Add {,Non}TopHosts variants of Precache.Latency.

Add Precache.Latency.NonPrefetch.TopHosts, which records the latency
only for non-prefetch requests whose referer is in the top hosts, and
Precache.Latency.NonPrefetch.NonTopHosts, which records the latency for
the rest of the non-prefetch requests. This allows us to evaluate the
effect of precaching on latency only on those requests it has a chance
of improving. Thus, we can better distinguish the effectiveness of the
host selection algorithm from the effectiveness of the manifests.

This adds a HostRankIfAvailable method to HistoryService, in order to expose
the pre-computed TopHosts, as requiring that computation for every request
would be untenable.

This also adds a Control group, where TopHosts is computed for users, but no
resources are prefetched.

BUG=499532

Review URL: https://codereview.chromium.org/1280673004

Cr-Commit-Position: refs/heads/master@{#343230}

7 files changed, 61 insertions, 7 deletions

diff --git a/components/history/core/browser/history_backend.cc b/components/history/core/browser/history_backend.cc
index d6dcb62..8e4cfaa 100644
--- a/components/history/core/browser/history_backend.cc
+++ b/components/history/core/browser/history_backend.cc
@@ -90,8 +90,6 @@ const int kMaxRedirectCount = 32;
 // and is deleted.
 const int kExpireDaysThreshold = 90;
 
-const int kMaxTopHostsForMetrics = 50;
-
 // Converts from PageUsageData to MostVisitedURL. |redirects| is a
 // list of redirects for this URL. Empty list means no redirects.
 MostVisitedURL MakeMostVisitedURL(const PageUsageData& page_data,
@@ -430,6 +428,11 @@ TopHostsList HistoryBackend::TopHosts(int num_hosts) const {
   return top_hosts;
 }
 
+int HistoryBackend::HostRankIfAvailable(const GURL& url) const {
+  auto it = host_ranks_.find(HostForTopHosts(url));
+  return it != host_ranks_.end() ? it->second : kMaxTopHosts;
+}
+
 void HistoryBackend::AddPage(const HistoryAddPageArgs& request) {
   if (!db_)
     return;
@@ -743,12 +746,9 @@ void HistoryBackend::CloseAllDatabases() {
 }
 
 void HistoryBackend::RecordTopHostsMetrics(const GURL& url) {
-  auto it = host_ranks_.find(HostForTopHosts(url));
-  int host_rank = it != host_ranks_.end() ? it->second : kMaxTopHostsForMetrics;
-
   // Convert index from 0-based to 1-based.
-  UMA_HISTOGRAM_ENUMERATION("History.TopHostsVisitsByRank", host_rank + 1,
-                            kMaxTopHostsForMetrics + 2);
+  UMA_HISTOGRAM_ENUMERATION("History.TopHostsVisitsByRank",
+                            HostRankIfAvailable(url) + 1, kMaxTopHosts + 2);
 }
 
 std::pair<URLID, VisitID> HistoryBackend::AddPageVisit(
diff --git a/components/history/core/browser/history_backend.h b/components/history/core/browser/history_backend.h
index 78de7d4..51f9519 100644
--- a/components/history/core/browser/history_backend.h
+++ b/components/history/core/browser/history_backend.h
@@ -212,6 +212,12 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
   // generating internal metrics.
   TopHostsList TopHosts(int num_hosts) const;
 
+  // Returns, for the given URL, a 0-based index into the list produced by
+  // TopHosts(), corresponding to that URL's host. If TopHosts() has not
+  // previously been run, or the host is not in the top kMaxTopHosts, returns
+  // kMaxTopHosts.
+  int HostRankIfAvailable(const GURL& url) const;
+
   // Navigation ----------------------------------------------------------------
 
   // |request.time| must be unique with high probability.
@@ -541,6 +547,7 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, TopHosts_OnlyLast30Days);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, TopHosts_MaxNumHosts);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, TopHosts_IgnoreUnusualURLs);
+  FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, HostRankIfAvailable);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, RecordTopHostsMetrics);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, UpdateVisitDuration);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, ExpireHistoryForTimes);
diff --git a/components/history/core/browser/history_backend_unittest.cc b/components/history/core/browser/history_backend_unittest.cc
index 54df3f2..a4a3d9d9 100644
--- a/components/history/core/browser/history_backend_unittest.cc
+++ b/components/history/core/browser/history_backend_unittest.cc
@@ -3263,6 +3263,27 @@ TEST_F(HistoryBackendTest, TopHosts_IgnoreUnusualURLs) {
   EXPECT_THAT(backend_->TopHosts(5), ElementsAre(std::make_pair("cnn.com", 3)));
 }
 
