What is the real effect of upstream_keepalive_pool_size?

Hi all,
I am tunning these three parameters:

and

upstream_keepalive_max_requests

upstream_keepalive_idle_timeout

I have clear what’s the meaning and what to expext of the last two, but I dont know how this pool size has effect:

  • I have set to 1 and I see that more than 1 keepalive connections for an upstream (with several ips returned) are created and holded according to idle timeout and max requests
  • If I leave the default value of 60, the behaviour creating/opening connections is the same

Is a kind of global parameter or something internal that is shared in the memory of every worker ?