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..ca750689ae56dc18698cf6d511fcc5b3879619e4 100644
--- a/chrome/browser/prerender/prerender_manager.cc
+++ b/chrome/browser/prerender/prerender_manager.cc
@@ -24,6 +24,12 @@ namespace prerender {
 base::TimeTicks PrerenderManager::last_prefetch_seen_time_;
 
 // static
+base::TimeTicks PrerenderManager::last_prerender_start_time_;
+
+// static
+bool PrerenderManager::rate_limit_enabled_ = true;
+
+// static
 PrerenderManager::PrerenderManagerMode PrerenderManager::mode_ =
     PRERENDER_MODE_ENABLED;
 
@@ -92,10 +98,21 @@ 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 +294,22 @@ bool PrerenderManager::ShouldRecordWindowedPPLT() {
   return elapsed_time <= base::TimeDelta::FromSeconds(kWindowedPPLTSeconds);
 }
 
+// static
+bool PrerenderManager::DoesRateLimitAllowPrerender() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  if (!rate_limit_enabled_)
+    return true;
+  if (last_prerender_start_time_.is_null())
+    return true;
+  base::TimeDelta elapsed_time =
+      base::TimeTicks::Now() - last_prerender_start_time_;
+  UMA_HISTOGRAM_ENUMERATION("Prerender.TimeBetweenRequests",

[tburkard, 2011/03/11 23:49:07]

To clarify the name, maybe call it TimeBetweenPrerenderRequests (as opposed to
user-initiated visibile requests).

+                            elapsed_time.InMilliseconds(),
+                            kMaxTimeBetweenPrerendersMs);
+  return elapsed_time >
+      base::TimeDelta::FromMilliseconds(kMinTimeBetweenPrerendersMs);
+}
+
 void PrerenderManager::StartSchedulingPeriodicCleanups() {
   if (repeating_timer_.IsRunning())
     return;