Reapply "A few fixes in the threadpool semaphore. Unify Windows/Unix implementation of LIFO policy." (#125193) by VSadov · Pull Request #125596 · dotnet/runtime

VSadov · 2026-03-16T01:42:21Z

TODO: need to confirm that NuGet restore performance is ok with the updated change.

dotnet-policy-service · 2026-03-16T01:43:26Z

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Pull request overview

Reapplies and reworks the ThreadPool LIFO semaphore changes (previously reverted due to NuGet restore regressions) by unifying the blocking/wake implementation across Windows and Unix using OS compare-and-wait primitives (WaitOnAddress / futex) with a monitor fallback.

Changes:

Adds low-level compare-and-wait interop for Windows (WaitOnAddress) and Linux (futex) and wires them through System.Native/CoreLib.
Replaces the prior per-OS LowLevelLifoSemaphore implementations with a unified managed implementation using a LIFO stack of per-thread blockers plus updated spin/backoff behavior.
Adjusts worker dispatch heuristics (missed-steal handling) and configuration plumbing (cooperative blocking env var alias).

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 6 comments.

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File	Description
src/native/libs/System.Native/pal_threading.h	Adds exported futex-related entrypoints to the System.Native PAL surface.
src/native/libs/System.Native/pal_threading.c	Implements Linux futex wait/wake wrappers; provides non-Linux stubs.
src/native/libs/System.Native/entrypoints.c	Registers the new futex entrypoints for managed interop.
src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolWorkQueue.cs	Adds a 1ms sleep before requesting workers when a steal was missed.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.WorkerThread.cs	Switches the worker wait to the new `LowLevelLifoSemaphore.Wait(timeout, activeThreadCount)` signature; removes old spin-limit wiring at the call site.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.Blocking.cs	Adds the `DOTNET_ThreadPool_CooperativeBlocking` env var alias for cooperative blocking.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelThreadBlocker.cs	Introduces a portable blocker abstraction (futex/WaitOnAddress or LowLevelMonitor fallback).
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs	Replaces OS-specific semaphore core with a unified managed LIFO implementation + updated spin heuristic and wake accounting.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Windows.cs	Removes the prior Windows IOCP-based LIFO semaphore implementation.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Unix.cs	Removes the prior Unix WaitSubsystem-based semaphore implementation.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Windows.cs	Adds Windows WaitOnAddress-based compare-and-wait wrapper.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Unix.cs	Adds Unix futex wrapper (currently Linux-only per comments).
src/libraries/System.Private.CoreLib/src/System/Threading/Backoff.cs	Updates exponential backoff to return spin count and reduces max backoff.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Wires new threading and interop files into CoreLib build (adds/removes Compile items).
src/libraries/Common/src/Interop/Windows/Mincore/Interop.WaitOnAddress.cs	Adds LibraryImport declarations for WaitOnAddress/WakeByAddressSingle.
src/libraries/Common/src/Interop/Windows/Kernel32/Interop.CriticalSection.cs	Adds `SuppressGCTransition` on `LeaveCriticalSection`.
src/libraries/Common/src/Interop/Windows/Interop.Libraries.cs	Adds the `Synch` API-set library constant for WaitOnAddress imports.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.LowLevelMonitor.cs	Adds `SuppressGCTransition` on `LowLevelMonitor_Release`.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.Futex.cs	Adds LibraryImport declarations for futex wait/wake entrypoints.
src/coreclr/tools/aot/ILCompiler/reproNative/reproNative.vcxproj	Adds `Synchronization.lib` to link set for NativeAOT repro project.
src/coreclr/nativeaot/BuildIntegration/WindowsAPIs.txt	Allows WaitOnAddress/WakeByAddressSingle through the NativeAOT Windows API allowlist.
src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Windows.targets	Adds `Synchronization.lib` to NativeAOT SDK library list.
docs/coding-guidelines/interop-guidelines.md	Updates interop guideline examples to match casing/structure and adds Synch library mention.

