Fix bug where a large number of extensions can starve the browser for renderer processes

BUG=98737
TEST=Install multiple extensions, run Chrome with --render-process-limit=6. All extensions should be placed in only 2 processes.

Review URL: http://codereview.chromium.org/9418010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@123072 0039d316-1c4b-4281-b951-d872f2087c98

16 files changed, 334 insertions, 139 deletions

diff --git a/chrome/browser/chrome_content_browser_client.cc b/chrome/browser/chrome_content_browser_client.cc
index dc3625f..3c36f12 100644
--- a/chrome/browser/chrome_content_browser_client.cc
+++ b/chrome/browser/chrome_content_browser_client.cc
@@ -25,7 +25,9 @@
 #include "chrome/browser/content_settings/tab_specific_content_settings.h"
 #include "chrome/browser/download/download_util.h"
 #include "chrome/browser/extensions/api/webrequest/webrequest_api.h"
+#include "chrome/browser/extensions/extension_host.h"
 #include "chrome/browser/extensions/extension_info_map.h"
+#include "chrome/browser/extensions/extension_process_manager.h"
 #include "chrome/browser/extensions/extension_message_handler.h"
 #include "chrome/browser/extensions/extension_service.h"
 #include "chrome/browser/extensions/extension_web_ui.h"
@@ -44,6 +46,7 @@
 #include "chrome/browser/printing/printing_message_filter.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_io_data.h"
+#include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/browser/renderer_host/chrome_render_message_filter.h"
 #include "chrome/browser/renderer_host/chrome_render_view_host_observer.h"
 #include "chrome/browser/renderer_host/plugin_info_message_filter.h"
@@ -508,6 +511,58 @@ bool ChromeContentBrowserClient::IsSuitableHost(
       privilege_required;
 }
 
