1 files changed, 634 insertions, 0 deletions
diff --git a/xpcom/base/AvailableMemoryWatcherMac.cpp b/xpcom/base/AvailableMemoryWatcherMac.cpp
new file mode 100644
index 0000000000..d6f2d16b30
--- /dev/null
+++ b/xpcom/base/AvailableMemoryWatcherMac.cpp
@@ -0,0 +1,634 @@
+/* -*- 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 <sys/sysctl.h>
+#include <sys/types.h>
+#include <time.h>
+
+#include "AvailableMemoryWatcher.h"
+#include "Logging.h"
+#include "mozilla/Preferences.h"
+#include "nsICrashReporter.h"
+#include "nsISupports.h"
+#include "nsITimer.h"
+#include "nsMemoryPressure.h"
+#include "nsPrintfCString.h"
+
+#define MP_LOG(...) MOZ_LOG(gMPLog, mozilla::LogLevel::Debug, (__VA_ARGS__))
+static mozilla::LazyLogModule gMPLog("MemoryPressure");
+
+namespace mozilla {
+
+/*
+ * The Mac AvailableMemoryWatcher works as follows. When the OS memory pressure
+ * level changes on macOS, nsAvailableMemoryWatcher::OnMemoryPressureChanged()
+ * is called with the new memory pressure level. The level is represented in
+ * Gecko by a MacMemoryPressureLevel instance and represents the states of
+ * normal, warning, or critical which correspond to the native levels. When the
+ * browser launches, the initial level is determined using a sysctl. Which
+ * actions are taken in the browser in response to memory pressure, and the
+ * level (warning or critical) which trigger the reponse is configurable with
+ * prefs to make it easier to perform experiments to study how the response
+ * affects the user experience.
+ *
+ * By default, the browser responds by attempting to reduce memory use when the
+ * OS transitions to the critical level and while it stays in the critical
+ * level. i.e., "critical" OS memory pressure is the default threshold for the
+ * low memory response. Setting pref "browser.lowMemoryResponseOnWarn" to true
+ * changes the memory response to occur at the "warning" level which is less
+ * severe than "critical". When entering the critical level, we begin polling
+ * the memory pressure level every 'n' milliseconds (specified via the pref
+ * "browser.lowMemoryPollingIntervalMS"). Each time the poller wakes up and
+ * finds the OS still under memory pressure, the low memory response is
+ * executed.
+ *
+ * By default, the memory pressure response is, in order, to
+ *   1) call nsITabUnloader::UnloadTabAsync(),
+ *   2) if no tabs could be unloaded, issue a Gecko
+ *      MemoryPressureState::LowMemory notification.
+ * The response can be changed via the pref "browser.lowMemoryResponseMask" to
+ * limit the actions to only tab unloading or Gecko memory pressure
+ * notifications.
+ *
+ * Polling occurs on the main thread because, at each polling interval, we
+ * call into the tab unloader which requires being on the main thread.
+ * Polling only occurs while under OS memory pressure at the critical (by
+ * default) level.
+ */
+class nsAvailableMemoryWatcher final : public nsITimerCallback,
+                                       public nsINamed,
+                                       public nsAvailableMemoryWatcherBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIOBSERVER
+  NS_DECL_NSITIMERCALLBACK
+  NS_DECL_NSINAMED
+
+  nsAvailableMemoryWatcher();
+  nsresult Init() override;
+
+  void OnMemoryPressureChanged(MacMemoryPressureLevel aLevel) override;
+  void AddChildAnnotations(
+      const UniquePtr<ipc::CrashReporterHost>& aCrashReporter) override;
+
+ private:
+  ~nsAvailableMemoryWatcher(){};
+
+  void OnMemoryPressureChangedInternal(MacMemoryPressureLevel aNewLevel,
+                                       bool aIsInitialLevel);
+
+  // Override OnUnloadAttemptCompleted() so that we can control whether
+  // or not a Gecko memory-pressure event is sent after a tab unload attempt.
