[Mongrel] scaling unicorn

snacktime snacktime at gmail.com
Mon Jun 21 15:58:58 EDT 2010


Interested in some feeback on this (does it sound right?), or maybe
this might be of interest to others.

We are launching a new facebook app in a couple weeks and we did some
load testing over the weekend on our unicorn web cluster.  The servers
are 8 way xeon's with 24gb ram.  Our app ended up being primarily cpu
bound.  So far the sweet spot for the number of unicorns seems to be
around 40.  This seemed to yield the most requests per second without
overloading the server or hitting memory bandwidth issues.  The
backlog is at the somaxconn default of 128, I'm still not sure if we
will bump that up or not.  Increasing the number of unicorns beyond a
certain point resulted in a noticable drop in the requests per second
the server could handle.   I'm pretty sure the cause is the box
running out of memory bandwidth.  The load average and resource usage
in general (except for memory) would keep going down but so did the
requests per second.  At 80 unicorns the requests per second dropped
by more then half.  I'm going to disable hyperthreading and rerun some
of the tests to see what impact that has.

Chris


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