Operating system limits
The operating system is often the last place we look to for discovering a problem. We assume that whoever set up the system has tuned the operating system for our workload and tested it under similar scenarios. This is often not the case. We sometimes need to look into the operating system itself to identify a bottleneck.
As with NGINX, there are two major areas where we can initially look for performance problems: file descriptor limits and network limits.
File descriptor limits
NGINX uses file descriptors in several different ways. The major use is to respond to client connections, each one using a file descriptor. Each outgoing connection (especially prevalent in proxy configurations) requires a unique IP:TCP port pair, which NGINX refers to using a file descriptor. If NGINX is serving any static files or a response from its cache, a file descriptor is used as well. As you can see, the number of file descriptors can climb quickly with the number of concurrent users...