+  // This method is called externally by the tab unloader after a tab unload
+  // attempt. It is used internally when tab unloading is disabled in
+  // mResponseMask.
+  nsresult OnUnloadAttemptCompleted(nsresult aResult) override;
+
+  void OnShutdown();
+  void OnPrefChange();
+
+  void InitParentAnnotations();
+  void UpdateParentAnnotations();
+
+  void AddParentAnnotation(CrashReporter::Annotation aAnnotation,
+                           nsAutoCString aString) {
+    CrashReporter::AnnotateCrashReport(aAnnotation, aString);
+  }
+  void AddParentAnnotation(CrashReporter::Annotation aAnnotation,
+                           uint32_t aData) {
+    CrashReporter::AnnotateCrashReport(aAnnotation, aData);
+  }
+
+  void LowMemoryResponse();
+  void StartPolling();
+  void StopPolling();
+  void RestartPolling();
+  inline bool IsPolling() { return mTimer; }
+
+  void ReadSysctls();
+
+  // This enum represents the allowed values for the pref that controls
+  // the low memory response - "browser.lowMemoryResponseMask". Specifically,
+  // whether or not we unload tabs and/or issue the Gecko "memory-pressure"
+  // internal notification. For tab unloading, the pref
+  // "browser.tabs.unloadOnLowMemory" must also be set.
+  enum ResponseMask {
+    eNone = 0x0,
+    eTabUnload = 0x1,
+    eInternalMemoryPressure = 0x2,
+    eAll = 0x3,
+  };
+  static constexpr char kResponseMask[] = "browser.lowMemoryResponseMask";
+  static const uint32_t kResponseMaskDefault;
+  static const uint32_t kResponseMaskMax;
+
+  // Pref for controlling how often we wake up during an OS memory pressure
+  // time period. At each wakeup, we unload tabs and issue the Gecko
+  // "memory-pressure" internal notification. When not under OS memory pressure,
+  // polling is disabled.
+  static constexpr char kPollingIntervalMS[] =
+      "browser.lowMemoryPollingIntervalMS";
+  static const uint32_t kPollingIntervalMaxMS;
+  static const uint32_t kPollingIntervalMinMS;
+  static const uint32_t kPollingIntervalDefaultMS;
+
+  static constexpr char kResponseOnWarn[] = "browser.lowMemoryResponseOnWarn";
+  static const bool kResponseLevelOnWarnDefault = false;
+
+  // Init has been called.
+  bool mInitialized;
+
+  // The memory pressure reported to the application by macOS.
+  MacMemoryPressureLevel mLevel;
+
+  // The OS memory pressure level that triggers the response.
+  MacMemoryPressureLevel mResponseLevel;
+
+  // The value of the kern.memorystatus_vm_pressure_level sysctl. The OS
+  // notifies the application when the memory pressure level changes,
+  // but the sysctl value can be read at any time. Unofficially, the sysctl
+  // value corresponds to the OS memory pressure level with 4=>critical,
+  // 2=>warning, and 1=>normal (values from kernel event.h file).
+  uint32_t mLevelSysctl;
+  static const int kSysctlLevelNormal = 0x1;
+  static const int kSysctlLevelWarning = 0x2;
+  static const int kSysctlLevelCritical = 0x4;
+
+  // The value of the kern.memorystatus_level sysctl. Unofficially,
+  // this is the percentage of available memory. (Also readable
+  // via the undocumented memorystatus_get_level syscall.)
+  int mAvailMemSysctl;
+
+  // The string representation of `mLevel`. i.e., normal, warning, or critical.
+  // Set to "unset" until a memory pressure change is reported to the process
+  // by the OS.
+  nsAutoCString mLevelStr;
+
+  // Timestamps for memory pressure level changes. Specifically, the Unix
+  // time in string form. Saved as Unix time to allow comparisons with
+  // the crash time.
