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= 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. 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.
* 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
* 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
* 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.