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.
See Unicorn::Configurator for details on the config file format. worker_processes is the most-commonly needed tuning parameter.
Unicorn::Configurator#worker_processesworker_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.
worker_processes should be at least the number of CPU cores on a dedicated server (unless you do not have enough memory). If your application has occasionally slow responses that are /not/ CPU-intensive, you may increase this to workaround those inefficiencies.
Under Ruby 2.2 or later, Etc.nprocessors may be used to determine the number of CPU cores present.
worker_processes may be increased for Unicorn::OobGC users to provide more consistent response times.
Never, ever, increase worker_processes to the point where the system runs out of physical memory and hits swap. Production servers should never see heavy swap activity.
Unicorn::Configurator#listen OptionsSetting 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. If your app only generates small responses or expects small requests, you may shrink the buffer sizes to save memory, too.
Having socket buffers too large can also be detrimental or have little effect. Huge buffers can put more pressure on the allocator and may also thrash CPU caches, cancelling out performance gains one would normally expect.
UNIX domain sockets are slightly faster than TCP sockets, but only work if nginx is on the same machine.
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 before any writes.
WARNING: Do not change system parameters unless you know what you're doing!
Transparent hugepages (THP) improves performance in many cases, but can also increase memory use when relying on a copy-on-write(CoW)-friendly GC (Ruby 2.0+) with "preload_app true". CoW operates at the page level, so writing to a huge page would trigger a 2 MB copy (x86-64), as opposed to a 4 KB copy on a regular (non-huge) page.
Consider only allowing THP to be used when it is requested via the madvise(2) syscall:
echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
Or disabling it system-wide, via "never".
n.b. "page" in this context only applies to the OS kernel, Ruby GC implementations also use this term for the same concept in a way that is agnostic to the OS.
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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