+  nsAutoCString mNormalTimeStr;
+  nsAutoCString mWarningTimeStr;
+  nsAutoCString mCriticalTimeStr;
+
+  nsCOMPtr<nsITimer> mTimer;  // non-null indicates the timer is active
+
+  // Saved pref values.
+  uint32_t mPollingInterval;
+  uint32_t mResponseMask;
+};
+
+const uint32_t nsAvailableMemoryWatcher::kResponseMaskDefault =
+    ResponseMask::eAll;
+const uint32_t nsAvailableMemoryWatcher::kResponseMaskMax = ResponseMask::eAll;
+
+// 10 seconds
+const uint32_t nsAvailableMemoryWatcher::kPollingIntervalDefaultMS = 10'000;
+// 10 minutes
+const uint32_t nsAvailableMemoryWatcher::kPollingIntervalMaxMS = 600'000;
+// 100 milliseconds
+const uint32_t nsAvailableMemoryWatcher::kPollingIntervalMinMS = 100;
+
+NS_IMPL_ISUPPORTS_INHERITED(nsAvailableMemoryWatcher,
+                            nsAvailableMemoryWatcherBase, nsIObserver,
+                            nsITimerCallback, nsINamed)
+
+nsAvailableMemoryWatcher::nsAvailableMemoryWatcher()
+    : mInitialized(false),
+      mLevel(MacMemoryPressureLevel::Value::eUnset),
+      mResponseLevel(MacMemoryPressureLevel::Value::eCritical),
+      mLevelSysctl(0xFFFFFFFF),
+      mAvailMemSysctl(-1),
+      mLevelStr("Unset"),
+      mNormalTimeStr("Unset"),
+      mWarningTimeStr("Unset"),
+      mCriticalTimeStr("Unset"),
+      mPollingInterval(0),
+      mResponseMask(ResponseMask::eAll) {}
+
+nsresult nsAvailableMemoryWatcher::Init() {
+  nsresult rv = nsAvailableMemoryWatcherBase::Init();
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // Users of nsAvailableMemoryWatcher should use
+  // nsAvailableMemoryWatcherBase::GetSingleton() and not call Init directly.
+  MOZ_ASSERT(!mInitialized);
+  if (mInitialized) {
+    return NS_ERROR_ALREADY_INITIALIZED;
+  }
+
+  // Read polling frequency pref
+  mPollingInterval =
+      Preferences::GetUint(kPollingIntervalMS, kPollingIntervalDefaultMS);
+  mPollingInterval = std::clamp(mPollingInterval, kPollingIntervalMinMS,
+                                kPollingIntervalMaxMS);
+
+  // Read response bitmask pref which (along with the main tab unloading
+  // preference) controls whether or not tab unloading and Gecko (internal)
+  // memory pressure notifications will be sent. The main tab unloading
+  // preference must also be enabled for tab unloading to occur.
+  mResponseMask = Preferences::GetUint(kResponseMask, kResponseMaskDefault);
+  if (mResponseMask > kResponseMaskMax) {
+    mResponseMask = kResponseMaskMax;
+  }
+
+  // Read response level pref
+  if (Preferences::GetBool(kResponseOnWarn, kResponseLevelOnWarnDefault)) {
+    mResponseLevel = MacMemoryPressureLevel::Value::eWarning;
+  } else {
+    mResponseLevel = MacMemoryPressureLevel::Value::eCritical;
+  }
+
+  ReadSysctls();
+  MP_LOG("Initial memory pressure sysctl: %d", mLevelSysctl);
+  MP_LOG("Initial available memory sysctl: %d", mAvailMemSysctl);
+
+  // Set the initial state of all annotations for parent crash reports.
+  // Content process crash reports are set when a crash occurs and
+  // AddChildAnnotations() is called.
