Reason I was trying to benchmark a TCP based file server on Mac OS X. The server is running latest Node.js (0.8.2), while the client to push the server is written in go. ‘Go’ can start thousands of concurrent goroutines easily. I got error when there are hundreds of concurrent clients. Steps increate max open files kern.maxfiles and kern.maxfilesperproc were small numbers, they need to be increased: after this, you can increase your account’s limit by ulimit -n: