java NIO的空轮询bug 以及Netty的解决办法_nio bug_Gogym的博客

这个bug是指 java的NIO在linux下selector.select()时，本来如果轮询的结果为空并且不调用wakeup的方法的话，这个selector.select()应该是一直阻塞的，但是java却会打破阻塞，继续执行，导致程序无限空转，造成CPU使用率100%

这个bug只出现在linux系统下，因为linux下NIO底层使用的是epoll来实现的，而java的epoll实现存在bug，导致selector出现了这种轮询为空却唤醒的情况。windows下NIO是使用的poll来实现selector的就不存在这种bug

Netty中解决该bug的方法

1、设置一个selector.select(timeout)，有一个超时时间，selector有4种情况会跳出阻塞

1. 有事件发生
2. wakeup
3. 超时
4. 空轮询bug

而前两种返回值不为0，可以跳出循环，超时有时间戳记录，所以每次空轮询，有专门 的计数器+1，如果空轮询的次数超过了512次，就认为其触发了空轮询bug。

2、触发bug后，netty直接重建一个selector，将原来的channel重新注册到新的selector上，将旧的 selector关掉

private void select(boolean oldWakenUp) throws IOException {//节选
    for(;;)
        int selectedKeys = selector.select(timeoutMillis);
        selectCnt ++;
        
        if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
            // - Selected something,
            // - waken up by user, or
            // - the task queue has a pending task.
            // - a scheduled task is ready for processing
            break;
        }
        long time = System.nanoTime();
        if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
            // timeoutMillis elapsed without anything selected.
            // 超时
            selectCnt = 1;
        } else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
            selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {//默认值512
            // The code exists in an extra method to ensure the method is not too big to inline as this
            // branch is not very likely to get hit very frequently.
            // 空轮询一次 cnt+1  如果一个周期内次数超过512，则假定发生了空轮询bug，重建selector
            selector = selectRebuildSelector(selectCnt);
            selectCnt = 1;
            break;
        }
    }
}
 
 
 
/**
 * Replaces the current {@link Selector} of this event loop with newly created {@link Selector}s to work
 * around the infamous epoll 100% CPU bug.
 * 新建一个selector来解决空轮询bug
 */
public void rebuildSelector() {
    if (!inEventLoop()) {
        execute(new Runnable() {
            @Override
            public void run() {
                rebuildSelector0();
            }
        });
        return;
    }
    rebuildSelector0();
}
 
private void rebuildSelector0() {
    final Selector oldSelector = selector;
    final SelectorTuple newSelectorTuple;
 
    //新建一个selector
    newSelectorTuple = openSelector();
 
 
    // 将旧的selector的channel全部拿出来注册到新的selector上
    int nChannels = 0;
    for (SelectionKey key: oldSelector.keys()) {
        Object a = key.attachment();
        if (!key.isValid() || key.channel().keyFor(newSelectorTuple.unwrappedSelector) != null) {
            continue;
        }
        int interestOps = key.interestOps();
        key.cancel();
        SelectionKey newKey = key.channel().register(newSelectorTuple.unwrappedSelector, interestOps, a);
        if (a instanceof AbstractNioChannel) {
            // Update SelectionKey
            ((AbstractNioChannel) a).selectionKey = newKey;
        }
        nChannels ++;
  
    }
 
    selector = newSelectorTuple.selector;
    unwrappedSelector = newSelectorTuple.unwrappedSelector;
    // time to close the old selector as everything else is registered to the new one
    //关掉旧的selector
     oldSelector.close();
}