Nginx log error like the following:
2014/04/06 23:54:32 [error] 13340#0: *25756 upstream timed out (110: Connection timed out) while reading response header from upstream

Add three timeouts, the problem is resolved.

nginx.conf:
http{
fastcgi_connect_timeout 300s;
fastcgi_send_timeout 300s;
fastcgi_read_timeout 300s;

#fastcgi_buffer_size 128k;
#fastcgi_buffers 8 128k;
#fastcgi_busy_buffers_size 256k;
#fastcgi_temp_file_write_size 256k;
#fastcgi_intercept_errors on;

}