Nginx socket reverse proxy got 503 responses on concurrent requests

I am using Nginx as a reverse proxy for my PHP base WebSocket application and I try to load test the WebSocket server with Nginx reverse proxy. And I got 503 errors when concurrent users reach around 1,000.

But when I test the PHP application directly without Nginx reverse proxy, the application can handle over 5,000 concurrent users.

My Nginx Version nginx version: nginx/1.18.0

The following is my Nginx configuration.

user nginx;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;
worker_rlimit_nofile 30000;

events {
        worker_connections 10000;
        multi_accept on;
}

http {
        ##
        # Basic Settings
        ##

        sendfile on;
        tcp_nopush on;
        types_hash_max_size 2048;
        # server_tokens off;

        server_names_hash_bucket_size 128;
        # server_name_in_redirect off;

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

        ##
        # SSL Settings
        ##

        ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3; # Dropping SSLv3, ref: POODLE
        ssl_prefer_server_ciphers on;

        ##
        # Logging Settings
        ##

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

        ##
        # Gzip Settings
        ##

        gzip on;

        # gzip_vary on;
        # gzip_proxied any;
        # gzip_comp_level 6;
        # gzip_buffers 16 8k;
        # gzip_http_version 1.1;
        # gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;

        ##
        # Virtual Host Configs
        ##

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

The following is my Nginx site configuration.

server {
  listen        80;
  listen        [::]:80;
  server_name   socket.mydomain.com;

  location / {
    proxy_pass                          http://127.0.0.1:6001;
    proxy_set_header Host               $host;
    proxy_set_header X-Real-IP          $remote_addr;

    proxy_set_header X-Forwarded-For    $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto  https;
    proxy_set_header X-VerifiedViaNginx yes;
    proxy_read_timeout                  60;
    proxy_connect_timeout               60;
    proxy_redirect                      off;

    # Specific for websockets: force the use of HTTP/1.1 and set the Upgrade header
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection 'upgrade';
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;
  }
}

Is something wrong with my configuration? Or do I miss to update anything?