Error 502 in nginx + php5-fpm

The issue is socket itself, its problems on high-load cases is well-known. Please consider using TCP\IP connection instead of unix socket, for that you need to make these changes:

• in php-fpm pool configuration replace listen = /var/run/php5-fpm.sock with listen = 127.0.0.1:7777
• in /etc/nginx/php_location replace fastcgi_pass unix:/var/run/php5-fpm.sock; with fastcgi_pass 127.0.0.1:7777;