1 files changed, 663 insertions, 0 deletions
diff --git a/xpcom/tests/gtest/TestAvailableMemoryWatcherWin.cpp b/xpcom/tests/gtest/TestAvailableMemoryWatcherWin.cpp
new file mode 100644
index 0000000000..409d547aaa
--- /dev/null
+++ b/xpcom/tests/gtest/TestAvailableMemoryWatcherWin.cpp
@@ -0,0 +1,663 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <algorithm>
+#include <windows.h>
+#include <memoryapi.h>
+#include "gtest/gtest.h"
+
+#include "AvailableMemoryWatcher.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/gtest/MozAssertions.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/SpinEventLoopUntil.h"
+#include "mozilla/Unused.h"
+#include "mozilla/Vector.h"
+#include "nsComponentManagerUtils.h"
+#include "nsIObserver.h"
+#include "nsIObserverService.h"
+#include "nsServiceManagerUtils.h"
+#include "nsITimer.h"
+#include "nsMemoryPressure.h"
+#include "nsWindowsHelpers.h"
+#include "nsIWindowsRegKey.h"
+#include "nsXULAppAPI.h"
+#include "TelemetryFixture.h"
+#include "TelemetryTestHelpers.h"
+
+using namespace mozilla;
+
+namespace {
+
+static constexpr size_t kBytesInMB = 1024 * 1024;
+
+template <typename ConditionT>
+bool WaitUntil(const ConditionT& aCondition, uint32_t aTimeoutMs) {
+  const uint64_t t0 = ::GetTickCount64();
+  bool isTimeout = false;
+
+  // The message queue can be empty and the loop stops
+  // waiting for a new event before detecting timeout.
+  // Creating a timer to fire a timeout event.
+  nsCOMPtr<nsITimer> timer;
+  NS_NewTimerWithFuncCallback(
+      getter_AddRefs(timer),
+      [](nsITimer*, void* isTimeout) {
+        *reinterpret_cast<bool*>(isTimeout) = true;
+      },
+      &isTimeout, aTimeoutMs, nsITimer::TYPE_ONE_SHOT, __func__);
+
+  SpinEventLoopUntil("xpcom-tests:WaitUntil"_ns, [&]() -> bool {
+    if (isTimeout) {
+      return true;
+    }
+
+    bool done = aCondition();
+    if (done) {
+      fprintf(stderr, "Done in %llu msec\n", ::GetTickCount64() - t0);
+    }
+    return done;
+  });
+
+  return !isTimeout;
+}
+
+class Spinner final : public nsIObserver {
+  nsCOMPtr<nsIObserverService> mObserverSvc;
+  nsDependentCString mTopicToWatch;
+  Maybe<nsDependentString> mSubTopicToWatch;
+  bool mTopicObserved;
+
+  ~Spinner() = default;
+
+ public:
+  NS_DECL_ISUPPORTS
+
+  Spinner(nsIObserverService* aObserverSvc, const char* const aTopic,
+          const char16_t* const aSubTopic)
+      : mObserverSvc(aObserverSvc),
+        mTopicToWatch(aTopic),
+        mSubTopicToWatch(aSubTopic ? Some(nsDependentString(aSubTopic))
+                                   : Nothing()),
+        mTopicObserved(false) {}
+
+  NS_IMETHOD Observe(nsISupports* aSubject, const char* aTopic,
+                     const char16_t* aData) override {
+    if (mTopicToWatch == aTopic) {
+      if ((mSubTopicToWatch.isNothing() && !aData) ||
+          mSubTopicToWatch.ref() == aData) {
+        mTopicObserved = true;
+        mObserverSvc->RemoveObserver(this, aTopic);
+
+        // Force the loop to move in case that there is no event in the queue.
+        nsCOMPtr<nsIRunnable> dummyEvent = new Runnable(__func__);
+        NS_DispatchToMainThread(dummyEvent);
+      }
+    } else {
+      fprintf(stderr, "Unexpected topic: %s\n", aTopic);
+    }
+
+    return NS_OK;
+  }
+
+  void StartListening() {
+    mTopicObserved = false;
+    mObserverSvc->AddObserver(this, mTopicToWatch.get(), false);
+  }
+
+  bool Wait(uint32_t aTimeoutMs) {
+    return WaitUntil([this]() { return this->mTopicObserved; }, aTimeoutMs);
+  }
+};
+
+NS_IMPL_ISUPPORTS(Spinner, nsIObserver)
+
+/**
+ * Starts a new thread with a message queue to process
+ * memory allocation/free requests
+ */
+class MemoryEater {
+  using PageT = UniquePtr<void, VirtualFreeDeleter>;
+
+  static DWORD WINAPI ThreadStart(LPVOID aParam) {
+    return reinterpret_cast<MemoryEater*>(aParam)->ThreadProc();
+  }
+
+  static void TouchMemory(void* aAddr, size_t aSize) {
+    constexpr uint32_t kPageSize = 4096;
+    volatile uint8_t x = 0;
+    auto base = reinterpret_cast<uint8_t*>(aAddr);
+    for (int64_t i = 0, pages = aSize / kPageSize; i < pages; ++i) {
+      // Pick a random place in every allocated page
+      // and dereference it.
