Tuning Unicorn

Unicorn performance is generally as good as a (mostly) Ruby web server can provide. Most often the performance bottleneck is in the web application running on Unicorn rather than Unicorn itself.

Unicorn Configuration

See Unicorn::Configurator for details on the config file format.

• Setting a very low value for the :backlog parameter in “listen” directives can allow failover to happen more quickly if your cluster is configured for it.
• If you’re doing extremely simple benchmarks and getting connection errors under high request rates, increasing your :backlog parameter above the already-generous default of 1024 can help avoid connection errors. Keep in mind this is not recommended for real traffic if you have another machine to failover to (see above).
• :rcvbuf and :sndbuf parameters generally do not need to be set for TCP listeners under Linux 2.6 because auto-tuning is enabled. UNIX domain sockets do not have auto-tuning buffer sizes; so increasing those will allow syscalls and task switches to be saved for larger requests and responses.
• Setting “preload_app true” can allow copy-on-write-friendly GC to be used to save memory. It will probably not work out of the box with applications that open sockets or perform random I/O on files. Databases like TokyoCabinet use concurrency-safe pread()/pwrite() functions for safe sharing of database file descriptors across processes.
• On POSIX-compliant filesystems, it is safe for multiple threads or processes to append to one log file as long as all the processes are have them unbuffered (File#sync = true) or they are record(line)-buffered in userspace.
• worker_processes should be scaled to the number of processes your backend system(s) can support. DO NOT scale it to the number of external network clients your application expects to be serving. Unicorn is NOT for serving slow clients, that is the job of nginx.

Kernel Parameters (Linux sysctl)

WARNING: Do not change system parameters unless you know what you’re doing!

• net.core.rmem_max and net.core.wmem_max can increase the allowed size of :rcvbuf and :sndbuf respectively. This is mostly only useful for UNIX domain sockets which do not have auto-tuning buffer sizes.
• For load testing/benchmarking with UNIX domain sockets, you should consider increasing net.core.somaxconn or else nginx will start failing to connect under heavy load. You may also consider setting a higher :backlog to listen on as noted earlier.
• If you’re running out of local ports, consider lowering net.ipv4.tcp_fin_timeout to 20-30 (default: 60 seconds). Also consider widening the usable port range by changing net.ipv4.ip_local_port_range.
• Setting net.ipv4.tcp_timestamps=1 will also allow setting net.ipv4.tcp_tw_reuse=1 and net.ipv4.tcp_tw_recycle=1, which along with the above settings can slow down port exhaustion. Not all networks are compatible with these settings, check with your friendly network administrator before changing these.
• Increasing the MTU size can reduce framing overhead for larger transfers. One often-overlooked detail is that the loopback device (usually “lo”) can have its MTU increased, too.

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