+// This function is trying to limit the amount of processes used by extensions
+// with background pages. It uses a globally set percentage of processes to
+// run such extensions and if the limit is exceeded, it returns true, to
+// indicate to the content module to group extensions together.
+bool ChromeContentBrowserClient::ShouldTryToUseExistingProcessHost(
+    content::BrowserContext* browser_context, const GURL& url) {
+  // It has to be a valid URL for us to check for an extension.
+  if (!url.is_valid())
+    return false;
+
+  Profile* profile = Profile::FromBrowserContext(browser_context);
+  ExtensionService* service = profile->GetExtensionService();
+  if (!service)
+    return false;
+
+  // We have to have a valid extension with background page to proceed.
+  const Extension* extension =
+      service->extensions()->GetExtensionOrAppByURL(ExtensionURLInfo(url));
+  if (!extension)
+    return false;
+  if (!extension->has_background_page())
+    return false;
+
+  std::set<int> process_ids;
+  size_t max_process_count =
+      content::RenderProcessHost::GetMaxRendererProcessCount();
+
+  // Go through all profiles to ensure we have total count of extension
+  // processes containing background pages, otherwise one profile can
+  // starve the other.
+  std::vector<Profile*> profiles = g_browser_process->profile_manager()->
+      GetLoadedProfiles();
+  for (size_t i = 0; i < profiles.size(); ++i) {
+    ExtensionProcessManager* epm = profiles[i]->GetExtensionProcessManager();
+    for (ExtensionProcessManager::const_iterator iter = epm->begin();
+       iter != epm->end();
+       ++iter) {
+      ExtensionHost* host = *iter;
+      if (host->extension()->has_background_page()) {
+        process_ids.insert(host->render_process_host()->GetID());
+      }
+    }
+  }
+
+  if (process_ids.size() >
+      (max_process_count * chrome::kMaxShareOfExtensionProcesses)) {
+    return true;
+  }
+
+  return false;
+}
+
 void ChromeContentBrowserClient::SiteInstanceGotProcess(
     SiteInstance* site_instance) {
   CHECK(site_instance->HasProcess());
diff --git a/chrome/browser/chrome_content_browser_client.h b/chrome/browser/chrome_content_browser_client.h
index 56ad007..8cc56ef 100644
--- a/chrome/browser/chrome_content_browser_client.h
+++ b/chrome/browser/chrome_content_browser_client.h
@@ -34,6 +34,8 @@ class ChromeContentBrowserClient : public content::ContentBrowserClient {
   virtual bool IsHandledURL(const GURL& url) OVERRIDE;
   virtual bool IsSuitableHost(content::RenderProcessHost* process_host,
                               const GURL& url) OVERRIDE;
+  virtual bool ShouldTryToUseExistingProcessHost(
+      content::BrowserContext* browser_context, const GURL& url) OVERRIDE;
   virtual void SiteInstanceGotProcess(
       content::SiteInstance* site_instance) OVERRIDE;
   virtual void SiteInstanceDeleting(content::SiteInstance* site_instance)
diff --git a/chrome/browser/extensions/isolated_app_browsertest.cc b/chrome/browser/extensions/isolated_app_browsertest.cc
index 0c17507..26f7952 100644
--- a/chrome/browser/extensions/isolated_app_browsertest.cc
+++ b/chrome/browser/extensions/isolated_app_browsertest.cc
@@ -169,135 +169,3 @@ IN_PROC_BROWSER_TEST_F(IsolatedAppTest, NoCookieIsolationWithoutApp) {
   EXPECT_TRUE(HasCookie(browser()->GetWebContentsAt(2), "app2=4"));
   EXPECT_TRUE(HasCookie(browser()->GetWebContentsAt(2), "nonAppFrame=6"));
 }
-
-// Ensure that an isolated app never shares a process with WebUIs, non-isolated
-// extensions, and normal webpages.  None of these should ever comingle
-// RenderProcessHosts even if we hit the process limit.
-IN_PROC_BROWSER_TEST_F(IsolatedAppTest, ProcessOverflow) {
-  // Set max renderers to 1 to force running out of processes.
-  content::RenderProcessHost::SetMaxRendererProcessCount(1);
-
-  host_resolver()->AddRule("*", "127.0.0.1");
-  ASSERT_TRUE(test_server()->Start());
-
-  ASSERT_TRUE(LoadExtension(test_data_dir_.AppendASCII("isolated_apps/app1")));
-  ASSERT_TRUE(LoadExtension(test_data_dir_.AppendASCII("isolated_apps/app2")));
-  ASSERT_TRUE(LoadExtension(test_data_dir_.AppendASCII("hosted_app")));
-  ASSERT_TRUE(
-      LoadExtension(test_data_dir_.AppendASCII("api_test/app_process")));
-
-  // The app under test acts on URLs whose host is "localhost",
-  // so the URLs we navigate to must have host "localhost".
-  GURL base_url = test_server()->GetURL(
-      "files/extensions/");
-  GURL::Replacements replace_host;
-  std::string host_str("localhost");  // Must stay in scope with replace_host.
-  replace_host.SetHostStr(host_str);
-  base_url = base_url.ReplaceComponents(replace_host);
-
-  // Load an extension before adding tabs.
-  const Extension* extension1 = LoadExtension(
-      test_data_dir_.AppendASCII("api_test/browser_action/basics"));
-  ASSERT_TRUE(extension1);
-  GURL extension1_url = extension1->url();
-
-  // Create multiple tabs for each type of renderer that might exist.
-  ui_test_utils::NavigateToURLWithDisposition(
-      browser(), base_url.Resolve("isolated_apps/app1/main.html"),
-      CURRENT_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
-  ui_test_utils::NavigateToURLWithDisposition(
-      browser(), GURL(chrome::kChromeUINewTabURL),