+      x ^= *(base + i * kPageSize + rand() % kPageSize);
+    }
+    (void)x;
+  }
+
+  static uint32_t GetAvailablePhysicalMemoryInMb() {
+    MEMORYSTATUSEX statex = {sizeof(statex)};
+    if (!::GlobalMemoryStatusEx(&statex)) {
+      return 0;
+    }
+
+    return static_cast<uint32_t>(statex.ullAvailPhys / kBytesInMB);
+  }
+
+  static bool AddWorkingSet(size_t aSize, Vector<PageT>& aOutput) {
+    constexpr size_t kMinGranularity = 64 * 1024;
+
+    size_t currentSize = aSize;
+    while (aSize >= kMinGranularity) {
+      if (!GetAvailablePhysicalMemoryInMb()) {
+        // If the available physical memory is less than 1MB, we finish
+        // allocation though there may be still the available commit space.
+        fprintf(stderr, "No enough physical memory.\n");
+        return false;
+      }
+
+      PageT page(::VirtualAlloc(nullptr, currentSize, MEM_RESERVE | MEM_COMMIT,
+                                PAGE_READWRITE));
+      if (!page) {
+        DWORD gle = ::GetLastError();
+        if (gle != ERROR_COMMITMENT_LIMIT) {
+          return false;
+        }
+
+        // Try again with a smaller allocation size.
+        currentSize /= 2;
+        continue;
+      }
+
+      aSize -= currentSize;
+
+      // VirtualAlloc consumes the commit space, but we need to *touch* memory
+      // to consume physical memory
+      TouchMemory(page.get(), currentSize);
+      Unused << aOutput.emplaceBack(std::move(page));
+    }
+    return true;
+  }
+
+  DWORD mThreadId;
+  nsAutoHandle mThread;
+  nsAutoHandle mMessageQueueReady;
+  Atomic<bool> mTaskStatus;
+
+  enum class TaskType : UINT {
+    Alloc = WM_USER,  // WPARAM = Allocation size
+    Free,
+
+    Last,
+  };
+
+  DWORD ThreadProc() {
+    Vector<PageT> stock;
+    MSG msg;
+
+    // Force the system to create a message queue
+    ::PeekMessage(&msg, nullptr, WM_USER, WM_USER, PM_NOREMOVE);
+
+    // Ready to get a message.  Unblock the main thread.
+    ::SetEvent(mMessageQueueReady.get());
+
+    for (;;) {
+      BOOL result = ::GetMessage(&msg, reinterpret_cast<HWND>(-1), WM_QUIT,
+                                 static_cast<UINT>(TaskType::Last));
+      if (result == -1) {
+        return ::GetLastError();
+      }
+      if (!result) {
+        // Got WM_QUIT
+        break;
+      }
+
+      switch (static_cast<TaskType>(msg.message)) {
+        case TaskType::Alloc:
+          mTaskStatus = AddWorkingSet(msg.wParam, stock);
+          break;
+        case TaskType::Free:
+          stock = Vector<PageT>();
+          mTaskStatus = true;
+          break;
+        default:
+          MOZ_ASSERT_UNREACHABLE("Unexpected message in the queue");
+          break;
+      }
+    }
+
+    return static_cast<DWORD>(msg.wParam);
+  }
+
+  bool PostTask(TaskType aTask, WPARAM aW = 0, LPARAM aL = 0) const {
+    return !!::PostThreadMessageW(mThreadId, static_cast<UINT>(aTask), aW, aL);
+  }
+
+ public:
+  MemoryEater()
+      : mThread(::CreateThread(nullptr, 0, ThreadStart, this, 0, &mThreadId)),
+        mMessageQueueReady(::CreateEventW(nullptr, /*bManualReset*/ TRUE,
+                                          /*bInitialState*/ FALSE, nullptr)) {
+    ::WaitForSingleObject(mMessageQueueReady.get(), INFINITE);
+  }
+
+  ~MemoryEater() {
+    ::PostThreadMessageW(mThreadId, WM_QUIT, 0, 0);
+    if (::WaitForSingleObject(mThread.get(), 30000) != WAIT_OBJECT_0) {
+      ::TerminateThread(mThread.get(), 0);
+    }
+  }
+
+  bool GetTaskStatus() const { return mTaskStatus; }
+  void RequestAlloc(size_t aSize) { PostTask(TaskType::Alloc, aSize); }
+  void RequestFree() { PostTask(TaskType::Free); }
+};
+
+class MockTabUnloader final : public nsITabUnloader {
+  ~MockTabUnloader() = default;
+
+  uint32_t mCounter;
+
+ public:
+  MockTabUnloader() : mCounter(0) {}
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+
+  void ResetCounter() { mCounter = 0; }
+  uint32_t GetCounter() const { return mCounter; }
+
+  NS_IMETHOD UnloadTabAsync() override {
+    ++mCounter;
+    // Issue a memory-pressure to verify OnHighMemory issues
+    // a memory-pressure-stop event.
