Question

I have developed a TCP server according to your advises: High performance TCP server in C#

It is based on asynchron pattern.

I also developed a stress test application to test its performance. My server can get thousands of connections paralelly from my stress test app, can parse data and save it to my database.

When I stress my server, I can get “System.Net.Sockets.SocketException “No connection could be made because the target machine actively refused it” error from my server, so I have to reconnect to it. If I test it with 5000 concurrent connections, I have to try connect again because of this problem 10-20% of the connections, if I test it with 10K concurrent connections, it can be 30-40%. Sometimes it can be – very rarely – more, than 50%. It seems it can not handle connection accepts: I make new connections from my stress test as heavily as my test machine can – about 120 connections/sec.

So, what can cause this kind of exception? How to handle it? What to do in server side implementation to avoid this problem? How to tune TCP connection accept?

Thanks in advance!

Answer 1

You are making connections faster than the software can listen for new connections, or in other words you are reaching the connections per second limit of that port. I think you can double the amount of connections per second by listening to a second port, client side you should just reconnect when you get the exception.

There are also limits applied to the amount of connection, for these see Chris O’s answer.