-      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
-  ui_test_utils::NavigateToURLWithDisposition(
-      browser(), base_url.Resolve("hosted_app/main.html"),
-      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
-  ui_test_utils::NavigateToURLWithDisposition(
-      browser(), base_url.Resolve("test_file.html"),
-      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
-
-  ui_test_utils::NavigateToURLWithDisposition(
-      browser(), base_url.Resolve("isolated_apps/app2/main.html"),
-      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
-  ui_test_utils::NavigateToURLWithDisposition(
-      browser(), GURL(chrome::kChromeUINewTabURL),
-      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
-  ui_test_utils::NavigateToURLWithDisposition(
-      browser(), base_url.Resolve("api_test/app_process/path1/empty.html"),
-      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
-  ui_test_utils::NavigateToURLWithDisposition(
-      browser(), base_url.Resolve("test_file_with_body.html"),
-      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
-
-  // Load another copy of isolated app 1.
-  ui_test_utils::NavigateToURLWithDisposition(
-      browser(), base_url.Resolve("isolated_apps/app1/main.html"),
-      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
-
-  // Load another extension.
-  const Extension* extension2 = LoadExtension(
-      test_data_dir_.AppendASCII("api_test/browser_action/close_background"));
-  ASSERT_TRUE(extension2);
-  GURL extension2_url = extension2->url();
-
-  // Get tab processes.
-  ASSERT_EQ(9, browser()->tab_count());
-  content::RenderProcessHost* isolated1_host =
-      browser()->GetWebContentsAt(0)->GetRenderProcessHost();
-  content::RenderProcessHost* ntp1_host =
-      browser()->GetWebContentsAt(1)->GetRenderProcessHost();
-  content::RenderProcessHost* hosted1_host =
-      browser()->GetWebContentsAt(2)->GetRenderProcessHost();
-  content::RenderProcessHost* web1_host =
-      browser()->GetWebContentsAt(3)->GetRenderProcessHost();
-
-  content::RenderProcessHost* isolated2_host =
-      browser()->GetWebContentsAt(4)->GetRenderProcessHost();
-  content::RenderProcessHost* ntp2_host =
-      browser()->GetWebContentsAt(5)->GetRenderProcessHost();
-  content::RenderProcessHost* hosted2_host =
-      browser()->GetWebContentsAt(6)->GetRenderProcessHost();
-  content::RenderProcessHost* web2_host =
-      browser()->GetWebContentsAt(7)->GetRenderProcessHost();
-
-  content::RenderProcessHost* second_isolated1_host =
-      browser()->GetWebContentsAt(8)->GetRenderProcessHost();
-
-  // Get extension processes.
-  ExtensionProcessManager* process_manager =
-    browser()->GetProfile()->GetExtensionProcessManager();
-  content::RenderProcessHost* extension1_host =
-      process_manager->GetSiteInstanceForURL(extension1_url)->GetProcess();
-  content::RenderProcessHost* extension2_host =
-      process_manager->GetSiteInstanceForURL(extension2_url)->GetProcess();
-
-  // An isolated app only shares with other instances of itself, not other
-  // isolated apps or anything else.
-  EXPECT_EQ(isolated1_host, second_isolated1_host);
-  EXPECT_NE(isolated1_host, isolated2_host);
-  EXPECT_NE(isolated1_host, ntp1_host);
-  EXPECT_NE(isolated1_host, hosted1_host);
-  EXPECT_NE(isolated1_host, web1_host);
-  EXPECT_NE(isolated1_host, extension1_host);
-  EXPECT_NE(isolated2_host, ntp1_host);
-  EXPECT_NE(isolated2_host, hosted1_host);
-  EXPECT_NE(isolated2_host, web1_host);
-  EXPECT_NE(isolated2_host, extension1_host);
-
-  // Everything else is clannish.  WebUI only shares with other WebUI.
-  EXPECT_EQ(ntp1_host, ntp2_host);
-  EXPECT_NE(ntp1_host, hosted1_host);
-  EXPECT_NE(ntp1_host, web1_host);
-  EXPECT_NE(ntp1_host, extension1_host);
-
-  // Hosted apps only share with each other.
-  EXPECT_EQ(hosted1_host, hosted2_host);
-  EXPECT_NE(hosted1_host, web1_host);
-  EXPECT_NE(hosted1_host, extension1_host);
-
-  // Web pages only share with each other.
-  EXPECT_EQ(web1_host, web2_host);
-  EXPECT_NE(web1_host, extension1_host);
-
-  // Extensions only share with each other.
-  EXPECT_EQ(extension1_host, extension2_host);
-}
diff --git a/chrome/browser/extensions/process_management_browsertest.cc b/chrome/browser/extensions/process_management_browsertest.cc
new file mode 100644
index 0000000..54022c6
--- /dev/null
+++ b/chrome/browser/extensions/process_management_browsertest.cc
@@ -0,0 +1,232 @@
+// Copyright (c) 2012 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 "base/utf_string_conversions.h"
+#include "chrome/browser/automation/automation_util.h"
+#include "chrome/browser/extensions/extension_apitest.h"
+#include "chrome/browser/extensions/extension_host.h"
+#include "chrome/browser/extensions/extension_process_manager.h"
+#include "chrome/browser/extensions/extension_service.h"
+#include "chrome/browser/profiles/profile.h"
+#include "chrome/browser/profiles/profile_manager.h"
+#include "chrome/browser/ui/browser.h"
+#include "chrome/browser/ui/tab_contents/tab_contents_wrapper.h"
+#include "chrome/common/chrome_switches.h"
+#include "chrome/common/url_constants.h"
+#include "chrome/test/base/ui_test_utils.h"
+#include "content/browser/renderer_host/render_view_host.h"