+    NS_NotifyOfEventualMemoryPressure(MemoryPressureState::LowMemory);
+    return NS_OK;
+  }
+};
+
+NS_IMPL_ISUPPORTS(MockTabUnloader, nsITabUnloader)
+
+}  // namespace
+
+class AvailableMemoryWatcherFixture : public TelemetryTestFixture {
+  static const char kPrefLowCommitSpaceThreshold[];
+
+  RefPtr<nsAvailableMemoryWatcherBase> mWatcher;
+  nsCOMPtr<nsIObserverService> mObserverSvc;
+
+ protected:
+  static bool IsPageFileExpandable() {
+    const auto kMemMgmtKey =
+        u"SYSTEM\\CurrentControlSet\\Control\\"
+        u"Session Manager\\Memory Management"_ns;
+
+    nsresult rv;
+    nsCOMPtr<nsIWindowsRegKey> regKey =
+        do_CreateInstance("@mozilla.org/windows-registry-key;1", &rv);
+    if (NS_FAILED(rv)) {
+      return false;
+    }
+
+    rv = regKey->Open(nsIWindowsRegKey::ROOT_KEY_LOCAL_MACHINE, kMemMgmtKey,
+                      nsIWindowsRegKey::ACCESS_READ);
+    if (NS_FAILED(rv)) {
+      return false;
+    }
+
+    nsAutoString pagingFiles;
+    rv = regKey->ReadStringValue(u"PagingFiles"_ns, pagingFiles);
+    if (NS_FAILED(rv)) {
+      return false;
+    }
+
+    // The value data is REG_MULTI_SZ and each element is "<path> <min> <max>".
+    // If the page file size is automatically managed for all drives, the <path>
+    // is set to "?:\pagefile.sys".
+    // If the page file size is configured per drive, for a drive whose page
+    // file is set to "system managed size", both <min> and <max> are set to 0.
+    return !pagingFiles.IsEmpty() &&
+           (pagingFiles[0] == u'?' || FindInReadable(u" 0 0"_ns, pagingFiles));
+  }
+
+  static size_t GetAllocationSizeToTriggerMemoryNotification() {
+    // The percentage of the used physical memory to the total physical memory
+    // size which is big enough to trigger a memory resource notification.
+    constexpr uint32_t kThresholdPercentage = 98;
+    // If the page file is not expandable, leave a little commit space.
+    const uint32_t kMinimumSafeCommitSpaceMb =
+        IsPageFileExpandable() ? 0 : 1024;
+
+    MEMORYSTATUSEX statex = {sizeof(statex)};
+    EXPECT_TRUE(::GlobalMemoryStatusEx(&statex));
+
+    // How much memory needs to be used to trigger the notification
+    const size_t targetUsedTotalMb =
+        (statex.ullTotalPhys / kBytesInMB) * kThresholdPercentage / 100;
+
+    // How much memory is currently consumed
+    const size_t currentConsumedMb =
+        (statex.ullTotalPhys - statex.ullAvailPhys) / kBytesInMB;
+
+    if (currentConsumedMb >= targetUsedTotalMb) {
+      fprintf(stderr, "The available physical memory is already low.\n");
+      return 0;
+    }
+
+    // How much memory we need to allocate to trigger the notification
+    const uint32_t allocMb = targetUsedTotalMb - currentConsumedMb;
+
+    // If we allocate the target amount, how much commit space will be
+    // left available.
