Boosted default nginx.conf, based on https://github.com/denji/nginx-tuning

nginx.conf

# default nginx runtime settings
user www-data;
pid /run/nginx.pid;

# you must set worker processes based on your CPU cores, nginx does not benefit from setting more than that
worker_processes auto; #some last versions calculate it automatically

# number of file descriptors used for nginx
# the limit for the maximum FDs on the server is usually set by the OS.
# if you don't set FD's then OS settings will be used which is by default 2000
worker_rlimit_nofile 100000;

# only log critical errors
error_log /var/log/nginx/error.log crit;

# provides the configuration file context in which the directives that affect connection processing are specified.
events {
    # determines how much clients will be served per worker
    # max clients = worker_connections * worker_processes
    # max clients is also limited by the number of socket connections available on the system (~64k)
    worker_connections 4000;

    # optmized to serve many clients with each thread, essential for linux -- for testing environment
    use epoll;

    # accept as many connections as possible, may flood worker connections if set too low -- for testing environment
    multi_accept on;
}

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

    # ssl settings
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
    ssl_prefer_server_ciphers on;

    # cache informations about FDs, frequently accessed files
    # can boost performance, but you need to test those values
    open_file_cache max=200000 inactive=20s; 
    open_file_cache_valid 30s; 
    open_file_cache_min_uses 2;
    open_file_cache_errors on;

    # to boost I/O on HDD we can disable access logs
    access_log off;

    # copies data between one FD and other from within the kernel
    # faster then read() + write()
    sendfile on;

    # send headers in one peace, its better then sending them one by one 
    tcp_nopush on;

    # don't buffer data sent, good for small data bursts in real time
    tcp_nodelay on;

    # reduce the data that needs to be sent over network -- for testing environment
    gzip on;
    gzip_min_length 10240;
    gzip_proxied expired no-cache no-store private auth;
    gzip_types text/plain text/css text/xml text/javascript application/x-javascript application/json application/xml;
    gzip_disable msie6;

    # allow the server to close connection on non responding client, this will free up memory
    reset_timedout_connection on;

    # if the request body size is more than the buffer size, then the entire (or partial)
    # request body is written into a temporary file
    client_body_buffer_size  128k;

    # headerbuffer size for the request header from client -- for testing environment
    client_header_buffer_size 3m;

    # maximum number and size of buffers for large headers to read from client request
    large_client_header_buffers 4 256k;

    # read timeout for the request body from client -- for testing environment
    client_body_timeout 3m;

    # how long to wait for the client to send a request header -- for testing environment
    client_header_timeout 3m;

    # if client stop responding, free up memory -- default 60
    send_timeout 2;

    # server will close connection after this time -- default 75
    keepalive_timeout 30;

    # number of requests client can make over keep-alive -- for testing environment
    keepalive_requests 100000;

    # limit the number of connections per single IP
    limit_conn_zone $binary_remote_addr zone=conn_limit_per_ip:10m;

    # limit the number of requests for a given session
    limit_req_zone $binary_remote_addr zone=req_limit_per_ip:10m rate=5r/s;

    # zone which we want to limit by upper values, we want limit whole server
    server {
        limit_conn conn_limit_per_ip 10;
        limit_req zone=req_limit_per_ip burst=10 nodelay;
    }

    # virtual host configs
    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
}