chrome/browser/net/referrer.cc

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "chrome/browser/net/referrer.h"
   6
   7 #include <limits.h>
   8
   9 #include "base/compiler_specific.h"
  10 #include "base/logging.h"
  11 #include "base/message_loop/message_loop.h"
  12 #include "base/values.h"
  13 #include "chrome/browser/net/predictor.h"
  14
  15 namespace chrome_browser_net {
  16
  17 //------------------------------------------------------------------------------
  18 // Smoothing parameter for updating subresource_use_rate_.
  19
  20 // We always combine our old expected value, weighted by some factor W (we use
  21 // kWeightingForOldConnectsExpectedValue), with the new expected value Enew.
  22 // The new "expected value" is the number of actual connections made due to the
  23 // current navigations.
  24 // That means that IF we end up needing to connect, we should apply the formula:
  25 // Eupdated = Eold * W  +  Enew * (1 - W)
  26 // If we visit the containing url, but don't end up needing a connection, then
  27 // Enew == 0, so we use the formula:
  28 // Eupdated = Eold * W
  29 // To achieve the above updating algorithm, we end up doing the multiplication
  30 // by W every time we contemplate doing a preconnection (i.e., when we navigate
  31 // to the containing URL, and consider doing a preconnection), and then IFF we
  32 // learn that we really needed a connection to the subresource, we complete the
  33 // above algorithm by adding the (1 - W) for each connection we make.
  34
  35 // We weight the new expected value by a factor which is in the range of 0.0 to
  36 // 1.0.
  37 static const double kWeightingForOldConnectsExpectedValue = 0.66;
  38
  39 // To estimate the expected value of the number of connections that we'll need
  40 // when a referrer is navigated to, we start with the following low initial
  41 // value.
  42 // Each time we do indeed (again) need the subresource, this value will get
  43 // increased.
  44 // Each time we navigate to the refererrer but never end up needing this
  45 // subresource, the value will decrease.
  46 // Very conservative is 0.0, which will mean that we have to wait for a while
  47 // before doing much speculative acvtivity.  We do persist results, so we'll
  48 // save the asymptotic (correct?) learned answer in the long run.
  49 // Some browsers blindly make 2 connections all the time, so we'll use that as
  50 // a starting point.
  51 static const double kInitialConnectsExpectedValue = 2.0;
  52
  53 Referrer::Referrer() : use_count_(1) {}
  54
  55 void Referrer::SuggestHost(const GURL& url) {
  56   // Limit how large our list can get, in case we make mistakes about what
  57   // hostnames are in sub-resources (example: Some advertisments have a link to
  58   // the ad agency, and then provide a "surprising" redirect to the advertised
  59   // entity, which then (mistakenly) appears to be a subresource on the page
  60   // hosting the ad).
  61   // TODO(jar): Do experiments to optimize the max count of suggestions.
  62   static const size_t kMaxSuggestions = 10;
  63
  64   if (!url.has_host())  // TODO(jar): Is this really needed????
  65     return;
  66   DCHECK(url == url.GetWithEmptyPath());
  67   SubresourceMap::iterator it = find(url);
  68   if (it != end()) {
  69     it->second.SubresourceIsNeeded();
  70     return;
  71   }
  72
  73   if (kMaxSuggestions <= size()) {
  74     DeleteLeastUseful();
  75     DCHECK(kMaxSuggestions > size());
  76   }
  77   (*this)[url].SubresourceIsNeeded();
  78 }
  79
  80 void Referrer::DeleteLeastUseful() {
  81   // Find the item with the lowest value.  Most important is preconnection_rate,
  82   // and least is lifetime (age).
  83   GURL least_useful_url;
  84   double lowest_rate_seen = 0.0;
  85   // We use longs for durations because we will use multiplication on them.
  86   int64 least_useful_lifetime = 0;  // Duration in milliseconds.
  87
  88   const base::Time kNow(base::Time::Now());  // Avoid multiple calls.
