rails server using nginx + puma on ubuntu - suddenly get socket connect failed errors

Question

I have ran jruby on rails in AWS ubuntu trusty 14.04 configuration. my server was going well during several hours. but suddenly get socket connect failed error.

-nginx error.log

2016/03/12 10:12:10 [error] 28444#0: *256477 connect() to unix:/home/deploy/shared/tmp/sockets/puma.sock failed (11: Resource temporarily unavailable) while connecting to upstream, ...
2016/03/12 10:12:11 [error] 28444#0: *256500 connect() to unix:/home/deploy/shared/tmp/sockets/puma.sock failed (111: Connection refused) while connecting to upstream, ...
2016/03/12 10:12:17 [crit] 28444#0: *256512 connect() to unix:/home/deploy/shared/tmp/sockets/puma.sock failed (2: No such file or directory) while connecting to upstream, ...

first, 11: Resource temporarily unavailable error appeared then, 111: Connection refused error, and finally, 2: No such file or directory error keep appearing.

I searched the problem and get this answer. -> Need to increase nginx throughput to an upstream unix socket -- linux kernel tuning?

so, I edited /etc/sysctl.conf

net.core.somaxconn = 1024
net.core.netdev_max_backlog = 1000

but the problem not solved.. still getting fxxking errors..

when I get connection failed errors, I restart puma server. then the server do its jobs well.

what's the problem? I can't keep restarting all day. help me, please.

nginx.conf

user www-data;
worker_processes 4;
pid /run/nginx.pid;

events {
    worker_connections 1536;
}

http {

    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 65;
    types_hash_max_size 2048;

    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;

    gzip on;
    gzip_disable "msie6";

    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
}