+#include "content/public/browser/render_process_host.h"
+#include "content/public/browser/site_instance.h"
+#include "content/public/browser/web_contents.h"
+#include "net/base/mock_host_resolver.h"
+
+using content::NavigationController;
+using content::WebContents;
+
+namespace {
+
+class ProcessManagementTest : public ExtensionBrowserTest {
+ private:
+  // This is needed for testing isolated apps, which are still experimental.
+  virtual void SetUpCommandLine(CommandLine* command_line) {
+    ExtensionBrowserTest::SetUpCommandLine(command_line);
+    command_line->AppendSwitch(switches::kEnableExperimentalExtensionApis);
+  }
+};
+
+}  // namespace
+
+// Ensure that an isolated app never shares a process with WebUIs, non-isolated
+// extensions, and normal webpages.  None of these should ever comingle
+// RenderProcessHosts even if we hit the process limit.
+IN_PROC_BROWSER_TEST_F(ProcessManagementTest, ProcessOverflow) {
+  // Set max renderers to 1 to force running out of processes.
+  content::RenderProcessHost::SetMaxRendererProcessCount(1);
+
+  host_resolver()->AddRule("*", "127.0.0.1");
+  ASSERT_TRUE(test_server()->Start());
+
+  ASSERT_TRUE(LoadExtension(test_data_dir_.AppendASCII("isolated_apps/app1")));
+  ASSERT_TRUE(LoadExtension(test_data_dir_.AppendASCII("isolated_apps/app2")));
+  ASSERT_TRUE(LoadExtension(test_data_dir_.AppendASCII("hosted_app")));
+  ASSERT_TRUE(
+      LoadExtension(test_data_dir_.AppendASCII("api_test/app_process")));
+
+  // The app under test acts on URLs whose host is "localhost",
+  // so the URLs we navigate to must have host "localhost".
+  GURL base_url = test_server()->GetURL(
+      "files/extensions/");
+  GURL::Replacements replace_host;
+  std::string host_str("localhost");  // Must stay in scope with replace_host.
+  replace_host.SetHostStr(host_str);
+  base_url = base_url.ReplaceComponents(replace_host);
+
+  // Load an extension before adding tabs.
+  const Extension* extension1 = LoadExtension(
+      test_data_dir_.AppendASCII("api_test/browser_action/basics"));
+  ASSERT_TRUE(extension1);
+  GURL extension1_url = extension1->url();
+
+  // Create multiple tabs for each type of renderer that might exist.
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), base_url.Resolve("isolated_apps/app1/main.html"),
+      CURRENT_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), GURL(chrome::kChromeUINewTabURL),
+      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), base_url.Resolve("hosted_app/main.html"),
+      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), base_url.Resolve("test_file.html"),
+      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), base_url.Resolve("isolated_apps/app2/main.html"),
+      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), GURL(chrome::kChromeUINewTabURL),
+      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), base_url.Resolve("api_test/app_process/path1/empty.html"),
+      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), base_url.Resolve("test_file_with_body.html"),
+      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+
+  // Load another copy of isolated app 1.
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), base_url.Resolve("isolated_apps/app1/main.html"),
+      NEW_FOREGROUND_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+
+  // Load another extension.
+  const Extension* extension2 = LoadExtension(
+      test_data_dir_.AppendASCII("api_test/browser_action/close_background"));
+  ASSERT_TRUE(extension2);
+  GURL extension2_url = extension2->url();
+
+  // Get tab processes.
+  ASSERT_EQ(9, browser()->tab_count());
+  content::RenderProcessHost* isolated1_host =
+      browser()->GetWebContentsAt(0)->GetRenderProcessHost();
+  content::RenderProcessHost* ntp1_host =
+      browser()->GetWebContentsAt(1)->GetRenderProcessHost();
+  content::RenderProcessHost* hosted1_host =
+      browser()->GetWebContentsAt(2)->GetRenderProcessHost();
+  content::RenderProcessHost* web1_host =
+      browser()->GetWebContentsAt(3)->GetRenderProcessHost();
+
+  content::RenderProcessHost* isolated2_host =
+      browser()->GetWebContentsAt(4)->GetRenderProcessHost();
+  content::RenderProcessHost* ntp2_host =
+      browser()->GetWebContentsAt(5)->GetRenderProcessHost();
+  content::RenderProcessHost* hosted2_host =
+      browser()->GetWebContentsAt(6)->GetRenderProcessHost();
+  content::RenderProcessHost* web2_host =
+      browser()->GetWebContentsAt(7)->GetRenderProcessHost();
+
+  content::RenderProcessHost* second_isolated1_host =
+      browser()->GetWebContentsAt(8)->GetRenderProcessHost();
+
+  // Get extension processes.
+  ExtensionProcessManager* process_manager =
+    browser()->GetProfile()->GetExtensionProcessManager();
+  content::RenderProcessHost* extension1_host =
+      process_manager->GetSiteInstanceForURL(extension1_url)->GetProcess();
+  content::RenderProcessHost* extension2_host =
+      process_manager->GetSiteInstanceForURL(extension2_url)->GetProcess();
+
+  // An isolated app only shares with other instances of itself, not other
+  // isolated apps or anything else.
+  EXPECT_EQ(isolated1_host, second_isolated1_host);
+  EXPECT_NE(isolated1_host, isolated2_host);
+  EXPECT_NE(isolated1_host, ntp1_host);
