Add in-memory caching of network quality estimates across network changes.

net/base/network_quality_estimator.h

Index: net/base/network_quality_estimator.h
diff --git a/net/base/network_quality_estimator.h b/net/base/network_quality_estimator.h
index 6f3a4674cfcf3509d3b0004eac010fcb5988f4ee..723d2cfda57f0ff1441b548f0e56ed4dcf199614 100644
--- a/net/base/network_quality_estimator.h
+++ b/net/base/network_quality_estimator.h
@@ -8,6 +8,8 @@
 #include <stdint.h>
 
 #include <deque>
+#include <map>
+#include <string>
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
@@ -15,11 +17,10 @@
 #include "base/time/time.h"
 #include "net/base/net_export.h"
 #include "net/base/network_change_notifier.h"
+#include "net/base/network_quality.h"
 
 namespace net {
 
-class NetworkQuality;
-
 // NetworkQualityEstimator provides network quality estimates (quality of the
 // full paths to all origins that have been connected to).
 // The estimates are based on the observed organic traffic.
@@ -51,13 +52,87 @@ class NET_EXPORT_PRIVATE NetworkQualityEstimator
                           int64_t cumulative_prefilter_bytes_read,
                           int64_t prefiltered_bytes_read);
 
+ protected:
+  // Returns true if the cached network quality estimate was successfully read.
+  bool ReadCachedNetworkQualityEstimate();
+
+  // NetworkChangeNotifier::ConnectionTypeObserver implementation.
+  void OnConnectionTypeChanged(
+      NetworkChangeNotifier::ConnectionType type) override;
+
+  // Returns the number of entries in the network quality cache.
+  // Used only for testing.
+  size_t GetNetworkQualityCacheSizeForTests() const;
+
  private:
   FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest, StoreObservations);
   FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest,
                            TestPeakKbpsFastestRTTUpdates);
   FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest, TestAddObservation);
+  FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest, TestCaching);
+  FRIEND_TEST_ALL_PREFIXES(NetworkQualityEstimatorTest,
+                           TestLRUCacheMaximumSize);
   FRIEND_TEST_ALL_PREFIXES(URLRequestTestHTTP, NetworkQualityEstimator);
 
+  // NetworkID is used to uniquely identify a network.
+  // For the purpose of network quality estimation and caching, a network is
+  // uniquely identified by a combination of |type| and
+  // |id|. This approach is unable to distinguish networks with
+  // same name (e.g., different Wi-Fi networks with same SSID).
+  struct NetworkID {
+    NetworkID(NetworkChangeNotifier::ConnectionType type, const std::string& id)
+        : type(type), id(id) {}
+
+    NetworkID(const NetworkID& other) : type(other.type), id(other.id) {}
+
+    ~NetworkID() {}
+
+    NetworkID& operator=(const NetworkID& other) {
+      type = other.type;
+      id = other.id;
+      return *this;
+    }
+
+    // Overloaded because NetworkID is used as key in a map.
+    bool operator<(const NetworkID& other) const {
+      return type < other.type || (type == other.type && id < other.id);
+    }
+
+    // Connection type of the network.
+    NetworkChangeNotifier::ConnectionType type;
+
+    // Name of this network as returned by |GetCurrentNetworkName| function.
+    std::string id;
+  };
+
+  // CachedNetworkQuality stores the quality of a previously seen network.
+  class CachedNetworkQuality {
+   public:
+    explicit CachedNetworkQuality(const NetworkQuality& network_quality);
+
+    ~CachedNetworkQuality();
+
+    // Returns the network quality associated with this cached entry.
+    const NetworkQuality network_quality() const { return network_quality_; }
+
+    // Updates the network quality to the specified |median_kbps| and
+    // |median_rtt|.
+    void UpdateNetworkQuality(int32_t median_kbps,
+                              const base::TimeDelta& median_rtt);
+
+    // Returns the time when this cached entry was last updated.
+    base::TimeTicks last_update_time() const { return last_update_time_; }

[pauljensen, 2015/06/17 14:23:26]

Slightly nicer encapsulation if we replaced this function with a "bool
older_than(const CachedNetworkQuality&)" function.

[tbansal1, 2015/06/17 21:18:46]

Done.

