xpcom/threads/nsMemoryPressure.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef nsMemoryPressure_h__
   8 #define nsMemoryPressure_h__
   9
  10 #include "nscore.h"
  11
  12 /*
  13  * These pre-defined strings are the topic to pass to the observer
  14  * service to declare the memory-pressure or lift the memory-pressure.
  15  *
  16  * 1. Notify kTopicMemoryPressure with kSubTopicLowMemoryNew
  17  * New memory pressure deteced
  18  * On a new memory pressure, we stop everything to start cleaning
  19  * aggresively the memory used, in order to free as much memory as
  20  * possible.
  21  *
  22  * 2. Notify kTopicMemoryPressure with kSubTopicLowMemoryOngoing
  23  * Repeated memory pressure.
  24  * A repeated memory pressure implies to clean softly recent allocations.
  25  * It is supposed to happen after a new memory pressure which already
  26  * cleaned aggressivley.  So there is no need to damage the reactivity of
  27  * Gecko by stopping the world again.
  28  *
  29  * In case of conflict with an new memory pressue, the new memory pressure
  30  * takes precedence over an ongoing memory pressure.  The reason being
  31  * that if no events are processed between 2 notifications (new followed
  32  * by ongoing, or ongoing followed by a new) we want to be as aggresive as
  33  * possible on the clean-up of the memory.  After all, we are trying to
  34  * keep Gecko alive as long as possible.
  35  *
  36  * 3. Notify kTopicMemoryPressureStop with nullptr
  37  * Memory pressure stopped.
  38  * We're no longer under acute memory pressure, so we might want to have a
  39  * chance of (cautiously) re-enabling some things we previously turned off.
  40  * As above, an already enqueued new memory pressure event takes precedence.
  41  * The priority ordering between concurrent attempts to queue both stopped
  42  * and ongoing memory pressure is currently not defined.
  43  */
  44 extern const char* const kTopicMemoryPressure;
  45 extern const char* const kTopicMemoryPressureStop;
  46 extern const char16_t* const kSubTopicLowMemoryNew;
  47 extern const char16_t* const kSubTopicLowMemoryOngoing;
  48
  49 enum class MemoryPressureState : uint32_t {
  50   None,  // For internal use.  Don't use this value.
  51   LowMemory,
  52   NoPressure,
  53 };
  54
  55 /**
  56  * This function causes the main thread to fire a memory pressure event
  57  * before processing the next event, but if there are no events pending in
  58  * the main thread's event queue, the memory pressure event would not be
  59  * dispatched until one is enqueued. It is infallible and does not allocate
  60  * any memory.
  61  *
  62  * You may call this function from any thread.
  63  */
  64 void NS_NotifyOfEventualMemoryPressure(MemoryPressureState aState);
  65
  66 /**
  67  * This function causes the main thread to fire a memory pressure event
  68  * before processing the next event. We wake up the main thread by adding a
  69  * dummy event to its event loop, so, unlike with
  70  * NS_NotifyOfEventualMemoryPressure, this memory-pressure event is always
  71  * fired relatively quickly, even if the event loop is otherwise empty.
  72  *
  73  * You may call this function from any thread.
  74  */
  75 nsresult NS_NotifyOfMemoryPressure(MemoryPressureState aState);
  76
  77 #endif  // nsMemoryPressure_h__