For beter understanding here is my config sniper

upstream super_upstream {
keepalive 128;
server be1 max_fails=45 fail_timeout=3s;
server be2 max_fails=45 fail_timeout=3s;
server be3 max_fails=45 fail_timeout=3s;
}

server {
server_name pytn.ru;
location ^~ /simple_requests {
proxy_read_timeout 2s;
proxy_send_timeout 2s;
proxy_connect_timeout 10ms;
proxy_next_upstream error timeout invalid_header http_500 http_502 http_503 http_504;
proxy_pass http://super_upstream;
}

location ^~ /very_heavy_requests {
send_timeout 60s;
proxy_read_timeout 60s;
proxy_send_timeout 60s;
proxy_connect_timeout 5ms;
proxy_next_upstream timeout;
proxy_pass http://super_upstream;
}