SF: add VSyncPredictor implementation (1678e2c1) · Commits · e / os / android_frameworks_native-old

services/surfaceflinger/Android.bp

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		@@ -165,8 +165,9 @@ filegroup {
		"Scheduler/PhaseOffsets.cpp",
		"Scheduler/Scheduler.cpp",
		"Scheduler/SchedulerUtils.cpp",
		"Scheduler/VSyncDispatchTimerQueue.cpp",
		"Scheduler/Timer.cpp",
		"Scheduler/VSyncDispatchTimerQueue.cpp",
		"Scheduler/VSyncPredictor.cpp",
		"Scheduler/VSyncModulator.cpp",
		"StartPropertySetThread.cpp",
		"SurfaceFlinger.cpp",

services/surfaceflinger/Scheduler/VSyncPredictor.cpp

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		/*
		* Copyright 2019 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.
		*/

		#define ATRACE_TAG ATRACE_TAG_GRAPHICS
		//#define LOG_NDEBUG 0
		#include "VSyncPredictor.h"
		#include <android-base/logging.h>
		#include <cutils/compiler.h>
		#include <utils/Log.h>
		#include <utils/Trace.h>
		#include <algorithm>
		#include <chrono>
		#include "SchedulerUtils.h"

		namespace android::scheduler {
		static auto constexpr kNeedsSamplesTag = "SamplesRequested";
		static auto constexpr kMaxPercent = 100u;

		VSyncPredictor::~VSyncPredictor() = default;

		VSyncPredictor::VSyncPredictor(nsecs_t idealPeriod, size_t historySize,
		size_t minimumSamplesForPrediction, uint32_t outlierTolerancePercent)
		: kHistorySize(historySize),
		kMinimumSamplesForPrediction(minimumSamplesForPrediction),
		kOutlierTolerancePercent(std::min(outlierTolerancePercent, kMaxPercent)),
		mIdealPeriod(idealPeriod) {
		mRateMap[mIdealPeriod] = {idealPeriod, 0};
		}

		inline size_t VSyncPredictor::next(int i) const {
		return (i + 1) % timestamps.size();
		}

		bool VSyncPredictor::validate(nsecs_t timestamp) const {
		if (lastTimestampIndex < 0 \|\| timestamps.empty()) {
		return true;
		}

		auto const aValidTimestamp = timestamps[lastTimestampIndex];
		auto const percent = (timestamp - aValidTimestamp) % mIdealPeriod * kMaxPercent / mIdealPeriod;
		return percent < kOutlierTolerancePercent \|\| percent > (kMaxPercent - kOutlierTolerancePercent);
		}

		void VSyncPredictor::addVsyncTimestamp(nsecs_t timestamp) {
		std::lock_guard<std::mutex> lk(mMutex);

		if (!validate(timestamp)) {
		ALOGW("timestamp was too far off the last known timestamp");
		return;
		}

		if (timestamps.size() != kHistorySize) {
		timestamps.push_back(timestamp);
		lastTimestampIndex = next(lastTimestampIndex);
		} else {
		lastTimestampIndex = next(lastTimestampIndex);
		timestamps[lastTimestampIndex] = timestamp;
		}

		if (timestamps.size() < kMinimumSamplesForPrediction) {
		mRateMap[mIdealPeriod] = {mIdealPeriod, 0};
		return;
		}

		// This is a 'simple linear regression' calculation of Y over X, with Y being the
		// vsync timestamps, and X being the ordinal of vsync count.
		// The calculated slope is the vsync period.
		// Formula for reference:
		// Sigma_i: means sum over all timestamps.
		// mean(variable): statistical mean of variable.
		// X: snapped ordinal of the timestamp
		// Y: vsync timestamp
		//
		// Sigma_i( (X_i - mean(X)) * (Y_i - mean(Y) )
		// slope = -------------------------------------------
		// Sigma_i ( X_i - mean(X) ) ^ 2
		//
		// intercept = mean(Y) - slope * mean(X)
		//
		std::vector<nsecs_t> vsyncTS(timestamps.size());
		std::vector<nsecs_t> ordinals(timestamps.size());

