DNS prefetch experiment extension: Consider different connection count limits

This CL both enables selection of a run-time selected limit
on the number of connections to a single host, and varies
that limit to see how it relates to DNS prefetch latency
in connections.

r=wtc
Review URL: http://codereview.chromium.org/62083

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@13339 0039d316-1c4b-4281-b951-d872f2087c98

4 files changed, 50 insertions, 11 deletions

diff --git a/chrome/browser/browser_main.cc b/chrome/browser/browser_main.cc
index 8dc32d4..0ef576f 100644
--- a/chrome/browser/browser_main.cc
+++ b/chrome/browser/browser_main.cc
@@ -48,6 +48,7 @@
 #include "grit/generated_resources.h"
 #include "grit/net_resources.h"
 #include "net/base/net_module.h"
+#include "net/http/http_network_session.h"
 
 #if defined(OS_POSIX)
 // TODO(port): get rid of this include. It's used just to provide declarations
@@ -316,7 +317,7 @@ int BrowserMain(const MainFunctionParams& parameters) {
 
   if (!parsed_command_line.HasSwitch(switches::kNoErrorDialogs)) {
     // Display a warning if the user is running windows 2000.
-    // TODO(port). We should probably change this to a "check for minimum
+    // TODO(port): We should probably change this to a "check for minimum
     // requirements" function, implemented by each platform.
     CheckForWin2000();
   }
@@ -439,14 +440,21 @@ int BrowserMain(const MainFunctionParams& parameters) {
   net::EnsureWinsockInit();
 #endif  // defined(OS_WIN)
 
-  // Set up a field trial.
+  // Set up a field trial to see what disabling DNS pre-resolution does to
+  // latency of network transactions.
   FieldTrial::Probability kDIVISOR = 100;
-  FieldTrial::Probability kDISABLE = 1;  // 1%.
+  FieldTrial::Probability k_PROBABILITY_PER_GROUP = 10;  // 10%.
   scoped_refptr<FieldTrial> dns_trial = new FieldTrial("DnsImpact", kDIVISOR);
-  int disabled_group = dns_trial->AppendGroup("_disabled_prefetch", kDISABLE);
+
+  dns_trial->AppendGroup("_disabled_prefetch", k_PROBABILITY_PER_GROUP);
+  int disabled_plus_4_connections = dns_trial->AppendGroup(
+      "_disabled_prefetch_4_connections", k_PROBABILITY_PER_GROUP);
+  int enabled_plus_4_connections = dns_trial->AppendGroup(
+      "_enabled_prefetch_4_connections", k_PROBABILITY_PER_GROUP);
 
   scoped_ptr<chrome_browser_net::DnsPrefetcherInit> dns_prefetch_init;
-  if (dns_trial->group() != disabled_group) {
+  if (dns_trial->group() == FieldTrial::kNotParticipating ||
+      dns_trial->group() == enabled_plus_4_connections) {
     // Initialize the DNS prefetch system
     dns_prefetch_init.reset(
         new chrome_browser_net::DnsPrefetcherInit(user_prefs));
@@ -454,6 +462,12 @@ int BrowserMain(const MainFunctionParams& parameters) {
     chrome_browser_net::RestoreSubresourceReferrers(local_state);
   }
 
+  if (dns_trial->group() == disabled_plus_4_connections ||
+      dns_trial->group() == enabled_plus_4_connections) {
+    net::HttpNetworkSession::set_max_sockets_per_group(4);
+  }
+
+
 #if defined(OS_WIN)
   // Init common control sex.
   INITCOMMONCONTROLSEX config;
diff --git a/net/http/http_network_session.cc b/net/http/http_network_session.cc
new file mode 100644
index 0000000..a498ed0
--- /dev/null
+++ b/net/http/http_network_session.cc
@@ -0,0 +1,22 @@
+// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "net/http/http_network_session.h"
+
+#include "base/logging.h"
+
+namespace net {
+
+// static
+int HttpNetworkSession::max_sockets_per_group_ = 6;
+
+// static
+void HttpNetworkSession::set_max_sockets_per_group(int socket_count) {
+  DCHECK(0 < socket_count);
+  // The following is a sanity check... but we should NEVER be near this value.
+  DCHECK(100 > socket_count);
+  max_sockets_per_group_ = socket_count;
+}
+
+} //  namespace net
diff --git a/net/http/http_network_session.h b/net/http/http_network_session.h
index 2dd1b89..7c4ac68 100644
--- a/net/http/http_network_session.h
+++ b/net/http/http_network_session.h
@@ -17,13 +17,8 @@ class ProxyService;
 // This class holds session objects used by HttpNetworkTransaction objects.
 class HttpNetworkSession : public base::RefCounted<HttpNetworkSession> {
  public:
-  // Allow up to 6 connections per host.
-  enum {
-    MAX_SOCKETS_PER_GROUP = 6
-  };
-
   explicit HttpNetworkSession(ProxyService* proxy_service)
-      : connection_pool_(new ClientSocketPool(MAX_SOCKETS_PER_GROUP)),
+      : connection_pool_(new ClientSocketPool(max_sockets_per_group_)),
         proxy_service_(proxy_service) {
     DCHECK(proxy_service);
   }
@@ -35,7 +30,14 @@ class HttpNetworkSession : public base::RefCounted<HttpNetworkSession> {
   SSLConfigService* ssl_config_service() { return &ssl_config_service_; }
 #endif
 
+  static void set_max_sockets_per_group(int socket_count);
+
  private:
+  // Default to allow up to 6 connections per host. Experiment and tuning may
+  // try other values (greater than 0).  Too large may cause many problems, such
+  // as home routers blocking the connections!?!?
+  static int max_sockets_per_group_;
+
   HttpAuthCache auth_cache_;
   scoped_refptr<ClientSocketPool> connection_pool_;
   ProxyService* proxy_service_;
diff --git a/net/net.gyp b/net/net.gyp
index ca972fc..cfd17f6 100644
--- a/net/net.gyp
+++ b/net/net.gyp
@@ -217,6 +217,7 @@
         'http/http_chunked_decoder.h',
         'http/http_network_layer.cc',
         'http/http_network_layer.h',
+        'http/http_network_session.cc',
         'http/http_network_session.h',
         'http/http_network_transaction.cc',
         'http/http_network_transaction.h',