Too Many Open Files (Mono .NET on Ubuntu)

Question

I know there are a lot of these questions on SO but I cannot see anything that matches my specific situation.

I am running a .NET console application via Mono on Ubuntu. The app runs as a server and accepts connections via a TcpListener (TcpListener.AcceptTcpClient()). The problem I have is that after a while the program starts throwing 'Too many open files' exceptions.

I have increased the max file limit in Ubuntu in the two places that I am aware of:

• /etc/security/limits.conf

root soft nofile 240000

root hard nofile 320000

(The process in question runs as root)

• /etc/sysctl.conf

fs.file-max = 2000000

Both are set to ~200000.

If check the number of open file descriptions on the system it is only 996 even though it is throwing the errors.

I have the same program running on several Windows servers with many more connections and they never have this problem.

Any idea what could be causing this error?

Answer 1

The reason the error does not occur on Windows could be related to when garbage collection is done. If you do not explicitly close your socket connections, the sockets will be closed when they are collected.

This is somewhat speculative but if Mono does not start the garbage collection until it sees you running out of memory and it never runs out of memory sockets are never closed. Windows .NET framework might run garbage collecting regularly or when it sees you running out of file descriptors, which could account for the difference.

As a test you could force a manual garbage collection (GB.Collect) at regular intervals and if this fixes your errors, the proper solution would be to not rely on garbage collection for closing sockets, making sure to close them manually in your code. Forcing manual garbage collection brings its own set of problems.

Answer 2

I first ran into a similar problem, but this is what I did to get it working:

sysctl -w net.inet.ip.portrange.first=3000  
sysctl -w net.inet.ip.portrange.hifirst=3000  
sysctl -w kern.maxfiles=900000  
sysctl -w kern.maxfilesperproc=900000  
ulimit -n 900000  

I ran the process under root instead of a normal user (different soft limit) and managed to create up to 100,000 connections. Note that ulimit is bound to a shell, so need to enter the mono (yourapp.exe) in the same shell as you do the ulimit, not from MonoDevelop.

For development running under OSX 10.7.5.