  89   for (SubresourceMap::iterator it = begin(); it != end(); ++it) {
  90     int64 lifetime = (kNow - it->second.birth_time()).InMilliseconds();
  91     double rate = it->second.subresource_use_rate();
  92     if (least_useful_url.has_host()) {
  93       if (rate > lowest_rate_seen)
  94         continue;
  95       if (lifetime <= least_useful_lifetime)
  96         continue;
  97     }
  98     least_useful_url = it->first;
  99     lowest_rate_seen = rate;
 100     least_useful_lifetime = lifetime;
 101   }
 102   if (least_useful_url.has_host())
 103     erase(least_useful_url);
 104 }
 105
 106 bool Referrer::Trim(double reduce_rate, double threshold) {
 107   std::vector<GURL> discarded_urls;
 108   for (SubresourceMap::iterator it = begin(); it != end(); ++it) {
 109     if (!it->second.Trim(reduce_rate, threshold))
 110       discarded_urls.push_back(it->first);
 111   }
 112   for (size_t i = 0; i < discarded_urls.size(); ++i)
 113     erase(discarded_urls[i]);
 114   return size() > 0;
 115 }
 116
 117 bool ReferrerValue::Trim(double reduce_rate, double threshold) {
 118   subresource_use_rate_ *= reduce_rate;
 119   return subresource_use_rate_ > threshold;
 120 }
 121
 122
 123 void Referrer::Deserialize(const base::Value& value) {
 124   if (value.GetType() != base::Value::TYPE_LIST)
 125     return;
 126   const base::ListValue* subresource_list(
 127       static_cast<const base::ListValue*>(&value));
 128   size_t index = 0;  // Bounds checking is done by subresource_list->Get*().
 129   while (true) {
 130     std::string url_spec;
 131     if (!subresource_list->GetString(index++, &url_spec))
 132       return;
 133     double rate;
 134     if (!subresource_list->GetDouble(index++, &rate))
 135       return;
 136
 137     GURL url(url_spec);
 138     // TODO(jar): We could be more direct, and change birth date or similar to
 139     // show that this is a resurrected value we're adding in.  I'm not yet sure
 140     // of how best to optimize the learning and pruning (Trim) algorithm at this
 141     // level, so for now, we just suggest subresources, which leaves them all
 142     // with the same birth date (typically start of process).
 143     SuggestHost(url);
 144     (*this)[url].SetSubresourceUseRate(rate);
 145   }
 146 }
 147
 148 base::Value* Referrer::Serialize() const {
 149   base::ListValue* subresource_list(new base::ListValue);
 150   for (const_iterator it = begin(); it != end(); ++it) {
 151     base::StringValue* url_spec(new base::StringValue(it->first.spec()));
 152     base::FundamentalValue* rate(new base::FundamentalValue(
 153         it->second.subresource_use_rate()));
 154
 155     subresource_list->Append(url_spec);
 156     subresource_list->Append(rate);
 157   }
 158   return subresource_list;
 159 }
 160
 161 //------------------------------------------------------------------------------
 162
 163 ReferrerValue::ReferrerValue()
 164     : birth_time_(base::Time::Now()),
 165       navigation_count_(0),
 166       preconnection_count_(0),
 167       preresolution_count_(0),
 168       subresource_use_rate_(kInitialConnectsExpectedValue) {
 169 }
 170
 171 void ReferrerValue::SubresourceIsNeeded() {
 172   DCHECK_GE(kWeightingForOldConnectsExpectedValue, 0);
 173   DCHECK_LE(kWeightingForOldConnectsExpectedValue, 1.0);
 174   ++navigation_count_;
 175   subresource_use_rate_ += 1 - kWeightingForOldConnectsExpectedValue;
 176 }
 177
 178 void ReferrerValue::ReferrerWasObserved() {
 179   subresource_use_rate_ *= kWeightingForOldConnectsExpectedValue;
 180   // Note: the use rate is temporarilly possibly incorect, as we need to find
 181   // out if we really end up connecting.  This will happen in a few hundred
 182   // milliseconds (when content arrives, etc.).
 183   // Value of subresource_use_rate_ should be sampled before this call.
 184 }
 185
 186 }  // namespace chrome_browser_net