+  CrashReporter::AnnotateCrashReport(
+      CrashReporter::Annotation::MacMemoryPressure, mLevelStr);
+  CrashReporter::AnnotateCrashReport(
+      CrashReporter::Annotation::MacMemoryPressureNormalTime, mNormalTimeStr);
+  CrashReporter::AnnotateCrashReport(
+      CrashReporter::Annotation::MacMemoryPressureWarningTime, mWarningTimeStr);
+  CrashReporter::AnnotateCrashReport(
+      CrashReporter::Annotation::MacMemoryPressureCriticalTime,
+      mCriticalTimeStr);
+  CrashReporter::AnnotateCrashReport(
+      CrashReporter::Annotation::MacMemoryPressureSysctl, mLevelSysctl);
+  CrashReporter::AnnotateCrashReport(
+      CrashReporter::Annotation::MacAvailableMemorySysctl, mAvailMemSysctl);
+
+  // To support running experiments, handle pref
+  // changes without requiring a browser restart.
+  rv = Preferences::AddStrongObserver(this, kResponseMask);
+  if (NS_FAILED(rv)) {
+    NS_WARNING(
+        nsPrintfCString("Failed to add %s observer", kResponseMask).get());
+  }
+  rv = Preferences::AddStrongObserver(this, kPollingIntervalMS);
+  if (NS_FAILED(rv)) {
+    NS_WARNING(
+        nsPrintfCString("Failed to add %s observer", kPollingIntervalMS).get());
+  }
+  rv = Preferences::AddStrongObserver(this, kResponseOnWarn);
+  if (NS_FAILED(rv)) {
+    NS_WARNING(
+        nsPrintfCString("Failed to add %s observer", kResponseOnWarn).get());
+  }
+
+  // Use the memory pressure sysctl to initialize our memory pressure state.
+  MacMemoryPressureLevel initialLevel;
+  switch (mLevelSysctl) {
+    case kSysctlLevelNormal:
+      initialLevel = MacMemoryPressureLevel::Value::eNormal;
+      break;
+    case kSysctlLevelWarning:
+      initialLevel = MacMemoryPressureLevel::Value::eWarning;
+      break;
+    case kSysctlLevelCritical:
+      initialLevel = MacMemoryPressureLevel::Value::eCritical;
+      break;
+    default:
+      initialLevel = MacMemoryPressureLevel::Value::eUnexpected;
+  }
+
+  OnMemoryPressureChangedInternal(initialLevel, /* aIsInitialLevel */ true);
+  mInitialized = true;
+  return NS_OK;
+}
+
+already_AddRefed<nsAvailableMemoryWatcherBase> CreateAvailableMemoryWatcher() {
+  // Users of nsAvailableMemoryWatcher should use
+  // nsAvailableMemoryWatcherBase::GetSingleton().
+  RefPtr watcher(new nsAvailableMemoryWatcher());
+  watcher->Init();
+  return watcher.forget();
+}
+
+// Update the memory pressure level, level change timestamps, and sysctl
+// level crash report annotations.
+void nsAvailableMemoryWatcher::UpdateParentAnnotations() {
+  // Generate a string representation of the current Unix time.
+  time_t timeChanged = time(NULL);
+  nsAutoCString timeChangedString;
+  timeChangedString =
+      nsPrintfCString("%" PRIu64, static_cast<uint64_t>(timeChanged));
+
+  nsAutoCString pressureLevelString;
+  Maybe<CrashReporter::Annotation> pressureLevelKey;
+
+  switch (mLevel.GetValue()) {
+    case MacMemoryPressureLevel::Value::eNormal:
+      mNormalTimeStr = timeChangedString;
+      pressureLevelString = "Normal";
+      pressureLevelKey.emplace(
+          CrashReporter::Annotation::MacMemoryPressureNormalTime);
+      break;
+    case MacMemoryPressureLevel::Value::eWarning:
+      mWarningTimeStr = timeChangedString;
+      pressureLevelString = "Warning";
+      pressureLevelKey.emplace(
+          CrashReporter::Annotation::MacMemoryPressureWarningTime);
+      break;
+    case MacMemoryPressureLevel::Value::eCritical:
+      mCriticalTimeStr = timeChangedString;
+      pressureLevelString = "Critical";
+      pressureLevelKey.emplace(
+          CrashReporter::Annotation::MacMemoryPressureCriticalTime);
+      break;
+    default:
+      pressureLevelString = "Unexpected";
+      break;
+  }
+
+  // Save the current memory pressure level.
