From 8b464ad877e74256bc858456a503a999ec99baef Mon Sep 17 00:00:00 2001
From: Luke Howard <lukeh@padl.com>
Date: Wed, 25 Mar 2026 12:23:53 +1100
Subject: [PATCH] Fix high CPU usage on Linux due to epoll busy-loop

Three fixes for 100% CPU spinning when HTTPServer is idle on Linux:

1. Use edge-triggered (EPOLLET) mode for all sockets, not just the
   canary eventfd. Level-triggered epoll causes epoll_wait() to return
   immediately when the listening socket is readable, creating a busy
   loop.

2. Change accept() to wait on .read events only instead of .connection
   ([.read, .write]). A listening socket is always writable (EPOLLOUT),
   so registering for write events causes immediate spurious wakeups.
   accept() only needs readability (a pending connection).

3. Skip redundant epoll_ctl(EPOLL_CTL_MOD) calls when the event mask
   hasn't changed. EPOLL_CTL_MOD re-arms edge-triggered events and can
   cause spurious wakeups when the condition is already met.

Fixes: #186
---
 FlyingSocks/Sources/AsyncSocket.swift      | 2 +-
 FlyingSocks/Sources/SocketPool+ePoll.swift | 5 ++++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/FlyingSocks/Sources/AsyncSocket.swift b/FlyingSocks/Sources/AsyncSocket.swift
index 8d235e96..31f9f009 100644
--- a/FlyingSocks/Sources/AsyncSocket.swift
+++ b/FlyingSocks/Sources/AsyncSocket.swift
@@ -131,7 +131,7 @@ public struct AsyncSocket: Sendable {
 
     @Sendable
     public func accept() async throws -> AsyncSocket {
-        try await pool.loopUntilReady(for: .connection, on: socket) {
+        try await pool.loopUntilReady(for: .read, on: socket) {
             let file = try socket.accept().file
             let socket = Socket(file: file)
             return try AsyncSocket(socket: socket, pool: pool)
diff --git a/FlyingSocks/Sources/SocketPool+ePoll.swift b/FlyingSocks/Sources/SocketPool+ePoll.swift
index a104e019..66ef808e 100644
--- a/FlyingSocks/Sources/SocketPool+ePoll.swift
+++ b/FlyingSocks/Sources/SocketPool+ePoll.swift
@@ -105,6 +105,7 @@ public struct ePoll: EventQueue {
     }
 
     mutating func setEvents(_ events: Socket.Events, for socket: Socket.FileDescriptor) throws {
+        if existing[socket] == events { return }
         var event = CSystemLinux.epoll_event()
         event.events = events.epollEvents.rawValue
         event.data.fd = socket.rawValue
@@ -242,7 +243,9 @@ private struct EPOLLEvents: OptionSet, Hashable {
 private extension Socket.Events {
 
     var epollEvents: EPOLLEvents {
-        reduce(EPOLLEvents()) { [$0, $1.epollEvent] }
+        var events = reduce(EPOLLEvents()) { [$0, $1.epollEvent] }
+        events.insert(.edgeTriggered)
+        return events
     }
 
     static func make(from pollevents: EPOLLEvents) -> Socket.Events {