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Pull request overview

Reapplies and extends the threadpool semaphore/LIFO-policy changes that were previously reverted due to NuGet restore performance regression, by unifying the Windows/Unix implementation around a shared managed LIFO waiter stack and adding low-level wait/wake primitives (Linux futex, Windows WaitOnAddress) plus supporting interop/AOT wiring.

Changes:

Add Linux futex exports in System.Native and corresponding managed interop; add Windows WaitOnAddress interop and link inputs for NativeAOT.
Replace platform-specific LowLevelLifoSemaphore implementations with a unified managed implementation built on LowLevelThreadBlocker.
Adjust threadpool behavior around missed steals (including a brief delay) and tweak a blocking config switch plumbing.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/native/libs/System.Native/pal_threading.h	Adds futex-related `PALEXPORT` declarations.
src/native/libs/System.Native/pal_threading.c	Implements Linux futex wait/wake syscalls (with non-Linux stubs).
src/native/libs/System.Native/entrypoints.c	Exposes futex entrypoints via `DllImportEntry`.
src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolWorkQueue.cs	Adds a delay before requesting a worker when missed steals occur.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.WorkerThread.cs	Switches semaphore construction/Wait signature; minor comment fix.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.Blocking.cs	Adds env-var name to cooperative blocking config lookup.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelThreadBlocker.cs	Introduces a portable blocker using futex/WaitOnAddress or monitor fallback.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs	Reworks semaphore into a single managed implementation with LIFO waiter stack + spin heuristic.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Windows.cs	Removes prior Windows IOCP-based implementation.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Unix.cs	Removes prior Unix WaitSubsystem-based implementation.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Windows.cs	Adds Windows WaitOnAddress/WakeByAddressSingle wrapper.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Unix.cs	Adds Linux futex wrapper (Linux-only).
src/libraries/System.Private.CoreLib/src/System/Threading/Backoff.cs	Changes exponential backoff to return spin count and reduces cap.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Wires new threading/interop files into CoreLib build and removes old semaphore OS-specific files.
src/libraries/Common/src/Interop/Windows/Mincore/Interop.WaitOnAddress.cs	Adds `LibraryImport` for WaitOnAddress/WakeByAddressSingle.
src/libraries/Common/src/Interop/Windows/Kernel32/Interop.CriticalSection.cs	Adds `SuppressGCTransition` to `LeaveCriticalSection`.
src/libraries/Common/src/Interop/Windows/Interop.Libraries.cs	Adds `Libraries.Synch` constant for the synch api-set.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.LowLevelMonitor.cs	Adds `SuppressGCTransition` to `LowLevelMonitor_Release`.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.Futex.cs	Adds `LibraryImport` declarations for System.Native futex exports.
src/coreclr/tools/aot/ILCompiler/reproNative/reproNative.vcxproj	Links `Synchronization.lib` for WaitOnAddress/WakeByAddressSingle.
src/coreclr/nativeaot/BuildIntegration/WindowsAPIs.txt	Adds WaitOnAddress/WakeByAddressSingle to the NativeAOT Windows API list.
src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Windows.targets	Adds `Synchronization.lib` to SDK native libraries for NativeAOT.
docs/coding-guidelines/interop-guidelines.md	Updates interop naming/examples (e.g., `Mincore`, `Synch`).

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Pull request overview

This PR re-applies and extends prior thread pool semaphore/LIFO work distribution changes by introducing a unified, portable blocking/wake mechanism (futex/WaitOnAddress where available) and reworking the CoreLib LowLevelLifoSemaphore implementation to use it.