+    const uint32_t estimtedAvailCommitSpace = std::max(
+        0,
+        static_cast<int32_t>((statex.ullAvailPageFile / kBytesInMB) - allocMb));
+
+    // If the available commit space will be too low, we should not continue
+    if (estimtedAvailCommitSpace < kMinimumSafeCommitSpaceMb) {
+      fprintf(stderr, "The available commit space will be short - %d\n",
+              estimtedAvailCommitSpace);
+      return 0;
+    }
+
+    fprintf(stderr,
+            "Total physical memory  = %ul\n"
+            "Available commit space = %ul\n"
+            "Amount to allocate     = %ul\n"
+            "Future available commit space after allocation = %d\n",
+            static_cast<uint32_t>(statex.ullTotalPhys / kBytesInMB),
+            static_cast<uint32_t>(statex.ullAvailPageFile / kBytesInMB),
+            allocMb, estimtedAvailCommitSpace);
+    return allocMb * kBytesInMB;
+  }
+
+  static void SetThresholdAsPercentageOfCommitSpace(uint32_t aPercentage) {
+    aPercentage = std::min(100u, aPercentage);
+
+    MEMORYSTATUSEX statex = {sizeof(statex)};
+    EXPECT_TRUE(::GlobalMemoryStatusEx(&statex));
+
+    const uint32_t newVal = static_cast<uint32_t>(
+        (statex.ullAvailPageFile / kBytesInMB) * aPercentage / 100);
+    fprintf(stderr, "Setting %s to %u\n", kPrefLowCommitSpaceThreshold, newVal);
+
+    Preferences::SetUint(kPrefLowCommitSpaceThreshold, newVal);
+  }
+
+  static constexpr uint32_t kStateChangeTimeoutMs = 20000;
+  static constexpr uint32_t kNotificationTimeoutMs = 20000;
+
+  RefPtr<Spinner> mHighMemoryObserver;
+  RefPtr<MockTabUnloader> mTabUnloader;
+  MemoryEater mMemEater;
+  nsAutoHandle mLowMemoryHandle;
+
+  void SetUp() override {
+    TelemetryTestFixture::SetUp();
+
+    mObserverSvc = do_GetService(NS_OBSERVERSERVICE_CONTRACTID);
+    ASSERT_TRUE(mObserverSvc);
+
+    mHighMemoryObserver =
+        new Spinner(mObserverSvc, "memory-pressure-stop", nullptr);
+    mTabUnloader = new MockTabUnloader;
+
+    mWatcher = nsAvailableMemoryWatcherBase::GetSingleton();
+    mWatcher->RegisterTabUnloader(mTabUnloader);
+
+    mLowMemoryHandle.own(
+        ::CreateMemoryResourceNotification(LowMemoryResourceNotification));
+    ASSERT_TRUE(mLowMemoryHandle);
+
+    // We set the threshold to 50% of the current available commit space.
+    // This means we declare low-memory when the available commit space
+    // gets lower than this threshold, otherwise we declare high-memory.
+    SetThresholdAsPercentageOfCommitSpace(50);
+  }
+
+  void TearDown() override {
+    StopUserInteraction();
+    Preferences::ClearUser(kPrefLowCommitSpaceThreshold);
+  }
+
+  bool WaitForMemoryResourceNotification() {
+    uint64_t t0 = ::GetTickCount64();
+    if (::WaitForSingleObject(mLowMemoryHandle, kNotificationTimeoutMs) !=
+        WAIT_OBJECT_0) {
+      fprintf(stderr, "The memory notification was not triggered.\n");
+      return false;
+    }
+    fprintf(stderr, "Notified in %llu msec\n", ::GetTickCount64() - t0);
+    return true;
+  }
+
+  void StartUserInteraction() {
+    mObserverSvc->NotifyObservers(nullptr, "user-interaction-active", nullptr);
+  }
+
+  void StopUserInteraction() {
+    mObserverSvc->NotifyObservers(nullptr, "user-interaction-inactive",
+                                  nullptr);
+  }
+};
+
+const char AvailableMemoryWatcherFixture::kPrefLowCommitSpaceThreshold[] =
+    "browser.low_commit_space_threshold_mb";
+
+class MemoryWatcherTelemetryEvent {
+  static nsLiteralString sEventCategory;
+  static nsLiteralString sEventMethod;
+  static nsLiteralString sEventObject;
+
+  uint32_t mLastCountOfEvents;
+
+ public:
+  explicit MemoryWatcherTelemetryEvent(JSContext* aCx) : mLastCountOfEvents(0) {
+    JS::RootedValue snapshot(aCx);
+    TelemetryTestHelpers::GetEventSnapshot(aCx, &snapshot);
+    nsTArray<nsString> eventValues = TelemetryTestHelpers::EventValuesToArray(
+        aCx, snapshot, sEventCategory, sEventMethod, sEventObject);
+    mLastCountOfEvents = eventValues.Length();
+  }
+
+  void ValidateLastEvent(JSContext* aCx) {
+    JS::RootedValue snapshot(aCx);
+    TelemetryTestHelpers::GetEventSnapshot(aCx, &snapshot);
+    nsTArray<nsString> eventValues = TelemetryTestHelpers::EventValuesToArray(
+        aCx, snapshot, sEventCategory, sEventMethod, sEventObject);
+
+    // A new event was generated.
