From afd240dba76ecb8626f4d913ee981bfd7d9c3870 Mon Sep 17 00:00:00 2001
From: Andrew Noyes <andrew@weaselab.dev>
Date: Mon, 15 Sep 2025 11:22:14 -0400
Subject: [PATCH] Remove vestigial "round-robin" code

---
 src/server.cpp | 16 ++++++----------
 src/server.hpp |  8 +++-----
 2 files changed, 9 insertions(+), 15 deletions(-)

diff --git a/src/server.cpp b/src/server.cpp
index f3c1f36..ae68e5c 100644
--- a/src/server.cpp
+++ b/src/server.cpp
@@ -164,7 +164,7 @@ int Server::create_local_connection() {
   struct sockaddr_storage addr{};
   addr.ss_family = AF_UNIX;
 
-  // Calculate epoll_index for connection distribution
+  // Use round-robin distribution for local connections across epoll instances
   size_t epoll_index =
       connection_distribution_counter_.fetch_add(1, std::memory_order_relaxed) %
       epoll_fds_.size();
@@ -219,7 +219,8 @@ void Server::setup_shutdown_pipe() {
 }
 
 void Server::create_epoll_instances() {
-  // Create multiple epoll instances to reduce contention
+  // Create one epoll instance per I/O thread (1:1 mapping) to eliminate
+  // contention
   epoll_fds_.resize(config_.server.io_threads);
 
   for (int i = 0; i < config_.server.io_threads; ++i) {
@@ -255,11 +256,6 @@ void Server::create_epoll_instances() {
   }
 }
 
-int Server::get_epoll_for_thread(int thread_id) const {
-  // Round-robin assignment of threads to epoll instances
-  return epoll_fds_[thread_id % epoll_fds_.size()];
-}
-
 void Server::start_io_threads(std::vector<std::thread> &threads) {
   int io_threads = config_.server.io_threads;
 
@@ -268,8 +264,8 @@ void Server::start_io_threads(std::vector<std::thread> &threads) {
       pthread_setname_np(pthread_self(),
                          ("io-" + std::to_string(thread_id)).c_str());
 
-      // Each thread uses its assigned epoll instance (round-robin)
-      int epollfd = get_epoll_for_thread(thread_id);
+      // Each thread uses its assigned epoll instance (1:1 mapping)
+      int epollfd = epoll_fds_[thread_id];
 
       std::vector<epoll_event> events(config_.server.event_batch_size);
       std::vector<Ref<Connection>> batch(config_.server.event_batch_size);
@@ -374,7 +370,7 @@ void Server::start_io_threads(std::vector<std::thread> &threads) {
             }
 
             // Transfer ownership from registry to batch processing
-            size_t epoll_index = thread_id % epoll_fds_.size();
+            size_t epoll_index = thread_id;
             batch[batch_count] = make_ref<Connection>(
                 addr, fd,
                 connection_id_.fetch_add(1, std::memory_order_relaxed),
diff --git a/src/server.hpp b/src/server.hpp
index 7722507..81b88b5 100644
--- a/src/server.hpp
+++ b/src/server.hpp
@@ -124,13 +124,14 @@ private:
   std::atomic<int64_t> connection_id_{0};
   std::atomic<int> active_connections_{0};
 
-  // Round-robin counter for connection distribution
+  // Round-robin counter for local connection distribution across epoll
+  // instances
   std::atomic<size_t> connection_distribution_counter_{0};
 
   // Shutdown coordination
   int shutdown_pipe_[2] = {-1, -1};
 
-  // Multiple epoll file descriptors to reduce contention
+  // Multiple epoll file descriptors (1:1 with I/O threads) to reduce contention
   std::vector<int> epoll_fds_;
   std::vector<int>
       listen_fds_; // FDs to accept connections on (Server owns these)
@@ -141,9 +142,6 @@ private:
   void create_epoll_instances();
   void start_io_threads(std::vector<std::thread> &threads);
 
-  // Helper to get epoll fd for a thread using round-robin
-  int get_epoll_for_thread(int thread_id) const;
-
   // Helper for processing connection I/O
   void process_connection_reads(Ref<Connection> &conn, int events);
   void process_connection_writes(Ref<Connection> &conn, int events);