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Multiple EventLoopGroup cause socketException

See original GitHub issue

Hi. Recently I make a server-stress-test client with netty bootstrap.

First, I create multiple bootstrap to connect server and it causes error: java.net.SocketException: No buffer space available (maximum connections reached?): connect when the bootstrap was created bout 1,000 times in my computer. At last i search in google find the right way to make to multiple connection with bootstrap.

And i looked at the bootstrap code and found nothing to cause SocketException. Now i wonder why it causes SocketException when i create multiple bootstrap object and the correct way to create multiple client connection to server.

Here is the incorrect code(cause SocketException) and correct(maybe?) code.

Incorrect Code.

for(int i = 0; i < 5000; i++) {
  EventLoopGroup workerGroup = new NioEventLoopGroup();
  Bootstrap bootstrap = new Bootstrap();
  bootstrap
    .group(workerGroup)
    .channel(NioSocketChannel.class)
    .option(ChannelOption.SO_KEEPALIVE, true)
    .option(ChannelOption.SO_REUSEADDR, true)
    .handler(new ChannelInitializer<SocketChannel>() {
      @Override
      protected void initChannel(SocketChannel ch) throws Exception {
        ch.pipeline().addLast(new EchoHandler());
      }
    });

  try {
    ChannelFuture future = bootstrap.connect("localhost", 10000).sync();
  } catch (InterruptedException e) {
    e.printStackTrace();
  }
}

Correct(?) Code

EventLoopGroup workerGroup = new NioEventLoopGroup();
Bootstrap bootstrap = new Bootstrap();
bootstrap
  .group(workerGroup)
  .channel(NioSocketChannel.class)
  .option(ChannelOption.SO_KEEPALIVE, true)
  .option(ChannelOption.SO_REUSEADDR, true)
  .handler(new ChannelInitializer<SocketChannel>() {
    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
      ch.pipeline().addLast(new EchoHandler());
    }
  });

List<ChannelFuture> list = new ArrayList<>();
for(int i = 0; i < 5000; i++) {
  ChannelFuture future = bootstrap.connect("localhost", 10000).sync();
  list.add(future);
}

Error Exception

Exception in thread "main" java.lang.IllegalStateException: failed to create a child event loop
	at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:88)
	at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:58)
	at io.netty.channel.MultithreadEventLoopGroup.<init>(MultithreadEventLoopGroup.java:52)
	at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:87)
	at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:82)
	at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:63)
	at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:51)
	at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:43)
	at com.example.demo.DemoApplication.multipleBootstrap(DemoApplication.java:54)
	at com.example.demo.DemoApplication.main(DemoApplication.java:31)
Caused by: io.netty.channel.ChannelException: failed to open a new selector
	at io.netty.channel.nio.NioEventLoop.openSelector(NioEventLoop.java:175)
	at io.netty.channel.nio.NioEventLoop.<init>(NioEventLoop.java:149)
	at io.netty.channel.nio.NioEventLoopGroup.newChild(NioEventLoopGroup.java:127)
	at io.netty.channel.nio.NioEventLoopGroup.newChild(NioEventLoopGroup.java:36)
	at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:84)
	... 9 more
Caused by: java.io.IOException: Unable to establish loopback connection
	at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:94)
	at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:61)
	at java.security.AccessController.doPrivileged(Native Method)
	at sun.nio.ch.PipeImpl.<init>(PipeImpl.java:171)
	at sun.nio.ch.SelectorProviderImpl.openPipe(SelectorProviderImpl.java:50)
	at java.nio.channels.Pipe.open(Pipe.java:155)
	at sun.nio.ch.WindowsSelectorImpl.<init>(WindowsSelectorImpl.java:127)
	at sun.nio.ch.WindowsSelectorProvider.openSelector(WindowsSelectorProvider.java:44)
	at io.netty.channel.nio.NioEventLoop.openSelector(NioEventLoop.java:173)
	... 13 more
Caused by: java.net.SocketException: No buffer space available (maximum connections reached?): connect
	at sun.nio.ch.Net.connect0(Native Method)
	at sun.nio.ch.Net.connect(Net.java:454)
	at sun.nio.ch.Net.connect(Net.java:446)
	at sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:648)
	at java.nio.channels.SocketChannel.open(SocketChannel.java:189)
	at sun.nio.ch.PipeImpl$Initializer$LoopbackConnector.run(PipeImpl.java:127)
	at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:76)
	... 21 more