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netty解决空轮询bug

netty解决空轮询bug

作者: loading_17 | 来源:发表于2018-06-25 23:23 被阅读0次

    selector在没有结果的情况下,依然被唤醒,导致一直空轮询,cpu100%
    直接定位到NioEventLoop

    @Override
        protected void run() {
            for (;;) {
                try {
                    switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
                        case SelectStrategy.CONTINUE:
                            continue;
                        case SelectStrategy.SELECT:
                            select(wakenUp.getAndSet(false));
                            if (wakenUp.get()) {
                                selector.wakeup();
                            }
                        default:
                            // fallthrough
                    }
    
                    cancelledKeys = 0;
                    needsToSelectAgain = false;
                    final int ioRatio = this.ioRatio;
                    if (ioRatio == 100) {
                        try {
                            processSelectedKeys();
                        } finally {
                            // Ensure we always run tasks.
                            runAllTasks();
                        }
                    } else {
                        final long ioStartTime = System.nanoTime();
                        try {
                            processSelectedKeys();
                        } finally {
                            // Ensure we always run tasks.
                            final long ioTime = System.nanoTime() - ioStartTime;
                            runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                        }
                    }
                } catch (Throwable t) {
                    handleLoopException(t);
                }
               
                try {
                    if (isShuttingDown()) {
                        closeAll();
                        if (confirmShutdown()) {
                            return;
                        }
                    }
                } catch (Throwable t) {
                    handleLoopException(t);
                }
            }
        }
    

    第二个case,SelectStrategy.SELECT ,其值为-1。跟进调用的select方法。

     private void select(boolean oldWakenUp) throws IOException {
            Selector selector = this.selector;
            try {
                int selectCnt = 0;
                long currentTimeNanos = System.nanoTime();
                long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos);
                for (;;) {
                    long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
                    if (timeoutMillis <= 0) {
                        if (selectCnt == 0) {
                            selector.selectNow();
                            selectCnt = 1;
                        }
                        break;
                    }
                    if (hasTasks() && wakenUp.compareAndSet(false, true)) {
                        selector.selectNow();
                        selectCnt = 1;
                        break;
                    }
    
                    int selectedKeys = selector.select(timeoutMillis);
                    selectCnt ++;
    
                    if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
                        break;
                    }
                    if (Thread.interrupted()) {
                        if (logger.isDebugEnabled()) {
                            logger.debug("Selector.select() returned prematurely because " +
                                    "Thread.currentThread().interrupt() was called. Use " +
                                    "NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");
                        }
                        selectCnt = 1;
                        break;
                    }
    
                    long time = System.nanoTime();
                    if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
                        // timeoutMillis elapsed without anything selected.
                        selectCnt = 1;
                    } else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
                            selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
                        // The selector returned prematurely many times in a row.
                        // Rebuild the selector to work around the problem.
                        logger.warn(
                                "Selector.select() returned prematurely {} times in a row; rebuilding Selector {}.",
                                selectCnt, selector);
    
                        rebuildSelector();
                        selector = this.selector;
    
                        // Select again to populate selectedKeys.
                        selector.selectNow();
                        selectCnt = 1;
                        break;
                    }
    
                    currentTimeNanos = time;
                }
    
                if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS) {
                    if (logger.isDebugEnabled()) {
                        logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",
                                selectCnt - 1, selector);
                    }
                }
            } catch (CancelledKeyException e) {
                if (logger.isDebugEnabled()) {
                    logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector {} - JDK bug?",
                            selector, e);
                }
                // Harmless exception - log anyway
            }
        }
    

    上述代码中在轮询之前,先定义当前时间currentTimeNanos。接着计算出一个执行最少需要的时间timeoutMillis。每次对selectCnt做++操作。
    在后面的代码中通过

    time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos
    

    进行判断,如果到达执行到最少时间,则seletCnt重置为1。一旦到达SELECTOR_AUTO_REBUILD_THRESHOLD这个阀值,就需要重建selector来解决这个问题。这个阀值默认是512。

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