Add a command line flag to change the default number of parallel DNS requests...

chrome/browser/io_thread.cc

 // Copyright (c) 2010 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 "chrome/browser/io_thread.h"
 #include "base/command_line.h"
 #include "base/leak_tracker.h"
 #include "base/logging.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/chrome_thread.h"
 #include "chrome/browser/gpu_process_host.h"
 #include "chrome/browser/net/chrome_net_log.h"
 #include "chrome/browser/net/predictor_api.h"
 #include "chrome/browser/net/passive_log_collector.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/net/url_fetcher.h"
 #include "net/base/mapped_host_resolver.h"
 #include "net/base/host_cache.h"
 #include "net/base/host_resolver.h"
 #include "net/base/host_resolver_impl.h"
 #include "net/base/net_util.h"
 #include "net/http/http_auth_filter.h"
 #include "net/http/http_auth_handler_factory.h"
 #include "net/http/http_auth_handler_negotiate.h"
 
 namespace {
 
 net::HostResolver* CreateGlobalHostResolver() {
   const CommandLine& command_line = *CommandLine::ForCurrentProcess();
-  net::HostResolver* global_host_resolver = net::CreateSystemHostResolver();
+
+  size_t parallelism = net::HostResolver::kDefaultParallelism;
+
+  // Use the concurrency override from the command-line, if any.
+  if (command_line.HasSwitch(switches::kHostResolverParallelism)) {
+    std::string s =
+        command_line.GetSwitchValueASCII(switches::kHostResolverParallelism);
+
+    // Parse the switch (it should be a positive integer formatted as decimal).
+    int n;
+    if (StringToInt(s, &n) && n > 0) {
+      parallelism = static_cast<size_t>(n);
+    } else {
+      LOG(ERROR) << "Invalid switch for host resolver parallelism: " << s;
+    }
+  }
+
+  net::HostResolver* global_host_resolver =
+      net::CreateSystemHostResolver(parallelism);
 
   // Determine if we should disable IPv6 support.
   if (!command_line.HasSwitch(switches::kEnableIPv6)) {
     if (command_line.HasSwitch(switches::kDisableIPv6)) {
       global_host_resolver->SetDefaultAddressFamily(net::ADDRESS_FAMILY_IPV4);
     } else {
       net::HostResolverImpl* host_resolver_impl =
           global_host_resolver->GetAsHostResolverImpl();
       if (host_resolver_impl != NULL) {
         // (optionally) Use probe to decide if support is warranted.
@@ skipping 310 matching lines @@
     net::HostCache* host_cache =
         globals_->host_resolver.get()->GetAsHostResolverImpl()->cache();
     if (host_cache)
       host_cache->clear();
   }
   // Clear all of the passively logged data.
   // TODO(eroman): this is a bit heavy handed, really all we need to do is
   //               clear the data pertaining to off the record context.
   globals_->net_log->passive_collector()->Clear();
 }