From f66497b0239a8b887485c1e8432f316737154552 Mon Sep 17 00:00:00 2001
From: Johannes Weiss <johannesweiss@apple.com>
Date: Fri, 7 Nov 2025 23:57:38 +0000
Subject: [PATCH] do not waste time when metrics delegate is not set

---
 .../MultiThreadedEventLoopGroup.swift         |  4 +-
 Sources/NIOPosix/SelectableEventLoop.swift    | 56 ++++++++++++-------
 2 files changed, 39 insertions(+), 21 deletions(-)

diff --git a/Sources/NIOPosix/MultiThreadedEventLoopGroup.swift b/Sources/NIOPosix/MultiThreadedEventLoopGroup.swift
index 63a0f55265..c4de5874ad 100644
--- a/Sources/NIOPosix/MultiThreadedEventLoopGroup.swift
+++ b/Sources/NIOPosix/MultiThreadedEventLoopGroup.swift
@@ -107,9 +107,9 @@ public final class MultiThreadedEventLoopGroup: EventLoopGroup {
                 canBeShutdownIndividually: canEventLoopBeShutdownIndividually,
                 metricsDelegate: metricsDelegate
             )
-            threadSpecificEventLoop.currentValue = loop
+            Self.threadSpecificEventLoop.currentValue = loop
             defer {
-                threadSpecificEventLoop.currentValue = nil
+                Self.threadSpecificEventLoop.currentValue = nil
             }
             callback(loop)
             try loop.run()
diff --git a/Sources/NIOPosix/SelectableEventLoop.swift b/Sources/NIOPosix/SelectableEventLoop.swift
index e696d505a2..3f12a251e1 100644
--- a/Sources/NIOPosix/SelectableEventLoop.swift
+++ b/Sources/NIOPosix/SelectableEventLoop.swift
@@ -187,9 +187,7 @@ internal final class SelectableEventLoop: EventLoop, @unchecked Sendable {
     private let promiseCreationStoreLock = NIOLock()
     private var _promiseCreationStore: [_NIOEventLoopFutureIdentifier: (file: StaticString, line: UInt)] = [:]
 
-    private let metricsDelegate: (any NIOEventLoopMetricsDelegate)?
-
-    private var lastTickEndTime: NIODeadline
+    private var metricsDelegateState: MetricsDelegateState?
 
     @usableFromInline
     internal func _promiseCreated(futureIdentifier: _NIOEventLoopFutureIdentifier, file: StaticString, line: UInt) {
@@ -261,7 +259,9 @@ internal final class SelectableEventLoop: EventLoop, @unchecked Sendable {
         canBeShutdownIndividually: Bool,
         metricsDelegate: NIOEventLoopMetricsDelegate?
     ) {
-        self.metricsDelegate = metricsDelegate
+        self.metricsDelegateState = metricsDelegate.map { delegate in
+            MetricsDelegateState(metricsDelegate: delegate, lastTickEndime: .now())
+        }
         self._uniqueID = uniqueID
         self._parentGroup = parentGroup
         self._selector = selector
@@ -270,7 +270,6 @@ internal final class SelectableEventLoop: EventLoop, @unchecked Sendable {
         self.msgBufferPool = Pool<PooledMsgBuffer>(maxSize: 16)
         self.tasksCopy.reserveCapacity(Self.tasksCopyBatchSize)
         self.canBeShutdownIndividually = canBeShutdownIndividually
-        self.lastTickEndTime = .now()
         // note: We are creating a reference cycle here that we'll break when shutting the SelectableEventLoop down.
         // note: We have to create the promise and complete it because otherwise we'll hit a loop in `makeSucceededFuture`. This is
         //       fairly dumb, but it's the only option we have.
@@ -747,21 +746,35 @@ internal final class SelectableEventLoop: EventLoop, @unchecked Sendable {
         return nextDeadline
     }
 
-    private func runLoop(selfIdentifier: ObjectIdentifier) -> NIODeadline? {
-        let tickStartTime: NIODeadline = .now()
-        let sleepTime: TimeAmount = tickStartTime - self.lastTickEndTime
+    private func runOneLoopTick(selfIdentifier: ObjectIdentifier) -> NIODeadline? {
+        let tickStartInfo:
+            (
+                metricsDelegate: any NIOEventLoopMetricsDelegate,
+                tickStartTime: NIODeadline,
+                sleepTime: TimeAmount
+            )? = self.metricsDelegateState.map { metricsDelegateState in
+                let tickStartTime = NIODeadline.now()  // Potentially expensive, only if delegate set.
+                return (
+                    metricsDelegate: metricsDelegateState.metricsDelegate,
+                    tickStartTime: tickStartTime,
+                    sleepTime: tickStartTime - metricsDelegateState.lastTickEndime
+                )
+            }
+
         var tasksProcessedInTick = 0
         defer {
-            let tickEndTime: NIODeadline = .now()
-            let tickInfo = NIOEventLoopTickInfo(
-                eventLoopID: selfIdentifier,
-                numberOfTasks: tasksProcessedInTick,
-                sleepTime: sleepTime,
-                startTime: tickStartTime,
-                endTime: tickEndTime
-            )
-            self.metricsDelegate?.processedTick(info: tickInfo)
-            self.lastTickEndTime = tickEndTime
+            if let tickStartInfo = tickStartInfo {
+                let tickEndTime = NIODeadline.now()  // Potentially expensive, only if delegate set.
+                let tickInfo = NIOEventLoopTickInfo(
+                    eventLoopID: selfIdentifier,
+                    numberOfTasks: tasksProcessedInTick,
+                    sleepTime: tickStartInfo.sleepTime,
+                    startTime: tickStartInfo.tickStartTime,
+                    endTime: tickEndTime
+                )
+                tickStartInfo.metricsDelegate.processedTick(info: tickInfo)
+                self.metricsDelegateState?.lastTickEndime = tickEndTime
+            }
         }
         while true {
             let nextReadyDeadline = self._tasksLock.withLock { () -> NIODeadline? in
@@ -905,7 +918,7 @@ internal final class SelectableEventLoop: EventLoop, @unchecked Sendable {
                     }
                 }
             }
-            nextReadyDeadline = runLoop(selfIdentifier: selfIdentifier)
+            nextReadyDeadline = self.runOneLoopTick(selfIdentifier: selfIdentifier)
         }
 
         // This EventLoop was closed so also close the underlying selector.
@@ -1036,6 +1049,11 @@ internal final class SelectableEventLoop: EventLoop, @unchecked Sendable {
     }
 }
 
+struct MetricsDelegateState {
+    var metricsDelegate: any NIOEventLoopMetricsDelegate
+    var lastTickEndime: NIODeadline
+}
+
 extension SelectableEventLoop: CustomStringConvertible, CustomDebugStringConvertible {
     @usableFromInline
     var description: String {