Limit the number of history urls indexed for omnibox suggestions.

components/omnibox/browser/url_index_private_data.cc

 // 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 "components/omnibox/browser/url_index_private_data.h"
 
 #include <stdint.h>
 
 #include <functional>
 #include <iterator>
@@ skipping 379 matching lines @@
   if (!history_db)
     return nullptr;
 
   base::TimeTicks beginning_time = base::TimeTicks::Now();
 
   scoped_refptr<URLIndexPrivateData>
       rebuilt_data(new URLIndexPrivateData);
   history::URLDatabase::URLEnumerator history_enum;
   if (!history_db->InitURLEnumeratorForSignificant(&history_enum))
     return nullptr;
+
   rebuilt_data->last_time_rebuilt_from_history_ = base::Time::Now();
+
+  // Limiting the number of URLs indexed degrades the quality of suggestions to
+  // save memory. This limit is only applied for urls indexed at startup and
+  // more urls can be indexed during the browsing session. The primary use case
+  // is for Android devices where the session is typically short.
+  const int max_urls_indexed =
+      OmniboxFieldTrial::MaxNumHQPUrlsIndexedAtStartup();
+  int num_urls_indexed = 0;
   for (history::URLRow row; history_enum.GetNextURL(&row);) {
+    // Do not use <= to account for case of -1 for unlimited urls.

[sdefresne, 2017/05/10 08:59:41]

nit: Should this comment be "// Do not use >= to account for case of -1 for
unlimited urls."?

[ssid, 2017/05/10 19:47:58]

Done.

+    if (num_urls_indexed++ == max_urls_indexed)
+      break;
     rebuilt_data->IndexRow(
         history_db, nullptr, row, scheme_whitelist, nullptr);
   }
 
   UMA_HISTOGRAM_TIMES("History.InMemoryURLIndexingTime",
                       base::TimeTicks::Now() - beginning_time);
   UMA_HISTOGRAM_COUNTS("History.InMemoryURLHistoryItems",
                        rebuilt_data->history_id_word_map_.size());
   UMA_HISTOGRAM_COUNTS_10000("History.InMemoryURLWords",
                              rebuilt_data->word_map_.size());
@@ skipping 854 matching lines @@
   // First cut: typed count, visit count, recency.
   // TODO(mrossetti): This is too simplistic. Consider an approach which ranks
   // recently visited (within the last 12/24 hours) as highly important. Get
   // input from mpearson.
   if (r1.typed_count() != r2.typed_count())
     return (r1.typed_count() > r2.typed_count());
   if (r1.visit_count() != r2.visit_count())
     return (r1.visit_count() > r2.visit_count());
   return (r1.last_visit() > r2.last_visit());
 }