java NIO的空轮询bug 以及Netty的解决办法

今天药忘吃喽~ 2023-01-12 10:23 224阅读 0赞

**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%  
[![java NIO的空轮询bug 以及Netty的解决办法\_第1张图片][java NIO_bug _Netty_1]][java NIO_bug _Netty_1_java NIO_bug _Netty_1]  
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();
    }

[java NIO_bug _Netty_1]: https://img-blog.csdnimg.cn/img_convert/102e043241154ab4645abdc3c5e0f883.png
[java NIO_bug _Netty_1_java NIO_bug _Netty_1]: https://img.it610.com/image/info8/b4fc9ed607914c339a2d8e7d7839eadd.jpg