+  EXPECT_NE(isolated1_host, hosted1_host);
+  EXPECT_NE(isolated1_host, web1_host);
+  EXPECT_NE(isolated1_host, extension1_host);
+  EXPECT_NE(isolated2_host, ntp1_host);
+  EXPECT_NE(isolated2_host, hosted1_host);
+  EXPECT_NE(isolated2_host, web1_host);
+  EXPECT_NE(isolated2_host, extension1_host);
+
+  // Everything else is clannish.  WebUI only shares with other WebUI.
+  EXPECT_EQ(ntp1_host, ntp2_host);
+  EXPECT_NE(ntp1_host, hosted1_host);
+  EXPECT_NE(ntp1_host, web1_host);
+  EXPECT_NE(ntp1_host, extension1_host);
+
+  // Hosted apps only share with each other.
+  EXPECT_EQ(hosted1_host, hosted2_host);
+  EXPECT_NE(hosted1_host, web1_host);
+  EXPECT_NE(hosted1_host, extension1_host);
+
+  // Web pages only share with each other.
+  EXPECT_EQ(web1_host, web2_host);
+  EXPECT_NE(web1_host, extension1_host);
+
+  // Extensions only share with each other.
+  EXPECT_EQ(extension1_host, extension2_host);
+}
+
+// Test to verify that the policy of maximum share of extension processes is
+// properly enforced.
+IN_PROC_BROWSER_TEST_F(ProcessManagementTest, ExtensionProcessBalancing) {
+  // Set max renderers to 6 so we can expect 2 extension processes to be
+  // allocated.
+  content::RenderProcessHost::SetMaxRendererProcessCount(6);
+
+  // The app under test acts on URLs whose host is "localhost",
+  // so the URLs we navigate to must have host "localhost".
+  GURL base_url = test_server()->GetURL(
+      "files/extensions/");
+  GURL::Replacements replace_host;
+  std::string host_str("localhost");  // Must stay in scope with replace_host.
+  replace_host.SetHostStr(host_str);
+  base_url = base_url.ReplaceComponents(replace_host);
+
+  host_resolver()->AddRule("*", "127.0.0.1");
+  ASSERT_TRUE(test_server()->Start());
+
+  ASSERT_TRUE(LoadExtension(
+      test_data_dir_.AppendASCII("api_test/browser_action/none")));
+  ASSERT_TRUE(LoadExtension(
+      test_data_dir_.AppendASCII("api_test/browser_action/basics")));
+  ASSERT_TRUE(LoadExtension(
+      test_data_dir_.AppendASCII("api_test/browser_action/remove_popup")));
+  ASSERT_TRUE(LoadExtension(
+      test_data_dir_.AppendASCII("api_test/browser_action/add_popup")));
+  ASSERT_TRUE(LoadExtension(
+      test_data_dir_.AppendASCII("api_test/browser_action/no_icon")));
+  ASSERT_TRUE(LoadExtension(
+      test_data_dir_.AppendASCII("isolated_apps/app1")));
+  ASSERT_TRUE(LoadExtension(
+      test_data_dir_.AppendASCII("api_test/management/test")));
+
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), base_url.Resolve("isolated_apps/app1/main.html"),
+      CURRENT_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+
+  ui_test_utils::NavigateToURLWithDisposition(
+      browser(), base_url.Resolve("api_test/management/test/basics.html"),
+      CURRENT_TAB, ui_test_utils::BROWSER_TEST_WAIT_FOR_NAVIGATION);
+
+  std::set<int> process_ids;
+  Profile* profile = browser()->GetProfile();
+  ExtensionProcessManager* epm = profile->GetExtensionProcessManager();
+
+  for (ExtensionProcessManager::const_iterator iter = epm->begin();
+       iter != epm->end();
+       ++iter) {
+    ExtensionHost* host = *iter;
+    if (host->extension()->has_background_page()) {
+      process_ids.insert(host->render_process_host()->GetID());
+    }
+  }
+
+  // We've loaded 5 extensions with background pages, 1 extension without
+  // background page, and one isolated app. We expect only 2 unique processes
+  // hosting those extensions.
+  ASSERT_EQ((size_t) 5, profile->GetExtensionService()->process_map()->size());
+  ASSERT_EQ((size_t) 2, process_ids.size());
+}
diff --git a/chrome/browser/prerender/prerender_manager.cc b/chrome/browser/prerender/prerender_manager.cc
index e4a82e9..681a76b 100644
--- a/chrome/browser/prerender/prerender_manager.cc
+++ b/chrome/browser/prerender/prerender_manager.cc
@@ -843,7 +843,8 @@ bool PrerenderManager::AddPrerender(
   // On Android we do reuse processes as we have a limited number of them and we
   // still want the benefits of prerendering even when several tabs are open.
 #if !defined(OS_ANDROID)
-  if (content::RenderProcessHost::ShouldTryToUseExistingProcessHost() &&
+  if (content::RenderProcessHost::ShouldTryToUseExistingProcessHost(
+          profile_, url) &&
       !content::RenderProcessHost::run_renderer_in_process()) {
     RecordFinalStatus(origin, experiment, FINAL_STATUS_TOO_MANY_PROCESSES);
     return false;
diff --git a/chrome/chrome_tests.gypi b/chrome/chrome_tests.gypi
index 6f57abf..89cb276 100644
--- a/chrome/chrome_tests.gypi
+++ b/chrome/chrome_tests.gypi
@@ -2677,6 +2677,7 @@
         'browser/extensions/notifications_apitest.cc',
         'browser/extensions/page_action_apitest.cc',
         'browser/extensions/platform_app_browsertest.cc',
+        'browser/extensions/process_management_browsertest.cc',
         'browser/extensions/settings/settings_apitest.cc',
         'browser/extensions/stubs_apitest.cc',
         'browser/extensions/system/system_apitest.cc',
diff --git a/chrome/common/chrome_constants.cc b/chrome/common/chrome_constants.cc
index 62f68d7..2010d53 100644
--- a/chrome/common/chrome_constants.cc
+++ b/chrome/common/chrome_constants.cc
@@ -185,6 +185,8 @@ const int kJavascriptMessageExpectedDelay = 1000;
 
