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Fix Thread pool priority alternation #123111

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Merged
eduardo-vp merged 2 commits into from
Jan 27, 2026
Merged

Fix Thread pool priority alternation #123111

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fd81865
Fix Thread pool priority alternation
Jan 13, 2026
382cd78
Remove dequeue with priority alternation
Jan 23, 2026
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30 changes: 2 additions & 28 deletions src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolWorkQueue.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -421,7 +421,6 @@ public int Count
(Environment.ProcessorCount + (ProcessorsPerAssignableWorkItemQueue - 1)) / ProcessorsPerAssignableWorkItemQueue;

private bool _loggingEnabled;
private bool _dispatchNormalPriorityWorkFirst;

// SOS's ThreadPool command depends on the following names
internal readonly WorkQueue workItems = new WorkQueue();
Expand Down Expand Up @@ -840,32 +839,6 @@ public long GlobalCount
// Dispatch (if YieldFromDispatchLoop is true), or performing periodic activities
public const uint DispatchQuantumMs = 30;

private static object? DequeueWithPriorityAlternation(ThreadPoolWorkQueue workQueue, ThreadPoolWorkQueueThreadLocals tl, out bool missedSteal)
{
object? workItem = null;

// Alternate between checking for high-prioriy and normal-priority work first, that way both sets of work
// items get a chance to run in situations where worker threads are starved and work items that run also
// take over the thread, sustaining starvation. For example, when worker threads are continually starved,
// high-priority work items may always be queued and normal-priority work items may not get a chance to run.
bool dispatchNormalPriorityWorkFirst = workQueue._dispatchNormalPriorityWorkFirst;
if (dispatchNormalPriorityWorkFirst && !tl.workStealingQueue.CanSteal)
{
workQueue._dispatchNormalPriorityWorkFirst = !dispatchNormalPriorityWorkFirst;
WorkQueue queue =
s_assignableWorkItemQueueCount > 0 ? tl.assignedGlobalWorkItemQueue : workQueue.workItems;
if (!queue.TryDequeue(out workItem) && s_assignableWorkItemQueueCount > 0)
{
workQueue.workItems.TryDequeue(out workItem);
}
}

missedSteal = false;
workItem ??= workQueue.Dequeue(tl, ref missedSteal);

return workItem;
}

/// <summary>
/// Dispatches work items to this thread.
/// </summary>
Expand All @@ -877,7 +850,8 @@ internal static bool Dispatch()
{
ThreadPoolWorkQueue workQueue = ThreadPool.s_workQueue;
ThreadPoolWorkQueueThreadLocals tl = workQueue.GetOrCreateThreadLocals();
object? workItem = DequeueWithPriorityAlternation(workQueue, tl, out bool missedSteal);
bool missedSteal = false;
object? workItem = workQueue.Dequeue(tl, ref missedSteal);
if (workItem == null)
{
// Missing a steal means there may be an item that we were unable to get.
Expand Down