diff --git a/services/surfaceflinger/tests/unittests/Android.bp b/services/surfaceflinger/tests/unittests/Android.bp
index 7574ff1..3c4a791 100644
--- a/services/surfaceflinger/tests/unittests/Android.bp
+++ b/services/surfaceflinger/tests/unittests/Android.bp
@@ -62,6 +62,7 @@
         "StrongTypingTest.cpp",
         "VSyncDispatchTimerQueueTest.cpp",
         "VSyncDispatchRealtimeTest.cpp",
+        "VSyncModulatorTest.cpp",
         "VSyncPredictorTest.cpp",
         "VSyncReactorTest.cpp",
         "mock/DisplayHardware/MockComposer.cpp",
diff --git a/services/surfaceflinger/tests/unittests/VSyncModulatorTest.cpp b/services/surfaceflinger/tests/unittests/VSyncModulatorTest.cpp
new file mode 100644
index 0000000..9c1ec07
--- /dev/null
+++ b/services/surfaceflinger/tests/unittests/VSyncModulatorTest.cpp
@@ -0,0 +1,301 @@
+/*
+ * Copyright 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#undef LOG_TAG
+#define LOG_TAG "LibSurfaceFlingerUnittests"
+#define LOG_NDEBUG 0
+
+#include "Scheduler/VSyncModulator.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+using namespace testing;
+
+namespace android::scheduler {
+
+class MockScheduler : public IPhaseOffsetControl {
+public:
+    void setPhaseOffset(ConnectionHandle handle, nsecs_t phaseOffset) {
+        mPhaseOffset[handle] = phaseOffset;
+    }
+
+    nsecs_t getOffset(ConnectionHandle handle) { return mPhaseOffset[handle]; }
+
+private:
+    std::unordered_map<ConnectionHandle, nsecs_t> mPhaseOffset;
+};
+
+class VSyncModulatorTest : public testing::Test {
+protected:
+    static constexpr auto MIN_EARLY_FRAME_COUNT_TRANSACTION =
+            VSyncModulator::MIN_EARLY_FRAME_COUNT_TRANSACTION;
+    // Add a 1ms slack to avoid strange timer race conditions.
+    static constexpr auto MARGIN_FOR_TX_APPLY = VSyncModulator::MARGIN_FOR_TX_APPLY + 1ms;
+
+    // Used to enumerate the different offsets we have
+    enum {
+        SF_LATE,
+        APP_LATE,
+        SF_EARLY,
+        APP_EARLY,
+        SF_EARLY_GL,
+        APP_EARLY_GL,
+    };
+
+    std::unique_ptr<VSyncModulator> mVSyncModulator;
+    MockScheduler mMockScheduler;
+    ConnectionHandle mAppConnection{1};
+    ConnectionHandle mSfConnection{2};
+    VSyncModulator::OffsetsConfig mOffsets = {{SF_EARLY, APP_EARLY},
+                                              {SF_EARLY_GL, APP_EARLY_GL},
+                                              {SF_LATE, APP_LATE}};
+
+    void SetUp() override {
+        mVSyncModulator = std::make_unique<VSyncModulator>(mMockScheduler, mAppConnection,
+                                                           mSfConnection, mOffsets);
+        mVSyncModulator->setPhaseOffsets(mOffsets);
+
+        EXPECT_EQ(APP_LATE, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_LATE, mMockScheduler.getOffset(mSfConnection));
+    };
+
+    void TearDown() override { mVSyncModulator.reset(); }
+};
+
+TEST_F(VSyncModulatorTest, Normal) {
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::Normal);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_LATE, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_LATE, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < MIN_EARLY_FRAME_COUNT_TRANSACTION; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_LATE, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_LATE, mMockScheduler.getOffset(mSfConnection));
+    }
+}
+
+TEST_F(VSyncModulatorTest, EarlyEnd) {
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyEnd);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < MIN_EARLY_FRAME_COUNT_TRANSACTION - 1; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->onRefreshed(false);
+    EXPECT_EQ(APP_LATE, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_LATE, mMockScheduler.getOffset(mSfConnection));
+}
+
+TEST_F(VSyncModulatorTest, EarlyStart) {
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyStart);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < 5 * MIN_EARLY_FRAME_COUNT_TRANSACTION; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyEnd);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < MIN_EARLY_FRAME_COUNT_TRANSACTION - 1; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->onRefreshed(false);
+    EXPECT_EQ(APP_LATE, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_LATE, mMockScheduler.getOffset(mSfConnection));
+}
+
+TEST_F(VSyncModulatorTest, EarlyStartWithEarly) {
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyStart);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < 5 * MIN_EARLY_FRAME_COUNT_TRANSACTION; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::Early);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < 5 * MIN_EARLY_FRAME_COUNT_TRANSACTION; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyEnd);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < MIN_EARLY_FRAME_COUNT_TRANSACTION - 1; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->onRefreshed(false);
+    EXPECT_EQ(APP_LATE, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_LATE, mMockScheduler.getOffset(mSfConnection));
+}
+
+TEST_F(VSyncModulatorTest, EarlyStartWithMoreTransactions) {
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyStart);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < 5 * MIN_EARLY_FRAME_COUNT_TRANSACTION; i++) {
+        mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::Normal);
+        std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyEnd);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < MIN_EARLY_FRAME_COUNT_TRANSACTION - 1; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->onRefreshed(false);
+    EXPECT_EQ(APP_LATE, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_LATE, mMockScheduler.getOffset(mSfConnection));
+}
+
+TEST_F(VSyncModulatorTest, EarlyStartAfterEarlyEnd) {
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyEnd);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < MIN_EARLY_FRAME_COUNT_TRANSACTION - 1; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyStart);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < 5 * MIN_EARLY_FRAME_COUNT_TRANSACTION; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyEnd);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < MIN_EARLY_FRAME_COUNT_TRANSACTION - 1; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->onRefreshed(false);
+    EXPECT_EQ(APP_LATE, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_LATE, mMockScheduler.getOffset(mSfConnection));
+}
+
+TEST_F(VSyncModulatorTest, EarlyStartAfterEarlyEndWithMoreTransactions) {
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyEnd);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < MIN_EARLY_FRAME_COUNT_TRANSACTION - 1; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyStart);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < 5 * MIN_EARLY_FRAME_COUNT_TRANSACTION; i++) {
+        mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::Normal);
+        std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->setTransactionStart(Scheduler::TransactionStart::EarlyEnd);
+    std::this_thread::sleep_for(MARGIN_FOR_TX_APPLY);
+    mVSyncModulator->onTransactionHandled();
+    EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+
+    for (int i = 0; i < MIN_EARLY_FRAME_COUNT_TRANSACTION - 1; i++) {
+        mVSyncModulator->onRefreshed(false);
+        EXPECT_EQ(APP_EARLY, mMockScheduler.getOffset(mAppConnection));
+        EXPECT_EQ(SF_EARLY, mMockScheduler.getOffset(mSfConnection));
+    }
+
+    mVSyncModulator->onRefreshed(false);
+    EXPECT_EQ(APP_LATE, mMockScheduler.getOffset(mAppConnection));
+    EXPECT_EQ(SF_LATE, mMockScheduler.getOffset(mSfConnection));
+}
+
+} // namespace android::scheduler