Trying to Understand Upstream Keepalive

[newnovice]

Maxim, 

http://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive

I would like to know what is the keepalive timeout for this connection pool?
Is it static? 

Also i want to understand - if there is a marriage between number of
connections nginx gets vs how many it opens to upstream? 

Thanks!

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,249924,254393#msg-254393