+  AddParentAnnotation(CrashReporter::Annotation::MacMemoryPressure,
+                      pressureLevelString);
+
+  // Save the time we transitioned to the current memory pressure level.
+  if (pressureLevelKey.isSome()) {
+    AddParentAnnotation(pressureLevelKey.value(), timeChangedString);
+  }
+
+  AddParentAnnotation(CrashReporter::Annotation::MacMemoryPressureSysctl,
+                      mLevelSysctl);
+  AddParentAnnotation(CrashReporter::Annotation::MacAvailableMemorySysctl,
+                      mAvailMemSysctl);
+}
+
+void nsAvailableMemoryWatcher::ReadSysctls() {
+  // Pressure level
+  uint32_t level;
+  size_t size = sizeof(level);
+  if (sysctlbyname("kern.memorystatus_vm_pressure_level", &level, &size, NULL,
+                   0) == -1) {
+    MP_LOG("Failure reading memory pressure sysctl");
+  }
+  mLevelSysctl = level;
+
+  // Available memory percent
+  int availPercent;
+  size = sizeof(availPercent);
+  if (sysctlbyname("kern.memorystatus_level", &availPercent, &size, NULL, 0) ==
+      -1) {
+    MP_LOG("Failure reading available memory level");
+  }
+  mAvailMemSysctl = availPercent;
+}
+
+/* virtual */
+void nsAvailableMemoryWatcher::OnMemoryPressureChanged(
+    MacMemoryPressureLevel aNewLevel) {
+  MOZ_ASSERT(mInitialized);
+  OnMemoryPressureChangedInternal(aNewLevel, /* aIsInitialLevel */ false);
+}
+
+void nsAvailableMemoryWatcher::OnMemoryPressureChangedInternal(
+    MacMemoryPressureLevel aNewLevel, bool aIsInitialLevel) {
+  MOZ_ASSERT(mInitialized || aIsInitialLevel);
+  MP_LOG("MemoryPressureChange: existing level: %s, new level: %s",
+         mLevel.ToString(), aNewLevel.ToString());
+
+  // If 'aNewLevel' is not one of normal, warning, or critical, ASSERT
+  // here so we can debug this scenario. For non-debug builds, ignore
+  // the unexpected value which will be logged in crash reports.
+  MOZ_ASSERT(aNewLevel.IsNormal() || aNewLevel.IsWarningOrAbove());
+
+  if (mLevel == aNewLevel) {
+    return;
+  }
+
+  // Start the memory pressure response if the new level is high enough
+  // and the existing level was not.
+  if ((mLevel < mResponseLevel) && (aNewLevel >= mResponseLevel)) {
+    UpdateLowMemoryTimeStamp();
+    LowMemoryResponse();
+    if (mResponseMask) {
+      StartPolling();
+    }
+  }
+
+  // End the memory pressure reponse if the new level is not high enough.
+  if ((mLevel >= mResponseLevel) && (aNewLevel < mResponseLevel)) {
+    {
+      MutexAutoLock lock(mMutex);
+      RecordTelemetryEventOnHighMemory(lock);
+    }
+    StopPolling();
+    MP_LOG("Issuing MemoryPressureState::NoPressure");
+    NS_NotifyOfMemoryPressure(MemoryPressureState::NoPressure);
+  }
+
+  mLevel = aNewLevel;
+
+  if (!aIsInitialLevel) {
+    // Sysctls are already read by ::Init().
+    ReadSysctls();
+    MP_LOG("level sysctl: %d, available memory: %d percent", mLevelSysctl,
+           mAvailMemSysctl);
+  }
+  UpdateParentAnnotations();
+}
+
+/* virtual */
+// Add all annotations to the provided crash reporter instance.