		// normalizing to the oldest timestamp cuts down on error in calculating the intercept.
		auto const oldest_ts = *std::min_element(timestamps.begin(), timestamps.end());
		auto it = mRateMap.find(mIdealPeriod);
		auto const currentPeriod = std::get<0>(it->second);
		// TODO (b/144707443): its important that there's some precision in the mean of the ordinals
		// for the intercept calculation, so scale the ordinals by 10 to continue
		// fixed point calculation. Explore expanding
		// scheduler::utils::calculate_mean to have a fixed point fractional part.
		static constexpr int kScalingFactor = 10;

		for (auto i = 0u; i < timestamps.size(); i++) {
		vsyncTS[i] = timestamps[i] - oldest_ts;
		ordinals[i] = ((vsyncTS[i] + (currentPeriod / 2)) / currentPeriod) * kScalingFactor;
		}

		auto meanTS = scheduler::calculate_mean(vsyncTS);
		auto meanOrdinal = scheduler::calculate_mean(ordinals);
		for (auto i = 0; i < vsyncTS.size(); i++) {
		vsyncTS[i] -= meanTS;
		ordinals[i] -= meanOrdinal;
		}

		auto top = 0ll;
		auto bottom = 0ll;
		for (auto i = 0; i < vsyncTS.size(); i++) {
		top += vsyncTS[i] * ordinals[i];
		bottom += ordinals[i] * ordinals[i];
		}

		if (CC_UNLIKELY(bottom == 0)) {
		it->second = {mIdealPeriod, 0};
		return;
		}

		nsecs_t const anticipatedPeriod = top / bottom * kScalingFactor;
		nsecs_t const intercept = meanTS - (anticipatedPeriod * meanOrdinal / kScalingFactor);

		it->second = {anticipatedPeriod, intercept};

		ALOGV("model update ts: %" PRId64 " slope: %" PRId64 " intercept: %" PRId64, timestamp,
		anticipatedPeriod, intercept);
		}

		nsecs_t VSyncPredictor::nextAnticipatedVSyncTimeFrom(nsecs_t timePoint) const {
		std::lock_guard<std::mutex> lk(mMutex);

		auto const [slope, intercept] = getVSyncPredictionModel(lk);

		if (timestamps.empty()) {
		auto const knownTimestamp = mKnownTimestamp ? *mKnownTimestamp : timePoint;
		auto const numPeriodsOut = ((timePoint - knownTimestamp) / mIdealPeriod) + 1;
		return knownTimestamp + numPeriodsOut * mIdealPeriod;
		}

		auto const oldest = *std::min_element(timestamps.begin(), timestamps.end());
		auto const ordinalRequest = (timePoint - oldest + slope) / slope;
		auto const prediction = (ordinalRequest * slope) + intercept + oldest;

		ALOGV("prediction made from: %" PRId64 " prediction: %" PRId64 " (+%" PRId64 ") slope: %" PRId64
		" intercept: %" PRId64,
		timePoint, prediction, prediction - timePoint, slope, intercept);
		return prediction;
		}

		std::tuple<nsecs_t, nsecs_t> VSyncPredictor::getVSyncPredictionModel() const {
		std::lock_guard<std::mutex> lk(mMutex);
		return VSyncPredictor::getVSyncPredictionModel(lk);
		}

		std::tuple<nsecs_t, nsecs_t> VSyncPredictor::getVSyncPredictionModel(
		std::lock_guard<std::mutex> const&) const {
		return mRateMap.find(mIdealPeriod)->second;
		}

		void VSyncPredictor::setPeriod(nsecs_t period) {
		ATRACE_CALL();

		std::lock_guard<std::mutex> lk(mMutex);
		static constexpr size_t kSizeLimit = 30;
		if (CC_UNLIKELY(mRateMap.size() == kSizeLimit)) {
		mRateMap.erase(mRateMap.begin());
		}

		mIdealPeriod = period;
		if (mRateMap.find(period) == mRateMap.end()) {
		mRateMap[mIdealPeriod] = {period, 0};
		}

		if (!timestamps.empty()) {
		mKnownTimestamp = *std::max_element(timestamps.begin(), timestamps.end());
		timestamps.clear();
		lastTimestampIndex = 0;
		}
		}

		bool VSyncPredictor::needsMoreSamples(nsecs_t now) const {
		using namespace std::literals::chrono_literals;
		std::lock_guard<std::mutex> lk(mMutex);
		bool needsMoreSamples = true;
		if (timestamps.size() >= kMinimumSamplesForPrediction) {
		nsecs_t constexpr aLongTime =
		std::chrono::duration_cast<std::chrono::nanoseconds>(500ms).count();
		if (!(lastTimestampIndex < 0 \|\| timestamps.empty())) {
		auto const lastTimestamp = timestamps[lastTimestampIndex];
		needsMoreSamples = !((lastTimestamp + aLongTime) > now);
		}
		}

