #8709

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31 changes: 29 additions & 2 deletions storm-server/src/main/java/org/apache/storm/scheduler/IsolationScheduler.java
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Original file line number Diff line number Diff line change
Expand Up @@ -303,7 +303,12 @@ private Map<String, Set<WorkerSlot>> hostToUsedSlots(Cluster cluster) {
return hostUsedSlots;
}

// returns list of list of slots, reverse sorted by number of slots
// Returns list of hosts with their assignable slots, sorted by:
// 1. (primary) total assignable slots, descending — same as before
// 2. (secondary) currently free slots, descending — prefer hosts that
// need fewer evictions
// 3. (tertiary) host name, ascending — deterministic order
// for testability
private LinkedList<HostAssignableSlots> hostAssignableSlots(Cluster cluster) {
List<WorkerSlot> assignableSlots = cluster.getAssignableSlots();
Map<String, List<WorkerSlot>> hostAssignableSlots = new HashMap<String, List<WorkerSlot>>();
Expand All @@ -316,14 +321,36 @@ private LinkedList<HostAssignableSlots> hostAssignableSlots(Cluster cluster) {
}
slots.add(slot);
}

final Map<String, Integer> hostFreeSlotCount = new HashMap<String, Integer>();
for (WorkerSlot slot : cluster.getAvailableSlots()) {
String host = cluster.getHost(slot.getNodeId());
if (hostAssignableSlots.containsKey(host)) {
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@reiabreu reiabreu May 30, 2026

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can we iterate over hostAssignableSlots in the outer loop? This will reduce the number of iterations, since hostAssignableSlots contains the target hosts

Integer count = hostFreeSlotCount.get(host);
hostFreeSlotCount.put(host, (count == null ? 0 : count) + 1);
}
}

List<HostAssignableSlots> sortHostAssignSlots = new ArrayList<HostAssignableSlots>();
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@reiabreu reiabreu May 30, 2026

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ran this through an LLM.

Suggested approach to avoid doing Map lookup in every sort iteration

 1. Add freeSlots to HostAssignableSlots:

  1     class HostAssignableSlots {
  2         private final int freeSlots; 
  3         // ... update constructor and add getter
  4     }

  2. Calculate once per host, then sort:

   1     // Build objects with pre-calculated metadata
   2     for (Map.Entry<String, List<WorkerSlot>> entry : hostAssignableSlots.entrySet()) {
   3         int free = (int) entry.getValue().stream().filter(cluster::isSlotAvailable).count();
   4         sortHostAssignSlots.add(new HostAssignableSlots(entry.getKey(), entry.getValue(), free));
   5     }
   6
   7     // Faster Comparator (Direct field access)
   8     Collections.sort(sortHostAssignSlots, (o1, o2) -> {
   9         int bySlots = o2.getWorkerSlots().size() - o1.getWorkerSlots().size();
  10         if (bySlots != 0) return bySlots;
  11         
  12         int byFree = o2.getFreeSlots() - o1.getFreeSlots();
  13         if (byFree != 0) return byFree;
  14         
  15         return o1.getHostName().compareTo(o2.getHostName());
  16     });
`

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@meetjain74 meetjain74 Jun 3, 2026

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Will pick and fix this and update the PR

for (Map.Entry<String, List<WorkerSlot>> entry : hostAssignableSlots.entrySet()) {
sortHostAssignSlots.add(new HostAssignableSlots(entry.getKey(), entry.getValue()));
}
Collections.sort(sortHostAssignSlots, new Comparator<HostAssignableSlots>() {
@Override
public int compare(HostAssignableSlots o1, HostAssignableSlots o2) {
return o2.getWorkerSlots().size() - o1.getWorkerSlots().size();
int bySlots = o2.getWorkerSlots().size() - o1.getWorkerSlots().size();
if (bySlots != 0) {
return bySlots;
}
int free1 = hostFreeSlotCount.containsKey(o1.getHostName())
? hostFreeSlotCount.get(o1.getHostName()) : 0;
int free2 = hostFreeSlotCount.containsKey(o2.getHostName())
? hostFreeSlotCount.get(o2.getHostName()) : 0;
int byFree = free2 - free1;
if (byFree != 0) {
return byFree;
}
return o1.getHostName().compareTo(o2.getHostName());
}
});

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