Changes:

Add low-level futex-style wait/wake exports on Linux in System.Native and corresponding managed interop (plus Windows WaitOnAddress interop).
Replace the platform-specific LowLevelLifoSemaphore implementations with a unified CoreLib implementation based on a LIFO stack of per-thread blockers.
Adjust thread pool worker dispatch/wait logic to differentiate spurious dispatches from regular dispatch and change spin/parking behavior accordingly.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/native/libs/System.Native/pal_threading.h	Adds new exported futex wait/wake APIs.
src/native/libs/System.Native/pal_threading.c	Implements Linux futex syscalls and non-Linux stubs.
src/native/libs/System.Native/entrypoints.c	Exposes the new futex entrypoints to managed code.
src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolWorkQueue.cs	Changes dispatch return type to a new `DispatchResult` enum.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.WorkerThread.cs	Updates worker loop to use `DispatchResult` and to park without spin when idle.
src/libraries/System.Private.CoreLib/src/System/Threading/PortableThreadPool.Blocking.cs	Adds DOTNET_ env-var alias for cooperative blocking config.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelThreadBlocker.cs	New portable thread blocker abstraction using futex/WaitOnAddress/monitor.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs	Replaces prior OS-based semaphore with unified LIFO blocker-stack implementation + new spin heuristic.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Windows.cs	New Windows WaitOnAddress wrapper for futex-like operations.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelFutex.Unix.cs	New Unix futex wrapper (Linux-backed via System.Native).
src/libraries/System.Private.CoreLib/src/System/Threading/Backoff.cs	Changes exponential backoff to return spin count and reduces max cap.
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Windows.cs	Deleted (replaced by unified implementation).
src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.Unix.cs	Deleted (replaced by unified implementation).
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Wires in new threading + interop sources.
src/libraries/Common/src/Interop/Windows/Mincore/Interop.WaitOnAddress.cs	Adds P/Invokes for WaitOnAddress/WakeByAddressSingle.
src/libraries/Common/src/Interop/Windows/Kernel32/Interop.CriticalSection.cs	Adds `SuppressGCTransition` to LeaveCriticalSection import.
src/libraries/Common/src/Interop/Windows/Interop.Libraries.cs	Adds `Libraries.Synch` constant for the synch API-set DLL.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.LowLevelMonitor.cs	Adds `SuppressGCTransition` to LowLevelMonitor_Release import.
src/libraries/Common/src/Interop/Unix/System.Native/Interop.Futex.cs	Adds System.Native futex interop declarations.
src/coreclr/tools/aot/ILCompiler/reproNative/reproNative.vcxproj	Adds Synchronization.lib to native repro project libs.
src/coreclr/nativeaot/BuildIntegration/WindowsAPIs.txt	Adds WaitOnAddress/WakeByAddressSingle to allowed API list.
src/coreclr/nativeaot/BuildIntegration/Microsoft.NETCore.Native.Windows.targets	Ensures Synchronization.lib is linked for NativeAOT.
docs/coding-guidelines/interop-guidelines.md	Updates documentation for Mincore casing/folder layout and Synch library constant.

Copilot

Pull request overview

Copilot reviewed 23 out of 23 changed files in this pull request and generated 2 comments.

Copilot

Pull request overview

Copilot reviewed 24 out of 24 changed files in this pull request and generated 5 comments.

 if (null != workStealingQueue)
 {
-TransferLocalWork();
+ThreadPoolWorkQueue.TransferAllLocalWorkItemsToHighPriorityGlobalQueue();


 // Currently where this type is used, queued work is expected to be processed
 // at high priority. The implementation could be modified to support different
 // priorities if necessary.


+ if (!HasWaitersToWake(countsBeforeUpdate))
+ break;
+
+ // CAS collision, but still have waters to wake, try again.


 public void Signal()
 {
 // Increment signal count. This enables one-shot acquire.
 Counts counts = _separated._counts.InterlockedIncrementSignalCount();


Copilot

Pull request overview

Copilot reviewed 24 out of 24 changed files in this pull request and generated 3 comments.

 {
- return counts.SignalCount != 0 || WaitForSignal(timeoutMs);
+ WakeOne();
+ break;


+ // LowLevelLock release is a full fence thus ordinary read of _pendingWake is ok
+ if (_pendingWake > 0)


+ if (!HasWaitersToWake(countsBeforeUpdate))
+ break;
+
+ // CAS collision, but still have waters to wake, try again.


