Android 中 MessageQueue 的 nativePollOnce

Android SDK 中的事件回圈已經是一個老生常談的問題了, 像 Handler Looper MessageQueue 這幾個類也是被大家研究透徹了.
但是再回頭看以前自己的分析, 總感覺差點什么, 不夠透徹. 心里隱隱感覺自己沒有把事情完全吃透, 于是今日又回顧 Android 中的事件回圈機制, 注意到
MessageQueue 中獲取下一條訊息時會執行一個 native 呼叫 nativePollOnce, 翻看 Android 系統原始碼發現有內容.

來自高手的解釋

首先, 先梭哈一把 stackoverflow 上高手對這個問題(android - what is message queue native poll once in android?)的回答原文:

Short answer:

The nativePollOnce method is used to "wait" till the next Message becomes available. If the time spent during this call is long, your main (UI) thread has no real work to do and waits for next events to process. There's no need to worry about that.

Explanation:

Because the "main" thread is responsible for drawing UI and handling various events, it's Runnable has a loop which processes all these events. The loop is managed by a Looper and its job is quite straightforward: it processes all Messages in the MessageQueue.

A Message is added to the queue for example in response to input events, as frame rendering callback or even your own Handler.post calls. Sometimes the main thread has no work to do (that is, no messages in the queue), which may happen e.g. just after finishing rendering single frame (the thread has just drawn one frame and is ready for the next one, just waits for a proper time). Two Java methods in the MessageQueue class are interesting to us: Message next() and boolean enqueueMessage(Message, long). Message next(), as its name suggest, takes and returns the next Message from the queue. If the queue is empty (and there's nothing to return), the method calls native void nativePollOnce(long, int) which blocks until a new message is added. At this point you might ask how does nativePollOnce know when to wake up. That's a very good question. When a Message is added to the queue, the framework calls the enqueueMessage method, which not only inserts the message into the queue, but also calls native static void nativeWake(long), if there's need to wake up the queue. The core magic of nativePollOnce and nativeWake happens in the native (actually, C++) code. Native MessageQueue utilizes a Linux system call named epoll, which allows to monitor a file descriptor for IO events. nativePollOnce calls epoll_wait on a certain file descriptor, whereas nativeWake writes to the descriptor, which is one of the IO operations, epoll_wait waits for. The kernel then takes out the epoll-waiting thread from the waiting state and the thread proceeds with handling the new message. If you're familiar with Java's Object.wait() and Object.notify() methods, you can imagine that nativePollOnce is a rough equivalent for Object.wait() and nativeWake for Object.notify(), except they're implemented completely differently: nativePollOnce uses epoll and Object.wait() uses futex Linux call. It's worth noticing that neither nativePollOnce nor Object.wait() waste CPU cycles, as when a thread enters either method, it becomes disabled for thread scheduling purposes (quoting the javadoc for the Object class). However, some profilers may mistakenly recognize epoll-waiting (or even Object-waiting) threads as running and consuming CPU time, which is incorrect. If those methods actually wasted CPU cycles, all idle apps would use 100% of the CPU, heating and slowing down the device.

Conclusion:

You shouldn't worry about nativePollOnce. It just indicates that processing of all Messages has been finished and the thread waits for the next one. Well, that simply means you don't give too much work to your main thread ??

translate.google 一下 ??:

簡短答案:

nativePollOnce 方法用于“等待”, 直到下一條訊息可用為止. 如果在此呼叫期間花費的時間很長, 則您的主執行緒沒有實際作業要做, 而是等待下一個事件處理.無需擔心.

說明:

