What is the reason for Broken Pipe on Unix Domain Sockets?
'Broken pipe' means you have written to a connection that had already been closed by the other end. It is detected somewhat asynchronously due to buffering. It basically means you have an error in your application protocol.
From the Linux Programmer's Manual (similar language is also in the socket man page on Mac):
The communications protocols which implement a SOCK_STREAM ensure that data is not lost or duplicated. If a piece of data for which the peer protocol has buffer space cannot be successfully transmitted within a reasonable length of time, then the connection is considered to be dead. When SO_KEEPALIVE is enabled on the socket the protocol checks in a protocol-specific manner if the other end is still alive. A SIGPIPE signal is raised if a process sends or receives on a broken stream; this causes naive processes, which do not handle the signal, to exit.
In other words, if data gets stuck in a stream socket for too long, you'll end up with a SIGPIPE. It's reasonable that you would end up with this if you can't keep up with your load test.