IsolatedPrerender Starts a Spare Renderer with Experiment Param

Adds an experiment param which will start a spare renderer after all
prefetching and NSPs are complete.

Change-Id: I102031892b169e9fb2416384c4a5684ed6ac8880
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424527
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813086}

chrome/browser/prerender/isolated/isolated_prerender_browsertest.cc

@@ -2573,6 +2573,50 @@ IN_PROC_BROWSER_TEST_F(IsolatedPrerenderWithNSPBrowserTest,
       "IsolatedPrerender.AfterClick.Subresources.UsedCache", true, 2);
 }
 
+IN_PROC_BROWSER_TEST_F(IsolatedPrerenderWithNSPBrowserTest,
+                       DISABLE_ON_WIN_MAC_CHROMEOS(StartsSpareRenderer)) {
+  // Enable low-end device mode to turn off automatic spare renderers. Note that
+  // this will also prevent NSPs from triggering, but the logic under test
+  // happens before that anyways.
+  base::CommandLine::ForCurrentProcess()->AppendSwitch(
+      switches::kEnableLowEndDeviceMode);
+  base::CommandLine::ForCurrentProcess()->AppendSwitch(
+      "isolated-prerender-start-spare-renderer");
+
+  SetDataSaverEnabled(true);
+  GURL starting_page = GetOriginServerURL("/simple.html");
+  ui_test_utils::NavigateToURL(browser(), starting_page);
+  WaitForUpdatedCustomProxyConfig();
+
+  IsolatedPrerenderTabHelper* tab_helper =
+      IsolatedPrerenderTabHelper::FromWebContents(GetWebContents());
+
+  GURL eligible_link =
+      GetOriginServerURL("/prerender/isolated/prefetch_page.html");
+
+  TestTabHelperObserver tab_helper_observer(tab_helper);
+  tab_helper_observer.SetExpectedSuccessfulURLs({eligible_link});
+
+  base::RunLoop prefetch_run_loop;
+  tab_helper_observer.SetOnPrefetchSuccessfulClosure(
+      prefetch_run_loop.QuitClosure());
+
+  GURL doc_url("https://www.google.com/search?q=test");
+  MakeNavigationPrediction(doc_url, {eligible_link});
+
+  base::HistogramTester histogram_tester;
+
+  // This run loop will quit when all the prefetch responses have been
+  // successfully done and processed.
+  prefetch_run_loop.Run();
+
+  // Navigate to trigger the histogram recording.
+  ui_test_utils::NavigateToURL(browser(), GURL("about:blank"));
+
+  histogram_tester.ExpectUniqueSample(
+      "IsolatedPrerender.SpareRenderer.CountStartedOnSRP", 1, 1);
+}
+
 namespace {
 std::unique_ptr<net::test_server::HttpResponse> HandleNonEligibleOrigin(
     const net::test_server::HttpRequest& request) {

chrome/browser/prerender/isolated/isolated_prerender_params.cc

@@ -156,3 +156,10 @@ size_t IsolatedPrerenderMaxSubresourcesPerPrerender() {
   return base::GetFieldTrialParamByFeatureAsInt(
       features::kIsolatePrerenders, "max_subresource_count_per_prerender", 50);
 }
+
+bool IsolatedPrerenderStartsSpareRenderer() {
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(
+             "isolated-prerender-start-spare-renderer") ||
+         base::GetFieldTrialParamByFeatureAsBool(features::kIsolatePrerenders,
+                                                 "start_spare_renderer", false);
+}

chrome/browser/prerender/isolated/isolated_prerender_params.h

@@ -73,4 +73,8 @@ bool IsolatedPrerenderMustHTTPProbeInsteadOfTLS();
 // The maximum number of subresources that will be fetched per prefetched page.
 size_t IsolatedPrerenderMaxSubresourcesPerPrerender();
 
+// Whether a spare renderer should be started after all prefetching and NSP is
+// complete.
+bool IsolatedPrerenderStartsSpareRenderer();
+
 #endif  // CHROME_BROWSER_PRERENDER_ISOLATED_ISOLATED_PRERENDER_PARAMS_H_

chrome/browser/prerender/isolated/isolated_prerender_tab_helper.cc

@@ -38,6 +38,7 @@
 #include "content/public/browser/navigation_handle.h"
 #include "content/public/browser/network_service_instance.h"
 #include "content/public/browser/render_frame_host.h"
+#include "content/public/browser/render_process_host.h"
 #include "content/public/browser/service_worker_context.h"
 #include "content/public/browser/storage_partition.h"
 #include "content/public/browser/web_contents.h"
@@ -147,6 +148,23 @@ void CookieSetHelper(base::RepeatingClosure run_me,
   run_me.Run();
 }
 
