Round results of multiplying QuicBandwidth and QuicTime::Delta rather than truncating. No behavior…

net/quic/core/congestion_control/rtt_stats_test.cc

 // Copyright 2014 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 "net/quic/core/congestion_control/rtt_stats.h"
 
+#include <cmath>
 #include <vector>
 
 #include "base/logging.h"
 #include "base/test/mock_log.h"
 #include "net/quic/core/quic_flags.h"
 #include "net/quic/test_tools/rtt_stats_peer.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 using logging::LOG_WARNING;
 using std::vector;
@@ skipping 29 matching lines @@
   EXPECT_EQ(QuicTime::Delta::FromMilliseconds(200), rtt_stats_.latest_rtt());
   EXPECT_EQ(QuicTime::Delta::FromMilliseconds(200), rtt_stats_.smoothed_rtt());
   // Verify that large erroneous ack_delay does not change Smoothed RTT.
   rtt_stats_.UpdateRtt(QuicTime::Delta::FromMilliseconds(200),
                        QuicTime::Delta::FromMilliseconds(300),
                        QuicTime::Zero());
   EXPECT_EQ(QuicTime::Delta::FromMilliseconds(200), rtt_stats_.latest_rtt());
   EXPECT_EQ(QuicTime::Delta::FromMilliseconds(200), rtt_stats_.smoothed_rtt());
 }
 
+// Ensure that the potential rounding artifacts in EWMA calculation do not cause
+// the SRTT to drift too far from the exact value.
+TEST_F(RttStatsTest, SmoothedRttStability) {
+  for (size_t time = 3; time < 20000; time++) {
+    RttStats stats;
+    for (size_t i = 0; i < 100; i++) {
+      stats.UpdateRtt(QuicTime::Delta::FromMicroseconds(time),
+                      QuicTime::Delta::FromMilliseconds(0), QuicTime::Zero());
+      int64_t time_delta_us = stats.smoothed_rtt().ToMicroseconds() - time;
+      ASSERT_LE(std::abs(time_delta_us), 1);
+    }
+  }
+}
+
 TEST_F(RttStatsTest, PreviousSmoothedRtt) {
   // Verify that ack_delay is corrected for in Smoothed RTT.
   rtt_stats_.UpdateRtt(QuicTime::Delta::FromMilliseconds(300),
                        QuicTime::Delta::FromMilliseconds(100),
                        QuicTime::Zero());
   EXPECT_EQ(QuicTime::Delta::FromMilliseconds(200), rtt_stats_.latest_rtt());
   EXPECT_EQ(QuicTime::Delta::FromMilliseconds(200), rtt_stats_.smoothed_rtt());
   EXPECT_EQ(QuicTime::Delta::Zero(), rtt_stats_.previous_srtt());
   // Ensure the previous SRTT is 200ms after a 100ms sample.
   rtt_stats_.UpdateRtt(QuicTime::Delta::FromMilliseconds(100),
@@ skipping 127 matching lines @@
   // Reset rtt stats on connection migrations.
   rtt_stats_.OnConnectionMigration();
   EXPECT_EQ(QuicTime::Delta::Zero(), rtt_stats_.latest_rtt());
   EXPECT_EQ(QuicTime::Delta::Zero(), rtt_stats_.smoothed_rtt());
   EXPECT_EQ(QuicTime::Delta::Zero(), rtt_stats_.min_rtt());
   EXPECT_EQ(QuicTime::Delta::Zero(), rtt_stats_.WindowedMinRtt());
 }
 
 }  // namespace test
 }  // namespace net