Add ccaConfig param for startup pace gain

Summary: As title.

Reviewed By: mjoras

Differential Revision: D56425968

fbshipit-source-id: 79d6566ae66aa1c63d44d33e83565d9b11e8d329

quic/QuicConstants.h

@@ -348,6 +348,7 @@ enum class QuicVersion : uint32_t {
   // MVFST_EXPERIMENTAL2 enables including and using cwnd hints in 0-rtt session
   // tickets in QuicServerWorker.cpp
   MVFST_EXPERIMENTAL2 = 0xfaceb011, // Experimental alias for MVFST
+  // MVFST_EXPERIMENTAL3 is used to apply a 2x pace scaling for BBRv2
   MVFST_EXPERIMENTAL3 = 0xfaceb013, // Experimental alias for MVFST
 };
 

quic/congestion_control/Bbr2.cpp

@@ -948,7 +948,10 @@ void Bbr2CongestionController::getStats(
 void Bbr2CongestionController::updatePacingAndCwndGain() {
   switch (state_) {
     case State::Startup:
-      pacingGain_ = kStartupPacingGain;
+      pacingGain_ =
+          conn_.transportSettings.ccaConfig.overrideStartupPacingGain > 0
+          ? conn_.transportSettings.ccaConfig.overrideStartupPacingGain
+          : kStartupPacingGain;
       cwndGain_ = kStartupCwndGain;
       break;
     case State::Drain:

quic/server/QuicServerWorker.cpp

@@ -653,6 +653,11 @@ QuicServerTransport::Ptr QuicServerWorker::makeTransport(
     if (quicVersion == QuicVersion::MVFST_EXPERIMENTAL) {
       transportSettings_.initCwndInMss = 30;
     }
+    if (quicVersion == QuicVersion::MVFST_EXPERIMENTAL3) {
+      // Use twice the default pacing gain to make BBRv2's startup behavior
+      // similar to BBRv1's.
+      transportSettings_.ccaConfig.overrideStartupPacingGain = 2 * 2.89;
+    }
 
     auto transportSettings = transportSettingsOverrideFn_
         ? transportSettingsOverrideFn_(

quic/server/state/ServerStateMachine.cpp

@@ -97,6 +97,8 @@ void maybeSetExperimentalSettings(QuicServerConnectionState& conn) {
     // QuicServerWorker.cpp before CC is initialized.
   } else if (conn.version == QuicVersion::MVFST_EXPERIMENTAL2) {
   } else if (conn.version == QuicVersion::MVFST_EXPERIMENTAL3) {
+    // MVFST_EXPERIMENTAL3 is used to apply a 2x pace scaling for BBRv2
+    // QuicServerWorker.cpp before CC is initialized.
   }
 }
 

quic/state/TransportSettings.h

@@ -89,6 +89,11 @@ struct CongestionControlConfig {
   // Use a different cwnd gain during ProbeBW Cruise and Refill.
   // If value < 0, use the default cwnd gain.
   float overrideCruiseCwndGain{-1.0};
+
+  // Used by: BBR2
+  // Use a different pacing gain during Startup.
+  // If value < 0, use the default pacing gain.
+  float overrideStartupPacingGain{-1.0};
 };
 
 struct DatagramConfig {

quic/state/TransportSettingsFunctions.cpp

@@ -85,9 +85,10 @@ quic::CongestionControlConfig parseCongestionControlConfig(
   }
 
   // Parse optional float fields
-  const std::array<std::pair<std::string, float&>, 2> floatFields = {
+  const std::array<std::pair<std::string, float&>, 3> floatFields = {
       {{"overrideCruisePacingGain", ccaConfig.overrideCruisePacingGain},
-       {"overrideCruiseCwndGain", ccaConfig.overrideCruiseCwndGain}}};
+       {"overrideCruiseCwndGain", ccaConfig.overrideCruiseCwndGain},
+       {"overrideStartupPacingGain", ccaConfig.overrideStartupPacingGain}}};
 
   for (const auto& [name, field] : floatFields) {
     if (auto val = ccaConfigDyn.get_ptr(name)) {

quic/state/test/TransportSettingsFunctionsTest.cpp

@@ -38,11 +38,13 @@ TEST_F(TransportSettingsFunctionsTest, ParseFloats) {
   std::string testString =
       "{"
       "\"overrideCruisePacingGain\": 7.9, "
-      "\"overrideCruiseCwndGain\": -0.1 "
+      "\"overrideCruiseCwndGain\": -0.1, "
+      "\"overrideStartupPacingGain\": 2.1 "
       "}";
   auto config = parseCongestionControlConfig(testString);
   EXPECT_EQ(config.overrideCruisePacingGain, 7.9f);
   EXPECT_EQ(config.overrideCruiseCwndGain, -0.1f);
+  EXPECT_EQ(config.overrideStartupPacingGain, 2.1f);
 }
 
 TEST_F(TransportSettingsFunctionsTest, FullConfig) {
@@ -66,7 +68,8 @@ TEST_F(TransportSettingsFunctionsTest, FullConfig) {
       "\"ignoreLoss\": true, "
       "\"enableRenoCoexistence\": true, "
       "\"overrideCruisePacingGain\": 7.9, "
-      "\"overrideCruiseCwndGain\": -0.1 "
+      "\"overrideCruiseCwndGain\": -0.1, "
+      "\"overrideStartupPacingGain\": -0.5 "
       "}";
   auto config = parseCongestionControlConfig(testString);
   EXPECT_EQ(config.conservativeRecovery, true);
@@ -82,6 +85,7 @@ TEST_F(TransportSettingsFunctionsTest, FullConfig) {
   EXPECT_EQ(config.enableRenoCoexistence, true);
   EXPECT_EQ(config.overrideCruisePacingGain, 7.9f);
   EXPECT_EQ(config.overrideCruiseCwndGain, -0.1f);
+  EXPECT_EQ(config.overrideStartupPacingGain, -0.5f);
 
   ASSERT_TRUE(config.ackFrequencyConfig.has_value());
   EXPECT_EQ(config.ackFrequencyConfig->ackElicitingThreshold, 99);
@@ -109,6 +113,7 @@ TEST_F(TransportSettingsFunctionsTest, UnspecifiedFieldsAreDefaulted) {
   EXPECT_EQ(config.enableRenoCoexistence, false);
   EXPECT_EQ(config.overrideCruisePacingGain, -1.0f);
   EXPECT_EQ(config.overrideCruiseCwndGain, -1.0f);
+  EXPECT_EQ(config.overrideStartupPacingGain, -1.0f);
 
   ASSERT_TRUE(config.ackFrequencyConfig.has_value());
   EXPECT_EQ(