Nginx not scaling more than 750 Connections when used as Reverse Proxy - nginx

Question

I have a mobile application in which we are using Nginx as a Reverse proxy which routes request to Nginx. The Nginx request in app server is passed to Node.js for processing.
We are getting 504 Gateway timeout error when we are hitting more than 750 Users.we are seeing below error in Nginx Logs.
upstream timed out (110: Connection timed out) while connecting to upstream, client: LoadGenerator_IP, server: WebserverDNS, request: "GET /api/sample/profile HTTP/1.1", upstream: "https://app_server:443/api/sample/profile", host: "webserver_IP"
I tried to hit App server Nginx directly.We could able to hit more than 1000 users.But if we are using reverse proxy we are getting that error.
I tried alot of Linux system and Nginx settings.But did not overcome this issue.
NGINX.CONF
user nginx;
worker_processes auto;
worker_rlimit_nofile 100000;
error_log /var/log/nginx/error.log error;
pid /var/run/nginx.pid;
events {
worker_connections 4096;
use epoll;
multi_accept on;
}
http {
include /etc/nginx/mime.types;
default_type application/octet-stream;
log_format main '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"'
'uct="$upstream_connect_time"'
'uht="$upstream_header_time"'
'urt="$upstream_response_time"'
'rt="$request_time "';
access_log /var/log/nginx/access.log main;
sendfile on;
#tcp_nopush on;
#tcp_nodelay on;
# to boost I/O on HDD we can disable access logs
access_log off;
keepalive_timeout 120;
keepalive_requests 10000;
# allow the server to close connection on non responding client, this will free up memory
reset_timedout_connection on;
# request timed out -- default 60
client_body_timeout 10;
# if client stop responding, free up memory -- default 60
send_timeout 2;
#gzip on;
# gzip_static on;
#gzip_min_length 10240;
#gzip_comp_level 1;
#gzip_vary on;
#gzip_disable msie6;
#gzip_proxied expired no-cache no-store private auth;
#gzip_types
# text/html is always compressed by HttpGzipModule
# text/css
# text/javascript
# text/xml
# text/plain
# text/x-component
# application/javascript
# application/x-javascript
# application/json
# application/xml
# application/rss+xml
# application/atom+xml
# font/truetype
# font/opentype
# application/vnd.ms-fontobject
# image/svg+xml;
include /etc/nginx/conf.d/Default.conf;
}
Default.conf
upstream app_server {
server app_server_ip:443;
}
server {
listen 80;
listen 443 ssl backlog=32768;
server_name someIP;
server_tokens off;
location ~ {
proxy_pass https://app_server_ip;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header Host $host;
#proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_connect_timeout 75;
proxy_read_timeout 300;
proxy_send_timeout 300;
proxy_http_version 1.1;
proxy_set_header Connection "";
}
ssl_certificate /etc/ssl/certs/uatweb.crt;
ssl_certificate_key /etc/ssl/certs/uatweb.key;
ssl_protocols TLSv1.1 TLSv1.2;
ssl_prefer_server_ciphers On;
ssl_ciphers ECDH+AESGCM:ECDH+CHACHA20:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:RSA+AESGCM:RSA+AES:!aNULL:!MD5:!DSS;
#charset koi8-r;
#access_log /var/log/nginx/host.access.log main;
location / {
root /usr/share/nginx/html;
index index.html index.htm;
}
#error_page 404 /404.html;
# redirect server error pages to the static page /50x.html
#
error_page 500 502 503 504 /50x.html;
location = /50x.html {
root /usr/share/nginx/html;
}
}
server {
listen 8114 ssl;
server_name someServerName;
ssl_certificate /etc/ssl/certs/uatweb.crt;
ssl_certificate_key /etc/ssl/certs/uatweb.key;
ssl_protocols TLSv1.1 TLSv1.2;
location ~ {
proxy_pass https://someIP;
}
error_page 500 502 503 504 /50x.html;
location = /50x.html {
root /usr/share/nginx/html;
}
}
Please let me know if any changes are required in Config Files.