Sinatra app fails to perform at scale

[Eliot Shepard <eshepard@slower.net>]

Hello,

We're attempting to move our set of Rails and Sinatra apps from
Passenger/REE to Unicorn/1.9.3. We've started with one of the Sinatra
apps, which typically sees about 50 req/s. It happens to talk to both
Redis (Resque) and MongoDB GridFS.

The basic setup works fine when tested under ab, but we're having
trouble getting the deploy into production. It performs fine for a
bit, then the nginx write queue fills up and begins returning 502s. A
colleague has posted a more detailed description of the issue and our
setup on ServerFault:
http://serverfault.com/questions/398972/need-to-increase-nginx-throughput-to-an-upstream-unix-socket-linux-kernel-tun

Additional information on the environment
curbed@app1:~$ uname -a
Linux app1 3.2.0-24-generic #39-Ubuntu SMP Mon May 21 16:52:17 UTC
2012 x86_64 x86_64 x86_64 GNU/Linux
curbed@app1:~$ ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]
curbed@app1:~$ unicorn -v
unicorn v4.3.1
curbed@app1:~$ nginx -V
nginx version: nginx/1.2.1
built by gcc 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)
TLS SNI support enabled
configure arguments: --prefix=/usr/local/nginx --with-http_ssl_module
--with-cc-opt=-Wno-error --with-http_gzip_static_module
--with-http_stub_status_module
--add-module=/home/curbed/src/nginx-modules/nginx-gridfs
--add-module=/home/curbed/src/nginx-modules/ngx_http_redis-0.3.6
--add-module=/home/curbed/src/nginx-modules/headers-more-nginx-module

Kernel tweaks:
net.core.rmem_default = 65536
net.core.wmem_default = 65536
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.ipv4.tcp_rmem = 4096 87380 16777216
net.ipv4.tcp_wmem = 4096 65536 16777216
net.ipv4.tcp_mem = 16777216 16777216 16777216
net.ipv4.tcp_window_scaling = 1
net.ipv4.route.flush = 1
net.ipv4.tcp_no_metrics_save = 1
net.ipv4.tcp_moderate_rcvbuf = 1
net.core.somaxconn = 8192
net.netfilter.nf_conntrack_max = 131072

Any suggestions on configuration, kernel tuning, etc. would be
welcomed (here or on SF). Please CC me if you answer through the list.
Thanks for your time.

Eliot

-- 
Eliot Shepard
Head of Tech, Curbed Network
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