		ATRACE_INT(kNeedsSamplesTag, needsMoreSamples);
		return needsMoreSamples;
		}

		} // namespace android::scheduler

services/surfaceflinger/Scheduler/VSyncPredictor.h

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		/*
		* Copyright 2019 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.
		*/

		#pragma once

		#include <android-base/thread_annotations.h>
		#include <mutex>
		#include <unordered_map>
		#include <vector>
		#include "VSyncTracker.h"

		namespace android::scheduler {

		class VSyncPredictor : public VSyncTracker {
		public:
		/*
		* \param [in] idealPeriod The initial ideal period to use.
		* \param [in] historySize The internal amount of entries to store in the model.
		* \param [in] minimumSamplesForPrediction The minimum number of samples to collect before
		* predicting. \param [in] outlierTolerancePercent a number 0 to 100 that will be used to filter
		* samples that fall outlierTolerancePercent from an anticipated vsync event.
		*/
		VSyncPredictor(nsecs_t idealPeriod, size_t historySize, size_t minimumSamplesForPrediction,
		uint32_t outlierTolerancePercent);
		~VSyncPredictor();

		void addVsyncTimestamp(nsecs_t timestamp) final;
		nsecs_t nextAnticipatedVSyncTimeFrom(nsecs_t timePoint) const final;

		/*
		* Inform the model that the period is anticipated to change to a new value.
		* model will use the period parameter to predict vsync events until enough
		* timestamps with the new period have been collected.
		*
		* \param [in] period The new period that should be used.
		*/
		void setPeriod(nsecs_t period);

		/* Query if the model is in need of more samples to make a prediction at timePoint.
		* \param [in] timePoint The timePoint to inquire of.
		* \return True, if model would benefit from more samples, False if not.
		*/
		bool needsMoreSamples(nsecs_t timePoint) const;

		std::tuple<nsecs_t /* slope /, nsecs_t / intercept */> getVSyncPredictionModel() const;

		private:
		VSyncPredictor(VSyncPredictor const&) = delete;
		VSyncPredictor& operator=(VSyncPredictor const&) = delete;

		size_t const kHistorySize;
		size_t const kMinimumSamplesForPrediction;
		size_t const kOutlierTolerancePercent;

		std::mutex mutable mMutex;
		size_t next(int i) const REQUIRES(mMutex);
		bool validate(nsecs_t timestamp) const REQUIRES(mMutex);
		std::tuple<nsecs_t, nsecs_t> getVSyncPredictionModel(std::lock_guard<std::mutex> const&) const
		REQUIRES(mMutex);

		nsecs_t mIdealPeriod GUARDED_BY(mMutex);
		std::optional<nsecs_t> mKnownTimestamp GUARDED_BY(mMutex);

		std::unordered_map<nsecs_t, std::tuple<nsecs_t, nsecs_t>> mutable mRateMap GUARDED_BY(mMutex);

		int lastTimestampIndex GUARDED_BY(mMutex) = 0;
		std::vector<nsecs_t> timestamps GUARDED_BY(mMutex);
		};

		} // namespace android::scheduler

services/surfaceflinger/tests/unittests/Android.bp

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		@@ -56,6 +56,7 @@ cc_test {
		"StrongTypingTest.cpp",
		"VSyncDispatchTimerQueueTest.cpp",
		"VSyncDispatchRealtimeTest.cpp",
		"VSyncPredictorTest.cpp",
		"mock/DisplayHardware/MockComposer.cpp",
		"mock/DisplayHardware/MockDisplay.cpp",
		"mock/DisplayHardware/MockPowerAdvisor.cpp",

services/surfaceflinger/tests/unittests/VSyncPredictorTest.cpp

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		/*
		* Copyright 2019 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/VSyncPredictor.h"

