beast: Deadlock problem under high concurrency

We write a http example with beast, and we use multiple threads to run io_service. Then we conduct a large number of concurrent connection tests. Each connection will only send and receive packets once, and then disconnect. Later, we found that it is difficult to connect to the opened ports. We think that a deadlock occurred in async_accept.Because if we use single thread, this problem will not happen. Is there any solution for this problem？ I would very appreciate it if who can help me.