…implementation of LIFO policy." (dotnet#125193) This reverts commit 51b1e92.

Copilot

Pull request overview

Copilot reviewed 25 out of 25 changed files in this pull request and generated 3 comments.

+ if (!HasWaitersToWake(countsBeforeUpdate))
+ break;
+
+ // CAS collision, but still have waters to wake, try again.


 internal const string Ucrtbase = "ucrtbase.dll";
 internal const string Xolehlp = "xolehlp.dll";
 internal const string Comdlg32 = "comdlg32.dll";
 internal const string Gdiplus = "gdiplus.dll";
- internal const string Oleaut32 = "oleaut32.dll";
 internal const string Winspool = "winspool.drv";
+ internal const string Synch = "api-ms-win-core-synch-l1-2-0.dll";
 }


@@ -94,12 +96,9 @@ internal static partial class Interop // contents of Common\src\Interop\Windows\
 private static class Libraries
 {
 internal const string Kernel32 = "kernel32.dll";
+ internal const string OleAut32 = "oleaut32.dll";
 internal const string Localization = "api-ms-win-core-localization-l1-2-0.dll";
- internal const string Handle = "api-ms-win-core-handle-l1-1-0.dll";
- internal const string ProcessThreads = "api-ms-win-core-processthreads-l1-1-0.dll";
- internal const string File = "api-ms-win-core-file-l1-1-0.dll";
- internal const string NamedPipe = "api-ms-win-core-namedpipe-l1-1-0.dll";
- internal const string IO = "api-ms-win-core-io-l1-1-0.dll";
+ internal const string Synch = "api-ms-win-core-synch-l1-2-0.dll";
 ...
 }
 }


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Pull request overview

Copilot reviewed 25 out of 25 changed files in this pull request and generated 4 comments.

+ private readonly short _maxSpinCount;
+ private readonly short _threadWakeCooldownUsec;
 private readonly Action _onWait;



+ // The spin count is chosen to be in the range of typical thread wake latency and some additional overhead,
+ // all assuming a single spin is calibrated to around 35 nanoseconds.
+ // The thread wake latency commonly measures at 2-10 microsecond (year 2026) and unlikely to drastically change.
+ private const int DefaultSemaphoreSpinCountLimit = 256;
+ // The cooldown roughly serves as detection that the thread did not spend time being blocked.
+ // If it woke in under 4 microseconds, it was likely a fast/trivial wake without blocking.
+ private const int DefaultWakeCooldown = 4;
+


- \Interop.DuplicateHandle_SafeTokenHandle.cs
- \Interop.DuplicateHandle_IntPtr.cs
+ \Kernel32
+ \Interop.DuplicateHandle_SafeFileHandle.cs


+#else// TARGET_LINUX
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-parameter"
+#pragma clang diagnostic ignored "-Wmissing-noreturn"
+void SystemNative_LowLevelFutex_WaitOnAddress(int32_t* address, int32_t comparand)
+{


VSadov · 2026-05-26T20:41:28Z

closing this as there is too much of now irrelevant comments/context. Will open a new one.

New: #128606

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The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008
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[android-arm64] The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#6408
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Comment threadsrc/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs
Comment threadsrc/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems Outdated

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VSadov added 6 commits May 25, 2026 15:03

Reapply "A few fixes in the threadpool semaphore. Unify Windows/Unix …
fb2b13b
…implementation of LIFO policy." (dotnet#125193) This reverts commit 51b1e92.

simplify LowLevelLock
0013084

USE_EVENT
75ed8bd

DOTNET_ThreadPool_CooperativeBlocking
2d127d0

ordering issue in work queue
0fde84e

lifo semaphore tweaks
f311719

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more tweaks
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priority boost is under #if TARGET_WINDOWS
dd11796

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move LowLevelThreadBlocker proj item
e297d4f

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		// LowLevelLock release is a full fence thus ordinary read of _pendingWake is ok
		if (_pendingWake > 0)

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