+void nsAvailableMemoryWatcher::AddChildAnnotations(
+    const UniquePtr<ipc::CrashReporterHost>& aCrashReporter) {
+  aCrashReporter->AddAnnotation(CrashReporter::Annotation::MacMemoryPressure,
+                                mLevelStr);
+  aCrashReporter->AddAnnotation(
+      CrashReporter::Annotation::MacMemoryPressureNormalTime, mNormalTimeStr);
+  aCrashReporter->AddAnnotation(
+      CrashReporter::Annotation::MacMemoryPressureWarningTime, mWarningTimeStr);
+  aCrashReporter->AddAnnotation(
+      CrashReporter::Annotation::MacMemoryPressureCriticalTime,
+      mCriticalTimeStr);
+  aCrashReporter->AddAnnotation(
+      CrashReporter::Annotation::MacMemoryPressureSysctl, mLevelSysctl);
+  aCrashReporter->AddAnnotation(
+      CrashReporter::Annotation::MacAvailableMemorySysctl, mAvailMemSysctl);
+}
+
+void nsAvailableMemoryWatcher::LowMemoryResponse() {
+  if (mResponseMask & ResponseMask::eTabUnload) {
+    MP_LOG("Attempting tab unload");
+    mTabUnloader->UnloadTabAsync();
+  } else {
+    // Re-use OnUnloadAttemptCompleted() to issue the internal
+    // memory pressure event.
+    OnUnloadAttemptCompleted(NS_ERROR_NOT_AVAILABLE);
+  }
+}
+
+NS_IMETHODIMP
+nsAvailableMemoryWatcher::Notify(nsITimer* aTimer) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mLevel >= mResponseLevel);
+  LowMemoryResponse();
+  return NS_OK;
+}
+
+// Override OnUnloadAttemptCompleted() so that we can issue Gecko memory
+// pressure notifications only if eInternalMemoryPressure is set in
+// mResponseMask. When called from the tab unloader, an |aResult| value of
+// NS_OK indicates the tab unloader successfully unloaded a tab.
+// NS_ERROR_NOT_AVAILABLE indicates the tab unloader did not unload any tabs.
+NS_IMETHODIMP
+nsAvailableMemoryWatcher::OnUnloadAttemptCompleted(nsresult aResult) {
+  // On MacOS we don't access these members offthread; however we do on other
+  // OSes and so they are guarded by the mutex.
+  MutexAutoLock lock(mMutex);
+  switch (aResult) {
+    // A tab was unloaded successfully.
+    case NS_OK:
+      MP_LOG("Tab unloaded");
+      ++mNumOfTabUnloading;
+      break;
+
+    // Either the tab unloader found no unloadable tabs OR we've been called
+    // locally to explicitly issue the internal memory pressure event because
+    // tab unloading is disabled in |mResponseMask|. In either case, attempt
+    // to reduce memory use using the internal memory pressure notification.
+    case NS_ERROR_NOT_AVAILABLE:
+      if (mResponseMask & ResponseMask::eInternalMemoryPressure) {
+        ++mNumOfMemoryPressure;
+        MP_LOG("Tab not unloaded");
+        MP_LOG("Issuing MemoryPressureState::LowMemory");
+        NS_NotifyOfEventualMemoryPressure(MemoryPressureState::LowMemory);
+      }
+      break;
+
+    // There was a pending task to unload a tab.
