NGINX 500 http error (4 replies)

Hi,

I have a lot of 500 http error in nginx access.log.
I think it's due to the number of requests that nginx receives. Because when I was testing the configuration with one ip adress I don't have this error but when I redirect all client traffic (sometimes more than 2000 simultaneous connexions) to nginx I'm getting 500 http error.
I don't have any error log in error.log.

I tried to modify this parameters : worker_connections, worker_rlimit_nofile, keepalive_timeout, proxy_connect_timeout ...

And then I start to get this logs in error.log :
worker process XXXXX exited on signal 11
an upstream response is buffered to a temporary file /var/lib/nginx/proxy/2/00/0000000002
failed (104: Connection reset by peer) while sending response to client


Here is my nginx.log conf :

user www-data;
worker_processes 4;
worker_rlimit_nofile 50000;
pid /var/run/nginx.pid;

events {
worker_connections 50000;
}

http {

sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 20;

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";

proxy_redirect off;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
client_max_body_size 10m;
client_body_buffer_size 50M;
proxy_connect_timeout 600;
proxy_send_timeout 600;
proxy_read_timeout 600;
proxy_buffer_size 128k;
proxy_buffers 4 254k;
proxy_busy_buffers_size 256k;

include /etc/nginx/sites-enabled/*;
}

When I execute top command, cpu and RAM usage are not hignt (max cpu 7% and ram 4%).

Any help would be appreciated.
Thanks.