		#include <gmock/gmock.h>
		#include <gtest/gtest.h>
		#include <algorithm>
		#include <chrono>
		#include <utility>

		using namespace testing;
		using namespace std::literals;

		namespace android::scheduler {

		MATCHER_P2(IsCloseTo, value, tolerance, "is within tolerance") {
		return arg <= value + tolerance && value >= value - tolerance;
		}

		std::vector<nsecs_t> generateVsyncTimestamps(size_t count, nsecs_t period, nsecs_t bias) {
		std::vector<nsecs_t> vsyncs(count);
		std::generate(vsyncs.begin(), vsyncs.end(),
		[&, n = 0]() mutable { return n++ * period + bias; });
		return vsyncs;
		}

		struct VSyncPredictorTest : testing::Test {
		nsecs_t mNow = 0;
		nsecs_t mPeriod = 1000;
		static constexpr size_t kHistorySize = 10;
		static constexpr size_t kMinimumSamplesForPrediction = 6;
		static constexpr size_t kOutlierTolerancePercent = 25;
		static constexpr nsecs_t mMaxRoundingError = 100;

		VSyncPredictor tracker{mPeriod, kHistorySize, kMinimumSamplesForPrediction,
		kOutlierTolerancePercent};
		};

		TEST_F(VSyncPredictorTest, reportsAnticipatedPeriod) {
		auto [slope, intercept] = tracker.getVSyncPredictionModel();

		EXPECT_THAT(slope, Eq(mPeriod));
		EXPECT_THAT(intercept, Eq(0));

		auto const changedPeriod = 2000;
		tracker.setPeriod(changedPeriod);
		std::tie(slope, intercept) = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, Eq(changedPeriod));
		EXPECT_THAT(intercept, Eq(0));
		}

		TEST_F(VSyncPredictorTest, reportsSamplesNeededWhenHasNoDataPoints) {
		for (auto i = 0u; i < kMinimumSamplesForPrediction; i++) {
		EXPECT_TRUE(tracker.needsMoreSamples(mNow += mPeriod));
		tracker.addVsyncTimestamp(mNow);
		}
		EXPECT_FALSE(tracker.needsMoreSamples(mNow));
		}

		TEST_F(VSyncPredictorTest, reportsSamplesNeededAfterExplicitRateChange) {
		for (auto i = 0u; i < kMinimumSamplesForPrediction; i++) {
		tracker.addVsyncTimestamp(mNow += mPeriod);
		}
		EXPECT_FALSE(tracker.needsMoreSamples(mNow));

		auto const changedPeriod = mPeriod * 2;
		tracker.setPeriod(changedPeriod);
		EXPECT_TRUE(tracker.needsMoreSamples(mNow));

		for (auto i = 0u; i < kMinimumSamplesForPrediction; i++) {
		EXPECT_TRUE(tracker.needsMoreSamples(mNow += changedPeriod));
		tracker.addVsyncTimestamp(mNow);
		}
		EXPECT_FALSE(tracker.needsMoreSamples(mNow));
		}

		TEST_F(VSyncPredictorTest, transitionsToModelledPointsAfterSynthetic) {
		auto last = mNow;
		auto const bias = 10;
		for (auto i = 0u; i < kMinimumSamplesForPrediction; i++) {
		EXPECT_THAT(tracker.nextAnticipatedVSyncTimeFrom(mNow), Eq(last + mPeriod));
		mNow += mPeriod - bias;
		last = mNow;
		tracker.addVsyncTimestamp(mNow);
		mNow += bias;
		}

		EXPECT_THAT(tracker.nextAnticipatedVSyncTimeFrom(mNow), Eq(mNow + mPeriod - bias));
		EXPECT_THAT(tracker.nextAnticipatedVSyncTimeFrom(mNow + 100), Eq(mNow + mPeriod - bias));
		EXPECT_THAT(tracker.nextAnticipatedVSyncTimeFrom(mNow + 990), Eq(mNow + 2 * mPeriod - bias));
		}

		TEST_F(VSyncPredictorTest, uponNotifiedOfInaccuracyUsesSynthetic) {
		auto const slightlyLessPeriod = mPeriod - 10;
		auto const changedPeriod = mPeriod - 1;
		for (auto i = 0u; i < kMinimumSamplesForPrediction; i++) {
		tracker.addVsyncTimestamp(mNow += slightlyLessPeriod);
		}

		EXPECT_THAT(tracker.nextAnticipatedVSyncTimeFrom(mNow), Eq(mNow + slightlyLessPeriod));
		tracker.setPeriod(changedPeriod);
		EXPECT_THAT(tracker.nextAnticipatedVSyncTimeFrom(mNow), Eq(mNow + changedPeriod));
		}