+    case NS_ERROR_ABORT:
+      break;
+
+    default:
+      MOZ_ASSERT_UNREACHABLE("Unexpected aResult");
+      break;
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsAvailableMemoryWatcher::Observe(nsISupports* aSubject, const char* aTopic,
+                                  const char16_t* aData) {
+  nsresult rv = nsAvailableMemoryWatcherBase::Observe(aSubject, aTopic, aData);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  if (strcmp(aTopic, "xpcom-shutdown") == 0) {
+    OnShutdown();
+  } else if (strcmp(aTopic, NS_PREFBRANCH_PREFCHANGE_TOPIC_ID) == 0) {
+    OnPrefChange();
+  }
+  return NS_OK;
+}
+
+void nsAvailableMemoryWatcher::OnShutdown() {
+  StopPolling();
+  Preferences::RemoveObserver(this, kResponseMask);
+  Preferences::RemoveObserver(this, kPollingIntervalMS);
+}
+
+void nsAvailableMemoryWatcher::OnPrefChange() {
+  MP_LOG("OnPrefChange()");
+  // Handle the polling interval changing.
+  uint32_t pollingInterval = Preferences::GetUint(kPollingIntervalMS);
+  if (pollingInterval != mPollingInterval) {
+    mPollingInterval = std::clamp(pollingInterval, kPollingIntervalMinMS,
+                                  kPollingIntervalMaxMS);
+    RestartPolling();
+  }
+
+  // Handle the response mask changing.
+  uint32_t responseMask = Preferences::GetUint(kResponseMask);
+  if (mResponseMask != responseMask) {
+    mResponseMask = std::min(responseMask, kResponseMaskMax);
+
+    // Do we need to turn on polling?
+    if (mResponseMask && (mLevel >= mResponseLevel) && !IsPolling()) {
+      StartPolling();
+    }
+
+    // Do we need to turn off polling?
+    if (!mResponseMask && IsPolling()) {
+      StopPolling();
+    }
+  }
+
+  // Handle the response level changing.
+  MacMemoryPressureLevel newResponseLevel;
+  if (Preferences::GetBool(kResponseOnWarn, kResponseLevelOnWarnDefault)) {
+    newResponseLevel = MacMemoryPressureLevel::Value::eWarning;
+  } else {
+    newResponseLevel = MacMemoryPressureLevel::Value::eCritical;
+  }
+  if (newResponseLevel == mResponseLevel) {
+    return;
+  }
+
+  // Do we need to turn on polling?
+  if (mResponseMask && (newResponseLevel <= mLevel)) {
+    UpdateLowMemoryTimeStamp();
+    LowMemoryResponse();
+    StartPolling();
+  }
+
+  // Do we need to turn off polling?
+  if (IsPolling() && (newResponseLevel > mLevel)) {
+    {
+      MutexAutoLock lock(mMutex);
+      RecordTelemetryEventOnHighMemory(lock);
+    }
+    StopPolling();
+    MP_LOG("Issuing MemoryPressureState::NoPressure");
+    NS_NotifyOfMemoryPressure(MemoryPressureState::NoPressure);
+  }
+  mResponseLevel = newResponseLevel;
+}
+
+void nsAvailableMemoryWatcher::StartPolling() {
+  MOZ_ASSERT(NS_IsMainThread());
+  if (!mTimer) {
+    MP_LOG("Starting poller");
+    mTimer = NS_NewTimer();
+    if (mTimer) {
+      mTimer->InitWithCallback(this, mPollingInterval,
+                               nsITimer::TYPE_REPEATING_SLACK);
+    }
+  }
+}
+
+void nsAvailableMemoryWatcher::StopPolling() {
+  MOZ_ASSERT(NS_IsMainThread());
+  if (mTimer) {
+    MP_LOG("Pausing poller");
+    mTimer->Cancel();
+    mTimer = nullptr;
+  }
+}
+
+void nsAvailableMemoryWatcher::RestartPolling() {
+  if (IsPolling()) {
+    StopPolling();
+    StartPolling();
+  } else {
+    MOZ_ASSERT(!mTimer);
+  }
+}
+
+NS_IMETHODIMP
+nsAvailableMemoryWatcher::GetName(nsACString& aName) {
+  aName.AssignLiteral("nsAvailableMemoryWatcher");
+  return NS_OK;
+}
+
+}  // namespace mozilla