因為主執行緒負責繪制 UI 和處理各種事件, 所以主執行緒擁有一個處理所有這些事件的回圈. 該回圈由 L??ooper 管理, 其作業非常簡單: 它處理 MessageQueue 中的所有 Message.
例如, 回應于輸入事件, 將訊息添加到佇列, 幀渲染回呼, 甚至您的 Handler.post 呼叫. 有時主執行緒無事可做（即佇列中沒有訊息), 例如在完成渲染單幀之后(執行緒付訓制了一幀, 并準備好下一幀, 等待適當的時間). MessageQueue 類中的兩個 Java 方法對我們很有趣: Message next()和 boolean enqueueMessage(Message, long). 顧名思義, Message next() 從佇列中獲取并回傳下一個訊息. 如果佇列為空(無回傳值), 則該方法將呼叫 native void nativePollOnce(long, int), 該方法將一直阻塞直到添加新訊息為止. 此時,您可能會問nativePollOnce 如何知道何時醒來. 這是一個很好的問題. 當將 Message 添加到佇列時, 框架呼叫 enqueueMessage 方法, 該方法不僅將訊息插入佇列, 而且還會呼叫native static void nativeWake(long). nativePollOnce 和 nativeWake 的核心魔術發生在 native 代碼中. native MessageQueue 利用名為 epoll 的 Linux 系統呼叫, 該系統呼叫可以監視檔案描述符中的 IO 事件. nativePollOnce 在某個檔案描述符上呼叫 epoll_wait, 而 nativeWake 寫入一個 IO 操作到描述符, epoll_wait 等待. 然后, 內核從等待狀態中取出 epoll 等待執行緒, 并且該執行緒繼續處理新訊息. 如果您熟悉 Java 的 Object.wait()和 Object.notify()方法,可以想象一下 nativePollOnce 大致等同于 Object.wait(), nativeWake 等同于 Object.notify(),但它們的實作完全不同: nativePollOnce 使用 epoll, 而 Object.wait 使用 futex Linux 呼叫. 值得注意的是, nativePollOnce 和 Object.wait 都不會浪費 CPU 周期, 因為當執行緒進入任一方法時, 出于執行緒調度的目的, 該執行緒將被禁用(參考Object類的javadoc). 但是, 某些事件探查器可能會錯誤地將等待 epoll 等待(甚至是 Object.wait)的執行緒識別為正在運行并消耗 CPU 時間, 這是不正確的. 如果這些方法實際上浪費了 CPU 周期, 則所有空閑的應用程式都將使用 100％ 的 CPU, 從而加熱并降低設備速度.

結論:

nativePollOnce. 它只是表明所有訊息的處理已完成, 執行緒正在等待下一個訊息.

我以前的理解分享

Linux 有多個 IO 模型:

• 阻塞 IO
• 非阻塞 IO
• IO 復用, 對應 select poll epoll 都屬于基于 IO 復用模式的呼叫
• 信號驅動 IO
• 異步 IO

看下原始碼

Java 這邊

enqueueMessage:

boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }
    synchronized (this) {
        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        boolean needWake;
        if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            // Inserted within the middle of the queue.  Usually we don't have to wake
            // up the event queue unless there is a barrier at the head of the queue
            // and the message is the earliest asynchronous message in the queue.
            needWake = mBlocked && p.target == null && msg.isAsynchronous();
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;
        }
        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            // 這里喚醒 nativePollOnce 的沉睡
            nativeWake(mPtr);
        }
    }
    return true;
}

next:

Message next() {
    //...
    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }
        // nativePollOnce 這里陷入沉睡, 等待喚醒
        nativePollOnce(ptr, nextPollTimeoutMillis);
        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {
                // Stalled by a barrier.  Find the next asynchronous message in the queue.
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    // Next message is not ready.  Set a timeout to wake up when it is ready.
                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                } else {
                    // Got a message.
                    mBlocked = false;
                    if (prevMsg != null) {
                        prevMsg.next = msg.next;
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    return msg;
                }
            } else {
                // No more messages.
                nextPollTimeoutMillis = -1;
            }
            //...
        }
        //...
    }
}

CPP 那邊

nativeWake

void NativeMessageQueue::wake() {
    mLooper->wake();
}

void Looper::wake() {
    uint64_t inc = 1;
    ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
    if (nWrite != sizeof(uint64_t)) {
        if (errno != EAGAIN) {
            LOG_ALWAYS_FATAL("Could not write wake signal to fd %d: %s",
                    mWakeEventFd, strerror(errno));
        }
    }
}

nativePollOnce:

void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
    mPollEnv = env;
    mPollObj = pollObj;
    mLooper->pollOnce(timeoutMillis);
    mPollObj = NULL;
    mPollEnv = NULL;

    if (mExceptionObj) {
        env->Throw(mExceptionObj);
        env->DeleteLocalRef(mExceptionObj);
        mExceptionObj = NULL;
    }
}