+
+   private:
+    // Time when this cache entry was last updated.
+    base::TimeTicks last_update_time_;
+
+    // Quality of this cached network.
+    NetworkQuality network_quality_;
+
+    DISALLOW_COPY_AND_ASSIGN(CachedNetworkQuality);
+  };
+
   // Records the round trip time or throughput observation, along with the time
   // the observation was made.
   struct Observation {
@@ -99,6 +174,9 @@ class NET_EXPORT_PRIVATE NetworkQualityEstimator
     DISALLOW_COPY_AND_ASSIGN(ObservationBuffer);
   };
 
+  typedef std::map<NetworkID, scoped_ptr<CachedNetworkQuality>>

[pauljensen, 2015/06/17 14:19:15]

I don't see an ordered traversal and assume we'll never need one, so please add
a comment or preferably use a hash table:

This does not use a unordered_map or hash_map for code simplicity (key just
implements operator<, rather than hash and equality) and because the map is
tiny.

[tbansal1, 2015/06/17 21:18:46]

Done.

+      CachedNetworkQualities;
+
   // Tiny transfer sizes may give inaccurate throughput results.
   // Minimum size of the transfer over which the throughput is computed.
   static const int kMinTransferSizeInBytes = 10000;
@@ -107,6 +185,11 @@ class NET_EXPORT_PRIVATE NetworkQualityEstimator
   // throughput is computed.
   static const int kMinRequestDurationMicroseconds = 1000;
 
+  // Maximum size of the cache that holds network quality estimates.
+  // Smaller size may reduce the cache hit rate due to frequent evictions.
+  // Larger size may affect performance.
+  static const size_t kMaximumNetworkQualityCacheSize;
+
   // Construct a NetworkQualityEstimator instance allowing for test
   // configuration.
   // Registers for network type change notifications so estimates can be kept
@@ -129,9 +212,18 @@ class NET_EXPORT_PRIVATE NetworkQualityEstimator
   // |expected_size|. Used for testing.
   bool VerifyBufferSizeForTests(size_t expected_size) const;

[pauljensen, 2015/06/17 14:19:15]

Can we remove this and have the tests compare to the internal sizes?

[tbansal1, 2015/06/17 21:18:46]

Removed this function but had to add 2 more functions. It seems that I can't
call estimator.kbps_observations_.Size() in the test code directly. It gives
linking error on windows (it does work on all other platforms):
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp....
I fixed the linking error by
https://codereview.chromium.org/1164713004/diff2/380001:400001/net/base/netwo....

 
-  // NetworkChangeNotifier::ConnectionTypeObserver implementation.
-  void OnConnectionTypeChanged(
-      NetworkChangeNotifier::ConnectionType type) override;
+  // Updates the current network name to:
+  // WiFi SSID (if the user is connected to a WiFi access point and the SSID

[pauljensen, 2015/06/17 14:19:15]

can you add " - " before "WiFi" on this line and before "the" on line 218 so
it's easier to read, and indent lines 217 and 219.

[tbansal1, 2015/06/17 21:18:46]

Done.

+  // name is available), or
+  // the MCC/MNC code of the cellular carrier if the device is connected to a
+  // cellular network.
+  // Updates the current network name to an empty string in all other cases or
+  // if the network name is not exposed by platform APIs.
+  // Virtualized for testing.
+  virtual std::string GetCurrentNetworkName() const;
+
+  // Writes the estimated quality of the current network to the cache.
+  void CacheNetworkQualityEstimate();
 
   // Determines if the requests to local host can be used in estimating the
   // network quality. Set to true only for tests.
@@ -145,14 +237,16 @@ class NET_EXPORT_PRIVATE NetworkQualityEstimator
   // Time when last connection change was observed.
   base::TimeTicks last_connection_change_;
 
-  // Last value passed to |OnConnectionTypeChanged|. This indicates the
-  // current connection type.
-  NetworkChangeNotifier::ConnectionType current_connection_type_;
+  // ID of the current network.
+  NetworkID current_network_id_;
 
   // Fastest round-trip-time (RTT) since last connectivity change. RTT measured
   // from URLRequest creation until first byte received.
   base::TimeDelta fastest_rtt_since_last_connection_change_;
 
+  // Cache that stores quality of previously seen networks.
+  CachedNetworkQualities cached_network_qualities_;
+
   // Rough measurement of downstream peak Kbps witnessed since last connectivity
   // change. The accuracy is decreased by ignoring these factors:
   // 1) Multiple URLRequests can occur concurrently.