+bool ShouldStartSpareRenderer() {
+  if (!IsolatedPrerenderStartsSpareRenderer()) {
+    return false;
+  }
+
+  for (content::RenderProcessHost::iterator iter(
+           content::RenderProcessHost::AllHostsIterator());
+       !iter.IsAtEnd(); iter.Advance()) {
+    if (iter.GetCurrentValue()->IsUnused()) {
+      // There is already a spare renderer.
+      return false;
+    }
+  }
+
+  return true;
+}
+
 }  // namespace
 
 IsolatedPrerenderTabHelper::PrefetchMetrics::PrefetchMetrics() = default;
@@ -168,6 +186,12 @@ IsolatedPrerenderTabHelper::CurrentPageLoad::CurrentPageLoad(
 }
 
 IsolatedPrerenderTabHelper::CurrentPageLoad::~CurrentPageLoad() {
+  if (IsolatedPrerenderStartsSpareRenderer()) {
+    UMA_HISTOGRAM_COUNTS_100(
+        "IsolatedPrerender.SpareRenderer.CountStartedOnSRP",
+        number_of_spare_renderers_started_);
+  }
+
   if (!profile_)
     return;
 
@@ -680,6 +704,12 @@ void IsolatedPrerenderTabHelper::StartSinglePrefetch() {
       IsolatedPrerenderMainframeBodyLengthLimit());
 
   page_->url_loaders_.emplace(std::move(loader));
+
+  // Start a spare renderer now so that it will be ready by the time it is
+  // useful to have.
+  if (ShouldStartSpareRenderer()) {
+    StartSpareRenderer();
+  }
 }
 
 void IsolatedPrerenderTabHelper::OnPrefetchRedirect(
@@ -922,6 +952,12 @@ void IsolatedPrerenderTabHelper::DoNoStatePrefetch() {
 void IsolatedPrerenderTabHelper::OnPrerenderDone(const GURL& url) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
 
+  // The completed NSP probably consumed a previously started spare renderer, so
+  // kick off another one if needed.
+  if (ShouldStartSpareRenderer()) {
+    StartSpareRenderer();
+  }
+
   // It is possible that this is run as a callback after a navigation has
   // already happened and |page_| is now a different instance than when the
   // prerender was started. In this case, just return.
@@ -943,6 +979,11 @@ void IsolatedPrerenderTabHelper::OnPrerenderDone(const GURL& url) {
   DoNoStatePrefetch();
 }
 
+void IsolatedPrerenderTabHelper::StartSpareRenderer() {
+  page_->number_of_spare_renderers_started_++;
+  content::RenderProcessHost::WarmupSpareRenderProcessHost(profile_);
+}
+
 void IsolatedPrerenderTabHelper::OnPredictionUpdated(
     const base::Optional<NavigationPredictorKeyedService::Prediction>
         prediction) {

chrome/browser/prerender/isolated/isolated_prerender_tab_helper.h

@@ -362,6 +362,9 @@ class IsolatedPrerenderTabHelper
     // The number of no state prefetch requests that have been made.
     size_t number_of_no_state_prefetch_attempts_ = 0;
 
+    // The number of spare renderers that were started during this page load.
+    size_t number_of_spare_renderers_started_ = 0;
+
     // All urls that are eligible to be no state prefetched. Once a no state
     // prefetch finishes, in success or in error, it is removed from this list.
     // If there is an active no state prefetch, its url will always be the first
@@ -445,6 +448,10 @@ class IsolatedPrerenderTabHelper
   // Starts a new no state prefetch for the next eligible url.
   void DoNoStatePrefetch();
 
+  // Starts a spare renderer. Should only be called when all NSPs and
+  // prefetching is complete.
+  void StartSpareRenderer();
+
   // Makes a clone of |this|'s prefetch response so that it can be used for
   // NoStatePrefetch now and later reused if the user navigates to that page.
   std::unique_ptr<PrefetchedMainframeResponseContainer>

tools/metrics/histograms/histograms_xml/others/histograms.xml

@@ -6862,6 +6862,17 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="IsolatedPrerender.SpareRenderer.CountStartedOnSRP"
+    units="count" expires_after="M90">
+  <owner>robertogden@chromium.org</owner>
+  <owner>ryansturm@chromium.org</owner>
+  <owner>tbansal@chromium.org</owner>
+  <summary>
+    Records the number of spare renderers that were attempted to be started on
+    the SRP, when the feature param is enabled.
+  </summary>
+</histogram>
+
 <histogram name="JSDialogs.OnBeforeUnloadStayVsLeave" enum="StayVsLeave"
     expires_after="M77">
   <owner>avi@chromium.org</owner>