 const bool kEnableTouchIcon = false;
 
+const float kMaxShareOfExtensionProcesses = 0.30f;
+
 #if defined(OS_LINUX)
 extern const int kLowestRendererOomScore = 300;
 extern const int kHighestRendererOomScore = 1000;
diff --git a/chrome/common/chrome_constants.h b/chrome/common/chrome_constants.h
index 99f9f16..a5ac026 100644
--- a/chrome/common/chrome_constants.h
+++ b/chrome/common/chrome_constants.h
@@ -108,6 +108,13 @@ extern const int kJavascriptMessageExpectedDelay;
 // Are touch icons enabled? False by default.
 extern const bool kEnableTouchIcon;
 
+// Fraction of the total number of processes to be used for hosting
+// extensions. If we have more extensions than this percentage, we will start
+// combining extensions in existing processes. This allows web pages to have
+// enough render processes and not be starved when a lot of extensions are
+// installed.
+extern const float kMaxShareOfExtensionProcesses;
+
 #if defined(OS_LINUX)
 // The highest and lowest assigned OOM score adjustment
 // (oom_score_adj) used by the OomPriority Manager.
diff --git a/content/browser/mock_content_browser_client.cc b/content/browser/mock_content_browser_client.cc
index 573b204..b7aab7b 100644
--- a/content/browser/mock_content_browser_client.cc
+++ b/content/browser/mock_content_browser_client.cc
@@ -63,6 +63,11 @@ bool MockContentBrowserClient::IsSuitableHost(
   return true;
 }
 