+    EXPECT_EQ(eventValues.Length(), mLastCountOfEvents + 1);
+    if (eventValues.IsEmpty()) {
+      return;
+    }
+
+    // Update mLastCountOfEvents for a subsequent call to ValidateLastEvent
+    ++mLastCountOfEvents;
+
+    nsTArray<nsString> tokens;
+    for (const nsAString& token : eventValues.LastElement().Split(',')) {
+      tokens.AppendElement(token);
+    }
+    EXPECT_EQ(tokens.Length(), 3U);
+    if (tokens.Length() != 3U) {
+      const wchar_t* valueStr = eventValues.LastElement().get();
+      fprintf(stderr, "Unexpected event value: %S\n", valueStr);
+      return;
+    }
+
+    // Since this test does not involve TabUnloader, the first two numbers
+    // are always expected to be zero.
+    EXPECT_STREQ(tokens[0].get(), L"0");
+    EXPECT_STREQ(tokens[1].get(), L"0");
+
+    // The third token should be a valid floating number.
+    nsresult rv;
+    tokens[2].ToDouble(&rv);
+    EXPECT_NS_SUCCEEDED(rv);
+  }
+};
+
+nsLiteralString MemoryWatcherTelemetryEvent::sEventCategory =
+    u"memory_watcher"_ns;
+nsLiteralString MemoryWatcherTelemetryEvent::sEventMethod =
+    u"on_high_memory"_ns;
+nsLiteralString MemoryWatcherTelemetryEvent::sEventObject = u"stats"_ns;
+
+TEST_F(AvailableMemoryWatcherFixture, AlwaysActive) {
+  AutoJSContextWithGlobal cx(mCleanGlobal);
+  MemoryWatcherTelemetryEvent telemetryEvent(cx.GetJSContext());
+  StartUserInteraction();
+
+  const size_t allocSize = GetAllocationSizeToTriggerMemoryNotification();
+  if (!allocSize) {
+    // Not enough memory to safely create a low-memory situation.
+    // Aborting the test without failure.
+    return;
+  }
+
+  mTabUnloader->ResetCounter();
+  mMemEater.RequestAlloc(allocSize);
+  if (!WaitForMemoryResourceNotification()) {
+    // If the notification was not triggered, abort the test without failure
+    // because it's not a fault in nsAvailableMemoryWatcher.
+    return;
+  }
+
+  EXPECT_TRUE(WaitUntil([this]() { return mTabUnloader->GetCounter() >= 1; },
+                        kStateChangeTimeoutMs));
+
+  mHighMemoryObserver->StartListening();
+  mMemEater.RequestFree();
+  EXPECT_TRUE(mHighMemoryObserver->Wait(kStateChangeTimeoutMs));
+
+  telemetryEvent.ValidateLastEvent(cx.GetJSContext());
+}
+
+TEST_F(AvailableMemoryWatcherFixture, InactiveToActive) {
+  AutoJSContextWithGlobal cx(mCleanGlobal);
+  MemoryWatcherTelemetryEvent telemetryEvent(cx.GetJSContext());
+  const size_t allocSize = GetAllocationSizeToTriggerMemoryNotification();
+  if (!allocSize) {
+    // Not enough memory to safely create a low-memory situation.
+    // Aborting the test without failure.
+    return;
+  }
+
+  mTabUnloader->ResetCounter();
+  mMemEater.RequestAlloc(allocSize);
+  if (!WaitForMemoryResourceNotification()) {
+    // If the notification was not triggered, abort the test without failure
+    // because it's not a fault in nsAvailableMemoryWatcher.