+TEST_F(HistoryBackendTest, HostRankIfAvailable) {
+  std::vector<GURL> urls;
+  urls.push_back(GURL("http://cnn.com/us"));
+  urls.push_back(GURL("http://cnn.com/intl"));
+  urls.push_back(GURL("http://dogtopia.com/"));
+  for (const auto& url : urls) {
+    backend_->AddPageVisit(url, base::Time::Now(), 0, ui::PAGE_TRANSITION_LINK,
+                           history::SOURCE_BROWSED);
+  }
+
+  EXPECT_EQ(kMaxTopHosts,
+            backend_->HostRankIfAvailable(GURL("http://cnn.com/")));
+
+  backend_->TopHosts(3);
+
+  EXPECT_EQ(0, backend_->HostRankIfAvailable(GURL("http://cnn.com/")));
+  EXPECT_EQ(1, backend_->HostRankIfAvailable(GURL("http://dogtopia.com/")));
+  EXPECT_EQ(kMaxTopHosts,
+            backend_->HostRankIfAvailable(GURL("http://catsylvania.com/")));
+}
+
 TEST_F(HistoryBackendTest, RecordTopHostsMetrics) {
   base::HistogramTester histogram;
 
diff --git a/components/history/core/browser/history_constants.cc b/components/history/core/browser/history_constants.cc
index 1b18dc7..842ed43 100644
--- a/components/history/core/browser/history_constants.cc
+++ b/components/history/core/browser/history_constants.cc
@@ -20,4 +20,6 @@ const base::FilePath::CharType kThumbnailsFilename[] =
 const base::FilePath::CharType kTopSitesFilename[] =
     FILE_PATH_LITERAL("Top Sites");
 
+const int kMaxTopHosts = 50;
+
 }  // namespace history
diff --git a/components/history/core/browser/history_constants.h b/components/history/core/browser/history_constants.h
index 875b099..14b0c24 100644
--- a/components/history/core/browser/history_constants.h
+++ b/components/history/core/browser/history_constants.h
@@ -16,6 +16,9 @@ extern const base::FilePath::CharType kHistoryFilename[];
 extern const base::FilePath::CharType kThumbnailsFilename[];
 extern const base::FilePath::CharType kTopSitesFilename[];
 
+// The maximum size of the list returned by history::HistoryService::TopHosts().
+extern const int kMaxTopHosts;
+
 }  // namespace history
 
 #endif  // COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_CONSTANTS_H_
diff --git a/components/history/core/browser/history_service.cc b/components/history/core/browser/history_service.cc
index 12e83d4..28bdd51 100644
--- a/components/history/core/browser/history_service.cc
+++ b/components/history/core/browser/history_service.cc
@@ -368,6 +368,17 @@ void HistoryService::TopHosts(int num_hosts,
       callback);
 }
 
+void HistoryService::HostRankIfAvailable(
+    const GURL& url,
+    const base::Callback<void(int)>& callback) const {
+  DCHECK(thread_) << "History service being called after cleanup";
+  DCHECK(thread_checker_.CalledOnValidThread());
+  PostTaskAndReplyWithResult(thread_->task_runner().get(), FROM_HERE,
+                             base::Bind(&HistoryBackend::HostRankIfAvailable,
+                                        history_backend_.get(), url),
+                             callback);
+}
+
 void HistoryService::AddPage(const GURL& url,
                              Time time,
                              ContextID context_id,
diff --git a/components/history/core/browser/history_service.h b/components/history/core/browser/history_service.h
index fbf1e72..f197d2b 100644
--- a/components/history/core/browser/history_service.h
+++ b/components/history/core/browser/history_service.h
@@ -166,6 +166,16 @@ class HistoryService : public syncer::SyncableService, public KeyedService {
   // Note: Virtual needed for mocking.
   virtual void TopHosts(int num_hosts, const TopHostsCallback& callback) const;
 
+  // Returns, for the given URL, a 0-based index into the list produced by
+  // TopHosts(), corresponding to that URL's host. If TopHosts() has not
+  // previously been run, or the host is not in the top kMaxTopHosts, returns
+  // kMaxTopHosts.
+  //
+  // Note: Virtual needed for mocking.
+  virtual void HostRankIfAvailable(
+      const GURL& url,
+      const base::Callback<void(int)>& callback) const;
+
   // Navigation ----------------------------------------------------------------
 
   // Adds the given canonical URL to history with the given time as the visit