+bool MockContentBrowserClient::ShouldTryToUseExistingProcessHost(
+    BrowserContext* browser_context, const GURL& url) {
+  return false;
+}
+
 void MockContentBrowserClient::SiteInstanceGotProcess(
     SiteInstance* site_instance) {
 }
diff --git a/content/browser/mock_content_browser_client.h b/content/browser/mock_content_browser_client.h
index 551f7f1..515d315 100644
--- a/content/browser/mock_content_browser_client.h
+++ b/content/browser/mock_content_browser_client.h
@@ -37,6 +37,8 @@ class MockContentBrowserClient : public ContentBrowserClient {
   virtual bool IsHandledURL(const GURL& url) OVERRIDE;
   virtual bool IsSuitableHost(RenderProcessHost* process_host,
                               const GURL& site_url) OVERRIDE;
+  virtual bool ShouldTryToUseExistingProcessHost(
+      BrowserContext* browser_context, const GURL& url) OVERRIDE;
   virtual void SiteInstanceGotProcess(SiteInstance* site_instance) OVERRIDE;
   virtual void SiteInstanceDeleting(SiteInstance* site_instance) OVERRIDE;
   virtual bool ShouldSwapProcessesForNavigation(const GURL& current_url,
diff --git a/content/browser/renderer_host/render_process_host_impl.cc b/content/browser/renderer_host/render_process_host_impl.cc
index 6e17b9c..ccd1ece 100644
--- a/content/browser/renderer_host/render_process_host_impl.cc
+++ b/content/browser/renderer_host/render_process_host_impl.cc
@@ -1138,17 +1138,22 @@ content::RenderProcessHost* content::RenderProcessHost::FromID(
 }
 