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
    int result = 0;
    for (;;) {
        while (mResponseIndex < mResponses.size()) {
            const Response& response = mResponses.itemAt(mResponseIndex++);
            int ident = response.request.ident;
            if (ident >= 0) {
                int fd = response.request.fd;
                int events = response.events;
                void* data = https://www.cnblogs.com/jiy-for-you/p/response.request.data;
                if (outFd != NULL) *outFd = fd;
                if (outEvents != NULL) *outEvents = events;
                if (outData != NULL) *outData = data;
                return ident;
            }
        }

        if (result != 0) {
            if (outFd != NULL) *outFd = 0;
            if (outEvents != NULL) *outEvents = 0;
            if (outData != NULL) *outData = NULL;
            return result;
        }

        result = pollInner(timeoutMillis);
    }
}

int Looper::pollInner(int timeoutMillis) {
    // Adjust the timeout based on when the next message is due.
    if (timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) {
        nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
        int messageTimeoutMillis = toMillisecondTimeoutDelay(now, mNextMessageUptime);
        if (messageTimeoutMillis >= 0
                && (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) {
            timeoutMillis = messageTimeoutMillis;
        }
    }

    // Poll.
    int result = POLL_WAKE;
    mResponses.clear();
    mResponseIndex = 0;

    // We are about to idle.
    mPolling = true;

    struct epoll_event eventItems[EPOLL_MAX_EVENTS];
    // 這里重點
    int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

    // No longer idling.
    mPolling = false;

    // Acquire lock.
    mLock.lock();

    // Rebuild epoll set if needed.
    if (mEpollRebuildRequired) {
        mEpollRebuildRequired = false;
        rebuildEpollLocked();
        goto Done;
    }

    // Check for poll error.
    if (eventCount < 0) {
        if (errno == EINTR) {
            goto Done;
        }
        ALOGW("Poll failed with an unexpected error: %s", strerror(errno));
        result = POLL_ERROR;
        goto Done;
    }

    // Check for poll timeout.
    if (eventCount == 0) {
        result = POLL_TIMEOUT;
        goto Done;
    }

    // Handle all events.
    for (int i = 0; i < eventCount; i++) {
        int fd = eventItems[i].data.fd;
        uint32_t epollEvents = eventItems[i].events;
        if (fd == mWakeEventFd) {
            if (epollEvents & EPOLLIN) {
                awoken();
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
            }
        } else {
            ssize_t requestIndex = mRequests.indexOfKey(fd);
            if (requestIndex >= 0) {
                int events = 0;
                if (epollEvents & EPOLLIN) events |= EVENT_INPUT;
                if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT;
                if (epollEvents & EPOLLERR) events |= EVENT_ERROR;
                if (epollEvents & EPOLLHUP) events |= EVENT_HANGUP;
                pushResponse(events, mRequests.valueAt(requestIndex));
            } else {
                ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
                        "no longer registered.", epollEvents, fd);
            }
        }
    }
Done: ;

    // Invoke pending message callbacks.
    mNextMessageUptime = LLONG_MAX;
    while (mMessageEnvelopes.size() != 0) {
        nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
        const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);
        if (messageEnvelope.uptime <= now) {
            // Remove the envelope from the list.
            // We keep a strong reference to the handler until the call to handleMessage
            // finishes.  Then we drop it so that the handler can be deleted *before*
            // we reacquire our lock.
            { // obtain handler
                sp<MessageHandler> handler = messageEnvelope.handler;
                Message message = messageEnvelope.message;
                mMessageEnvelopes.removeAt(0);
                mSendingMessage = true;
                mLock.unlock();
                handler->handleMessage(message);
            } // release handler

            mLock.lock();
            mSendingMessage = false;
            result = POLL_CALLBACK;
        } else {
            // The last message left at the head of the queue determines the next wakeup time.
            mNextMessageUptime = messageEnvelope.uptime;
            break;
        }
    }

    // Release lock.
    mLock.unlock();

    // Invoke all response callbacks.
    for (size_t i = 0; i < mResponses.size(); i++) {
        Response& response = mResponses.editItemAt(i);
        if (response.request.ident == POLL_CALLBACK) {
            int fd = response.request.fd;
            int events = response.events;
            void* data = https://www.cnblogs.com/jiy-for-you/p/response.request.data;
            // Invoke the callback.  Note that the file descriptor may be closed by
            // the callback (and potentially even reused) before the function returns so
            // we need to be a little careful when removing the file descriptor afterwards.
            int callbackResult = response.request.callback->handleEvent(fd, events, data);
            if (callbackResult == 0) {
                removeFd(fd, response.request.seq);
            }

            // Clear the callback reference in the response structure promptly because we
            // will not clear the response vector itself until the next poll.
            response.request.callback.clear();
            result = POLL_CALLBACK;
        }
    }
    return result;
}

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