+    return;
+  }
+
+  mHighMemoryObserver->StartListening();
+  EXPECT_TRUE(WaitUntil([this]() { return mTabUnloader->GetCounter() >= 1; },
+                        kStateChangeTimeoutMs));
+
+  mMemEater.RequestFree();
+
+  // OnHighMemory should not be triggered during no user interaction
+  // eve after all memory was freed.  Expecting false.
+  EXPECT_FALSE(mHighMemoryObserver->Wait(3000));
+
+  StartUserInteraction();
+
+  // After user is active, we expect true.
+  EXPECT_TRUE(mHighMemoryObserver->Wait(kStateChangeTimeoutMs));
+
+  telemetryEvent.ValidateLastEvent(cx.GetJSContext());
+}
+
+TEST_F(AvailableMemoryWatcherFixture, HighCommitSpace_AlwaysActive) {
+  // Setting a low threshold simulates a high commit space.
+  SetThresholdAsPercentageOfCommitSpace(1);
+  StartUserInteraction();
+
+  const size_t allocSize = GetAllocationSizeToTriggerMemoryNotification();
+  if (!allocSize) {
+    // Not enough memory to safely create a low-memory situation.
+    // Aborting the test without failure.
+    return;
+  }
+
+  mTabUnloader->ResetCounter();
+  mMemEater.RequestAlloc(allocSize);
+  if (!WaitForMemoryResourceNotification()) {
+    // If the notification was not triggered, abort the test without failure
+    // because it's not a fault in nsAvailableMemoryWatcher.
+    return;
+  }
+
+  // Tab unload will not be triggered because the commit space is not low.
+  EXPECT_FALSE(WaitUntil([this]() { return mTabUnloader->GetCounter() >= 1; },
+                         kStateChangeTimeoutMs / 2));
+
+  mMemEater.RequestFree();
+  ::Sleep(kStateChangeTimeoutMs / 2);
+
+  // Set a high threshold and make sure the watcher will trigger the tab
+  // unloader next time.
+  SetThresholdAsPercentageOfCommitSpace(50);
+
+  mMemEater.RequestAlloc(allocSize);
+  if (!WaitForMemoryResourceNotification()) {
+    return;
+  }
+
+  EXPECT_TRUE(WaitUntil([this]() { return mTabUnloader->GetCounter() >= 1; },
+                        kStateChangeTimeoutMs));
+
+  mHighMemoryObserver->StartListening();
+  mMemEater.RequestFree();
+  EXPECT_TRUE(mHighMemoryObserver->Wait(kStateChangeTimeoutMs));
+}
+
+TEST_F(AvailableMemoryWatcherFixture, HighCommitSpace_InactiveToActive) {
+  // Setting a low threshold simulates a high commit space.
+  SetThresholdAsPercentageOfCommitSpace(1);
+
+  const size_t allocSize = GetAllocationSizeToTriggerMemoryNotification();
+  if (!allocSize) {
+    // Not enough memory to safely create a low-memory situation.
+    // Aborting the test without failure.
+    return;
+  }
+
+  mTabUnloader->ResetCounter();
+  mMemEater.RequestAlloc(allocSize);
+  if (!WaitForMemoryResourceNotification()) {
+    // If the notification was not triggered, abort the test without failure
+    // because it's not a fault in nsAvailableMemoryWatcher.
+    return;
+  }
+
+  // Tab unload will not be triggered because the commit space is not low.
+  EXPECT_FALSE(WaitUntil([this]() { return mTabUnloader->GetCounter() >= 1; },
+                         kStateChangeTimeoutMs / 2));
+
+  mMemEater.RequestFree();
+  ::Sleep(kStateChangeTimeoutMs / 2);
+
+  // Set a high threshold and make sure the watcher will trigger the tab
+  // unloader next time.
+  SetThresholdAsPercentageOfCommitSpace(50);
+
+  // When the user becomes active, the watcher will resume the timer.
+  StartUserInteraction();
+
+  mMemEater.RequestAlloc(allocSize);
+  if (!WaitForMemoryResourceNotification()) {
+    return;
+  }
+
+  EXPECT_TRUE(WaitUntil([this]() { return mTabUnloader->GetCounter() >= 1; },
+                        kStateChangeTimeoutMs));
+
+  mHighMemoryObserver->StartListening();
+  mMemEater.RequestFree();
+  EXPECT_TRUE(mHighMemoryObserver->Wait(kStateChangeTimeoutMs));
+}