		TEST_F(VSyncPredictorTest, adaptsToFenceTimelines_60hzHighVariance) {
		// these are precomputed simulated 16.6s vsyncs with uniform distribution +/- 1.6ms error
		std::vector<nsecs_t> const simulatedVsyncs{
		15492949, 32325658, 49534984, 67496129, 84652891,
		100332564, 117737004, 132125931, 149291099, 165199602,
		};
		auto constexpr idealPeriod = 16600000;
		auto constexpr expectedPeriod = 16639242;
		auto constexpr expectedIntercept = 1049341;

		tracker.setPeriod(idealPeriod);
		for (auto const& timestamp : simulatedVsyncs) {
		tracker.addVsyncTimestamp(timestamp);
		}
		auto [slope, intercept] = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, IsCloseTo(expectedPeriod, mMaxRoundingError));
		EXPECT_THAT(intercept, IsCloseTo(expectedIntercept, mMaxRoundingError));
		}

		TEST_F(VSyncPredictorTest, adaptsToFenceTimelines_90hzLowVariance) {
		// these are precomputed simulated 11.1 vsyncs with uniform distribution +/- 1ms error
		std::vector<nsecs_t> const simulatedVsyncs{
		11167047, 22603464, 32538479, 44938134, 56321268,
		66730346, 78062637, 88171429, 99707843, 111397621,
		};
		auto idealPeriod = 11110000;
		auto expectedPeriod = 11089413;
		auto expectedIntercept = 94421;

		tracker.setPeriod(idealPeriod);
		for (auto const& timestamp : simulatedVsyncs) {
		tracker.addVsyncTimestamp(timestamp);
		}
		auto [slope, intercept] = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, IsCloseTo(expectedPeriod, mMaxRoundingError));
		EXPECT_THAT(intercept, IsCloseTo(expectedIntercept, mMaxRoundingError));
		}

		TEST_F(VSyncPredictorTest, adaptsToFenceTimelinesDiscontinuous_22hzLowVariance) {
		// these are 11.1s vsyncs with low variance, randomly computed, between -1 and 1ms
		std::vector<nsecs_t> const simulatedVsyncs{
		45259463, // 0
		91511026, // 1
		136307650, // 2
		1864501714, // 40
		1908641034, // 41
		1955278544, // 42
		4590180096, // 100
		4681594994, // 102
		5499224734, // 120
		5591378272, // 122
		};
		auto idealPeriod = 45454545;
		auto expectedPeriod = 45450152;
		auto expectedIntercept = 469647;

		tracker.setPeriod(idealPeriod);
		for (auto const& timestamp : simulatedVsyncs) {
		tracker.addVsyncTimestamp(timestamp);
		}
		auto [slope, intercept] = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, IsCloseTo(expectedPeriod, mMaxRoundingError));
		EXPECT_THAT(intercept, IsCloseTo(expectedIntercept, mMaxRoundingError));
		}

		TEST_F(VSyncPredictorTest, againstOutliersDiscontinuous_500hzLowVariance) {
		std::vector<nsecs_t> const simulatedVsyncs{
		1992548, // 0
		4078038, // 1
		6165794, // 2
		7958171, // 3
		10193537, // 4
		2401840200, // 1200
		2403000000, // an outlier that should be excluded (1201 and a half)
		2405803629, // 1202
		2408028599, // 1203
		2410121051, // 1204
		};
		auto idealPeriod = 2000000;
		auto expectedPeriod = 1999892;
		auto expectedIntercept = 175409;

		tracker.setPeriod(idealPeriod);
		for (auto const& timestamp : simulatedVsyncs) {
		tracker.addVsyncTimestamp(timestamp);
		}

		auto [slope, intercept] = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, IsCloseTo(expectedPeriod, mMaxRoundingError));
		EXPECT_THAT(intercept, IsCloseTo(expectedIntercept, mMaxRoundingError));
		}

		TEST_F(VSyncPredictorTest, handlesVsyncChange) {
		auto const fastPeriod = 100;
		auto const fastTimeBase = 100;
		auto const slowPeriod = 400;
		auto const slowTimeBase = 800;
		auto const simulatedVsyncsFast =
		generateVsyncTimestamps(kMinimumSamplesForPrediction, fastPeriod, fastTimeBase);
		auto const simulatedVsyncsSlow =
		generateVsyncTimestamps(kMinimumSamplesForPrediction, slowPeriod, slowTimeBase);

		tracker.setPeriod(fastPeriod);
		for (auto const& timestamp : simulatedVsyncsFast) {
		tracker.addVsyncTimestamp(timestamp);
		}

		auto const mMaxRoundingError = 100;
		auto [slope, intercept] = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, IsCloseTo(fastPeriod, mMaxRoundingError));
		EXPECT_THAT(intercept, IsCloseTo(0, mMaxRoundingError));

		tracker.setPeriod(slowPeriod);
		for (auto const& timestamp : simulatedVsyncsSlow) {
		tracker.addVsyncTimestamp(timestamp);
		}
		std::tie(slope, intercept) = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, IsCloseTo(slowPeriod, mMaxRoundingError));
		EXPECT_THAT(intercept, IsCloseTo(0, mMaxRoundingError));
		}

