Add an index to thread pipeline iterators for load balancing

src/http_handler.hpp

@@ -61,33 +61,46 @@ struct HttpConnectionState {
  * Supports the WeaselDB REST API endpoints with enum-based routing.
  */
 class HttpHandler : public ConnectionHandler {
-  ThreadPipeline<std::unique_ptr<Connection>> ok_pipeline{10, {1}};
-  std::thread ok_thread{[this]() {
-    pthread_setname_np(pthread_self(), "stage-0");
-    for (;;) {
-      auto guard = ok_pipeline.acquire(0, 0);
-      for (auto &c : guard.batch) {
-        if (!c) {
-          return;
-        }
-        auto *state = static_cast<HttpConnectionState *>(c->user_data);
-        TRACE_EVENT("http", "pipeline thread",
-                    perfetto::Flow::Global(state->request_id));
-        Server::releaseBackToServer(std::move(c));
-      }
-    }
-  }};
+  static constexpr int kFinalStageThreads = 2;
+  static constexpr int kLogSize = 12;
+  ThreadPipeline<std::unique_ptr<Connection>> pipeline{kLogSize,
+                                                       {kFinalStageThreads}};
+  std::vector<std::thread> finalStageThreads;
 
 public:
-  HttpHandler() = default;
+  HttpHandler() {
+    for (int threadId = 0; threadId < kFinalStageThreads; ++threadId) {
+      finalStageThreads.emplace_back([this, threadId]() {
+        pthread_setname_np(pthread_self(),
+                           ("stage-0-" + std::to_string(threadId)).c_str());
+        for (;;) {
+          auto guard = pipeline.acquire(0, threadId);
+          for (auto it = guard.batch.begin(); it != guard.batch.end(); ++it) {
+            if ((it.index() % kFinalStageThreads) == threadId) {
+              auto &c = *it;
+              if (!c) {
+                return;
+              }
+              auto *state = static_cast<HttpConnectionState *>(c->user_data);
+              TRACE_EVENT("http", "pipeline thread",
+                          perfetto::Flow::Global(state->request_id));
+              Server::releaseBackToServer(std::move(c));
+            }
+          }
+        }
+      });
+    }
+  }
   ~HttpHandler() {
     {
-      auto guard = ok_pipeline.push(1, true);
+      auto guard = pipeline.push(kFinalStageThreads, true);
       for (auto &c : guard.batch) {
         c = {};
       }
     }
-    ok_thread.join();
+    for (auto &thread : finalStageThreads) {
+      thread.join();
+    }
   }
 
   void on_connection_established(Connection &conn) override;
@@ -95,6 +108,7 @@ public:
   void on_data_arrived(std::string_view data,
                        std::unique_ptr<Connection> &conn_ptr) override;
   void on_write_progress(std::unique_ptr<Connection> &conn_ptr) override;
+  void on_post_batch(std::span<std::unique_ptr<Connection>> /*batch*/) override;
 
   // Route parsing (public for testing)
   static HttpRoute parseRoute(std::string_view method, std::string_view url);