From f507b5eee4b40a6621b39e8519c22a8d741794de Mon Sep 17 00:00:00 2001
From: Tatsuhiro Tsujikawa <tatsuhiro.t@gmail.com>
Date: Thu, 19 Oct 2017 21:15:08 +0900
Subject: [PATCH] nghttpx: Use an existing h2 backend connection as much as
 possible

h2load measurement reveals that this strategy is 3 times faster than
the previous implementations.
---
 src/shrpx_client_handler.cc | 76 +++++++++++++++----------------------
 1 file changed, 30 insertions(+), 46 deletions(-)

diff --git a/src/shrpx_client_handler.cc b/src/shrpx_client_handler.cc
index bf81f715..582b57da 100644
--- a/src/shrpx_client_handler.cc
+++ b/src/shrpx_client_handler.cc
@@ -804,61 +804,40 @@ bool load_lighter(const DownstreamAddr *lhs, const DownstreamAddr *rhs) {
 Http2Session *ClientHandler::select_http2_session(
     const std::shared_ptr<DownstreamAddrGroup> &group) {
   auto &shared_addr = group->shared_addr;
+  auto &http2_avail_freelist = shared_addr->http2_avail_freelist;
+
+  for (auto session = http2_avail_freelist.head; session;) {
+    auto next = session->dlnext;
+
+    session->remove_from_freelist();
+
+    // session may be in graceful shutdown period now.
+    if (session->max_concurrency_reached(0)) {
+      if (LOG_ENABLED(INFO)) {
+        CLOG(INFO, this)
+            << "Maximum streams have been reached for Http2Session(" << session
+            << ").  Skip it";
+      }
+
+      session = next;
 
-  // First count the working backend addresses.
-  size_t min = 0;
-  for (const auto &addr : shared_addr->addrs) {
-    if (addr.proto != PROTO_HTTP2 || addr.connect_blocker->blocked()) {
       continue;
     }
 
-    ++min;
-  }
-
-  if (min == 0) {
     if (LOG_ENABLED(INFO)) {
-      CLOG(INFO, this) << "No working backend address found";
+      CLOG(INFO, this) << "Use Http2Session " << session
+                       << " from http2_avail_freelist";
     }
 
-    return nullptr;
-  }
-
-  auto &http2_avail_freelist = shared_addr->http2_avail_freelist;
-
-  if (http2_avail_freelist.size() >= min) {
-    for (auto session = http2_avail_freelist.head; session;) {
-      auto next = session->dlnext;
-
-      session->remove_from_freelist();
-
-      // session may be in graceful shutdown period now.
-      if (session->max_concurrency_reached(0)) {
-        if (LOG_ENABLED(INFO)) {
-          CLOG(INFO, this)
-              << "Maximum streams have been reached for Http2Session("
-              << session << ").  Skip it";
-        }
-
-        session = next;
-
-        continue;
-      }
-
+    if (session->max_concurrency_reached(1)) {
       if (LOG_ENABLED(INFO)) {
-        CLOG(INFO, this) << "Use Http2Session " << session
-                         << " from http2_avail_freelist";
+        CLOG(INFO, this) << "Maximum streams are reached for Http2Session("
+                         << session << ").";
       }
-
-      if (session->max_concurrency_reached(1)) {
-        if (LOG_ENABLED(INFO)) {
-          CLOG(INFO, this) << "Maximum streams are reached for Http2Session("
-                           << session << ").";
-        }
-      } else {
-        session->add_to_avail_freelist();
-      }
-      return session;
+    } else {
+      session->add_to_avail_freelist();
     }
+    return session;
   }
 
   DownstreamAddr *selected_addr = nullptr;
@@ -901,7 +880,12 @@ Http2Session *ClientHandler::select_http2_session(
     }
   }
 
-  assert(selected_addr);
+  if (selected_addr == nullptr) {
+    if (LOG_ENABLED(INFO)) {
+      CLOG(INFO, this) << "No working backend address found";
+    }
+    return nullptr;
+  }
 
   if (LOG_ENABLED(INFO)) {
     CLOG(INFO, this) << "Selected DownstreamAddr=" << selected_addr