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| -rw-r--r-- | xpcom/threads/ThrottledEventQueue.h | 118 |
1 files changed, 118 insertions, 0 deletions
diff --git a/xpcom/threads/ThrottledEventQueue.h b/xpcom/threads/ThrottledEventQueue.h new file mode 100644 index 0000000000..cf37a10a6d --- /dev/null +++ b/xpcom/threads/ThrottledEventQueue.h @@ -0,0 +1,118 @@ +/* -*- 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/. */ + +// nsIEventTarget wrapper for throttling event dispatch. + +#ifndef mozilla_ThrottledEventQueue_h +#define mozilla_ThrottledEventQueue_h + +#include "nsISerialEventTarget.h" + +#define NS_THROTTLEDEVENTQUEUE_IID \ + { \ + 0x8f3cf7dc, 0xfc14, 0x4ad5, { \ + 0x9f, 0xd5, 0xdb, 0x79, 0xbc, 0xe6, 0xd5, 0x08 \ + } \ + } + +namespace mozilla { + +// A ThrottledEventQueue is an event target that can be used to throttle +// events being dispatched to another base target. It maintains its +// own queue of events and only dispatches one at a time to the wrapped +// target. This can be used to avoid flooding the base target. +// +// Flooding is avoided via a very simple principle. Runnables dispatched +// to the ThrottledEventQueue are only dispatched to the base target +// one at a time. Only once that runnable has executed will we dispatch +// the next runnable to the base target. This in effect makes all +// runnables passing through the ThrottledEventQueue yield to other work +// on the base target. +// +// ThrottledEventQueue keeps runnables waiting to be dispatched to the +// base in its own internal queue. Code can query the length of this +// queue using IsEmpty() and Length(). Further, code implement back +// pressure by checking the depth of the queue and deciding to stop +// issuing runnables if they see the ThrottledEventQueue is backed up. +// Code running on other threads could even use AwaitIdle() to block +// all operation until the ThrottledEventQueue drains. +// +// Note, this class is similar to TaskQueue, but also differs in a few +// ways. First, it is a very simple nsIEventTarget implementation. It +// does not use the AbstractThread API. +// +// In addition, ThrottledEventQueue currently dispatches its next +// runnable to the base target *before* running the current event. This +// allows the event code to spin the event loop without stalling the +// ThrottledEventQueue. In contrast, TaskQueue only dispatches its next +// runnable after running the current event. That approach is necessary +// for TaskQueue in order to work with thread pool targets. +// +// So, if you are targeting a thread pool you probably want a TaskQueue. +// If you are targeting a single thread or other non-concurrent event +// target, you probably want a ThrottledEventQueue. +// +// If you drop a ThrottledEventQueue while its queue still has events to be run, +// they will continue to be dispatched as usual to the base. Only once the last +// event has run will all the ThrottledEventQueue's memory be freed. +class ThrottledEventQueue final : public nsISerialEventTarget { + class Inner; + RefPtr<Inner> mInner; + + explicit ThrottledEventQueue(already_AddRefed<Inner> aInner); + ~ThrottledEventQueue() = default; + + public: + // Create a ThrottledEventQueue for the given target. + static already_AddRefed<ThrottledEventQueue> Create( + nsISerialEventTarget* aBaseTarget, const char* aName, + uint32_t aPriority = nsIRunnablePriority::PRIORITY_NORMAL); + + // Determine if there are any events pending in the queue. + bool IsEmpty() const; + + // Determine how many events are pending in the queue. + uint32_t Length() const; + + already_AddRefed<nsIRunnable> GetEvent(); + + // Block the current thread until the queue is empty. This may not be called + // on the main thread or the base target. The ThrottledEventQueue must not be + // paused. + void AwaitIdle() const; + + // If |aIsPaused| is true, pause execution of events from this queue. No + // events from this queue will be run until this is called with |aIsPaused| + // false. + // + // To un-pause a ThrottledEventQueue, we need to dispatch a runnable to the + // underlying event target. That operation may fail, so this method is + // fallible as well. + // + // Note that, although ThrottledEventQueue's behavior is descibed as queueing + // events on the base target, an event queued on a TEQ is never actually moved + // to any other queue. What is actually dispatched to the base is an + // "executor" event which, when run, removes an event from the TEQ and runs it + // immediately. This means that you can pause a TEQ even after the executor + // has been queued on the base target, and even so, no events from the TEQ + // will run. When the base target gets around to running the executor, the + // executor will see that the TEQ is paused, and do nothing. + [[nodiscard]] nsresult SetIsPaused(bool aIsPaused); + + // Return true if this ThrottledEventQueue is paused. + bool IsPaused() const; + + NS_DECL_THREADSAFE_ISUPPORTS + NS_DECL_NSIEVENTTARGET_FULL + + NS_DECLARE_STATIC_IID_ACCESSOR(NS_THROTTLEDEVENTQUEUE_IID); +}; + +NS_DEFINE_STATIC_IID_ACCESSOR(ThrottledEventQueue, NS_THROTTLEDEVENTQUEUE_IID); + +} // namespace mozilla + +#endif // mozilla_ThrottledEventQueue_h |