Limit prerender by only allowing one every X ms (X currently set to 500).

chrome/browser/prerender/prerender_manager.cc

Index: chrome/browser/prerender/prerender_manager.cc
diff --git a/chrome/browser/prerender/prerender_manager.cc b/chrome/browser/prerender/prerender_manager.cc
index d30bc9135c9e81f2c3f35701dae39a972a8a6b4c..9748380a2eb8e4f1f7120e10d6e77b0dedb362d1 100644
--- a/chrome/browser/prerender/prerender_manager.cc
+++ b/chrome/browser/prerender/prerender_manager.cc
@@ -54,11 +54,14 @@ struct PrerenderManager::PrerenderContentsData {
 };
 
 PrerenderManager::PrerenderManager(Profile* profile)
-    : profile_(profile),
+    : rate_limit_enabled_(true),
+      profile_(profile),
       max_prerender_age_(base::TimeDelta::FromSeconds(
           kDefaultMaxPrerenderAgeSeconds)),
       max_elements_(kDefaultMaxPrerenderElements),
-      prerender_contents_factory_(PrerenderContents::CreateFactory()) {
+      prerender_contents_factory_(PrerenderContents::CreateFactory()),
+      last_prerender_start_time_(base::TimeTicks::Now() -
+          base::TimeDelta::FromMilliseconds(kMinTimeBetweenPrerendersMs)) {
 }
 
 PrerenderManager::~PrerenderManager() {
@@ -92,10 +95,21 @@ bool PrerenderManager::AddPreload(const GURL& url,
     RecordFinalStatus(FINAL_STATUS_TOO_MANY_PROCESSES);
     return false;
   }
+
+  // Check if enough time has passed since the last prerender.
+  if (!DoesRateLimitAllowPrerender()) {
+    // Cancel the prerender. We could add it to the pending prerender list but
+    // this doesn't make sense as the next prerender request will be triggered
+    // by a navigation and is unlikely to be the same site.
+    RecordFinalStatus(FINAL_STATUS_RATE_LIMIT_EXCEEDED);
+    return false;
+  }
+
   // TODO(cbentzel): Move invalid checks here instead of PrerenderContents?
   PrerenderContentsData data(CreatePrerenderContents(url, alias_urls, referrer),
                              GetCurrentTime());
   prerender_list_.push_back(data);
+  last_prerender_start_time_ = base::TimeTicks::Now();
   data.contents_->StartPrerendering();
   while (prerender_list_.size() > max_elements_) {
     data = prerender_list_.front();
@@ -277,6 +291,18 @@ bool PrerenderManager::ShouldRecordWindowedPPLT() {
   return elapsed_time <= base::TimeDelta::FromSeconds(kWindowedPPLTSeconds);
 }
 
+bool PrerenderManager::DoesRateLimitAllowPrerender() const {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  base::TimeDelta elapsed_time =
+      base::TimeTicks::Now() - last_prerender_start_time_;

[cbentzel, 2011/03/16 01:06:01]

This should use GetCurrentTime instead of base::TimeTicks::Now - that way you
can reliably test this in a unit test.

The same holds for the other callers of base::TimeTicks::Now

[dominich, 2011/03/16 19:58:39]

I added GetCurrentTimeTicks() and used that everywhere instead. I didn't change
the recorded window stuff as that is static and is not unit tested (currently?)

+  UMA_HISTOGRAM_TIMES("Prerender.TimeBetweenPrerenderRequests",
+                      elapsed_time);
+  if (!rate_limit_enabled_)
+    return true;
+  return elapsed_time >
+      base::TimeDelta::FromMilliseconds(kMinTimeBetweenPrerendersMs);
+}
+
 void PrerenderManager::StartSchedulingPeriodicCleanups() {
   if (repeating_timer_.IsRunning())
     return;