		TEST_F(VSyncPredictorTest, willBeAccurateUsingPriorResultsForRate) {
		auto const fastPeriod = 101000;
		auto const fastTimeBase = fastPeriod - 500;
		auto const fastPeriod2 = 99000;

		auto const slowPeriod = 400000;
		auto const slowTimeBase = 800000 - 201;
		auto const simulatedVsyncsFast =
		generateVsyncTimestamps(kMinimumSamplesForPrediction, fastPeriod, fastTimeBase);
		auto const simulatedVsyncsSlow =
		generateVsyncTimestamps(kMinimumSamplesForPrediction, slowPeriod, slowTimeBase);
		auto const simulatedVsyncsFast2 =
		generateVsyncTimestamps(kMinimumSamplesForPrediction, fastPeriod2, fastTimeBase);

		auto idealPeriod = 100000;
		tracker.setPeriod(idealPeriod);
		for (auto const& timestamp : simulatedVsyncsFast) {
		tracker.addVsyncTimestamp(timestamp);
		}
		auto [slope, intercept] = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, Eq(fastPeriod));
		EXPECT_THAT(intercept, Eq(0));

		tracker.setPeriod(slowPeriod);
		for (auto const& timestamp : simulatedVsyncsSlow) {
		tracker.addVsyncTimestamp(timestamp);
		}

		// we had a model for 100ns mPeriod before, use that until the new samples are
		// sufficiently built up
		tracker.setPeriod(idealPeriod);
		std::tie(slope, intercept) = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, Eq(fastPeriod));
		EXPECT_THAT(intercept, Eq(0));

		for (auto const& timestamp : simulatedVsyncsFast2) {
		tracker.addVsyncTimestamp(timestamp);
		}
		std::tie(slope, intercept) = tracker.getVSyncPredictionModel();
		EXPECT_THAT(slope, Eq(fastPeriod2));
		EXPECT_THAT(intercept, Eq(0));
		}

		TEST_F(VSyncPredictorTest, willBecomeInaccurateAfterA_longTimeWithNoSamples) {
		auto const simulatedVsyncs = generateVsyncTimestamps(kMinimumSamplesForPrediction, mPeriod, 0);

		for (auto const& timestamp : simulatedVsyncs) {
		tracker.addVsyncTimestamp(timestamp);
		}
		auto const mNow = *simulatedVsyncs.rbegin();
		EXPECT_FALSE(tracker.needsMoreSamples(mNow));

		// TODO: would be better to decay this as a result of the variance of the samples
		static auto constexpr aLongTimeOut = 1000000000;
		EXPECT_TRUE(tracker.needsMoreSamples(mNow + aLongTimeOut));
		}

		TEST_F(VSyncPredictorTest, idealModelPredictionsBeforeRegressionModelIsBuilt) {
		auto const simulatedVsyncs =
		generateVsyncTimestamps(kMinimumSamplesForPrediction + 1, mPeriod, 0);
		nsecs_t const mNow = 0;
		EXPECT_THAT(tracker.nextAnticipatedVSyncTimeFrom(mNow), Eq(mPeriod));

		nsecs_t const aBitOfTime = 422;

		for (auto i = 0; i < kMinimumSamplesForPrediction; i++) {
		tracker.addVsyncTimestamp(simulatedVsyncs[i]);
		EXPECT_THAT(tracker.nextAnticipatedVSyncTimeFrom(simulatedVsyncs[i] + aBitOfTime),
		Eq(mPeriod + simulatedVsyncs[i]));
		}

		for (auto i = kMinimumSamplesForPrediction; i < simulatedVsyncs.size(); i++) {
		tracker.addVsyncTimestamp(simulatedVsyncs[i]);
		EXPECT_THAT(tracker.nextAnticipatedVSyncTimeFrom(simulatedVsyncs[i] + aBitOfTime),
		Eq(mPeriod + simulatedVsyncs[i]));
		}
		}

		} // namespace android::scheduler