 // static
-bool content::RenderProcessHost::ShouldTryToUseExistingProcessHost() {
-  size_t renderer_process_count = g_all_hosts.Get().size();
+bool content::RenderProcessHost::ShouldTryToUseExistingProcessHost(
+    BrowserContext* browser_context, const GURL& url) {
+
+  if (run_renderer_in_process())
+    return true;
 
   // NOTE: Sometimes it's necessary to create more render processes than
   //       GetMaxRendererProcessCount(), for instance when we want to create
   //       a renderer process for a browser context that has no existing
   //       renderers. This is OK in moderation, since the
   //       GetMaxRendererProcessCount() is conservative.
+  if (g_all_hosts.Get().size() >= GetMaxRendererProcessCount())
+    return true;
 
-  return run_renderer_in_process() ||
-         (renderer_process_count >= GetMaxRendererProcessCount());
+  return content::GetContentClient()->browser()->
+      ShouldTryToUseExistingProcessHost(browser_context, url);
 }
 
 // static
diff --git a/content/browser/site_instance_impl.cc b/content/browser/site_instance_impl.cc
index ee9d042..792afc5 100644
--- a/content/browser/site_instance_impl.cc
+++ b/content/browser/site_instance_impl.cc
@@ -77,7 +77,8 @@ content::RenderProcessHost* SiteInstanceImpl::GetProcess() {
   // Create a new process if ours went away or was reused.
   if (!process_) {
     // See if we should reuse an old process
-    if (content::RenderProcessHost::ShouldTryToUseExistingProcessHost())
+    if (content::RenderProcessHost::ShouldTryToUseExistingProcessHost(
+            browsing_instance_->browser_context(), site_))
       process_ = content::RenderProcessHost::GetExistingProcessHost(
           browsing_instance_->browser_context(), site_);
 
diff --git a/content/public/browser/content_browser_client.h b/content/public/browser/content_browser_client.h
index d8c842f..dab9721 100644
--- a/content/public/browser/content_browser_client.h
+++ b/content/public/browser/content_browser_client.h
@@ -120,6 +120,12 @@ class ContentBrowserClient {
   virtual bool IsSuitableHost(content::RenderProcessHost* process_host,
                               const GURL& site_url) = 0;
 
+  // Returns whether a new process should be created or an existing one should
+  // be reused based on the URL we want to load. This should return false,
+  // unless there is a good reason otherwise.
+  virtual bool ShouldTryToUseExistingProcessHost(
+      BrowserContext* browser_context, const GURL& url) = 0;
+
   // Called when a site instance is first associated with a process.
   virtual void SiteInstanceGotProcess(
       content::SiteInstance* site_instance) = 0;
diff --git a/content/public/browser/render_process_host.h b/content/public/browser/render_process_host.h
index adb08728..189ede6 100644
--- a/content/public/browser/render_process_host.h
+++ b/content/public/browser/render_process_host.h
@@ -208,7 +208,8 @@ class CONTENT_EXPORT RenderProcessHost : public IPC::Message::Sender,
 
   // Returns true if the caller should attempt to use an existing
   // RenderProcessHost rather than creating a new one.
-  static bool ShouldTryToUseExistingProcessHost();
+  static bool ShouldTryToUseExistingProcessHost(
+      content::BrowserContext* browser_context, const GURL& site_url);
 
   // Get an existing RenderProcessHost associated with the given browser
   // context, if possible.  The renderer process is chosen randomly from
diff --git a/content/shell/shell_content_browser_client.cc b/content/shell/shell_content_browser_client.cc
index 14aa1dd..d1b46a4 100644
--- a/content/shell/shell_content_browser_client.cc
+++ b/content/shell/shell_content_browser_client.cc
@@ -85,6 +85,11 @@ bool ShellContentBrowserClient::IsSuitableHost(
   return true;
 }
 
+bool ShellContentBrowserClient::ShouldTryToUseExistingProcessHost(
+      BrowserContext* browser_context, const GURL& url) {
+  return false;
+}
+
 void ShellContentBrowserClient::SiteInstanceGotProcess(
     SiteInstance* site_instance) {
 }
diff --git a/content/shell/shell_content_browser_client.h b/content/shell/shell_content_browser_client.h
index 4cd84ab..d5c0913 100644
--- a/content/shell/shell_content_browser_client.h
+++ b/content/shell/shell_content_browser_client.h
@@ -41,6 +41,8 @@ class ShellContentBrowserClient : public ContentBrowserClient {
   virtual bool IsHandledURL(const GURL& url) OVERRIDE;
   virtual bool IsSuitableHost(RenderProcessHost* process_host,
                               const GURL& site_url) OVERRIDE;
+  virtual bool ShouldTryToUseExistingProcessHost(
+      BrowserContext* browser_context, const GURL& url) OVERRIDE;
   virtual void SiteInstanceGotProcess(SiteInstance* site_instance) OVERRIDE;
   virtual void SiteInstanceDeleting(SiteInstance* site_instance) OVERRIDE;
   virtual bool ShouldSwapProcessesForNavigation(const GURL& current_url,