I'm somewhat unclear about how the keepalive functionality works within the upstream module. My nginx install currently handles several hundred domains all of which point to different origin servers. I would imagine I can improve performance by enabling keepalive, however the documentation says "The connections parameter sets the maximum number of idle keepalive connections to upstream servers that are preserved in the cache of each worker process. When this number is exceeded, the least recently used connections are closed. "

Does that mean that if I have 10 domains and then set keepalive to 32, that there will potentially be up to 320 open connections from my server to the backend servers per worker at any given point or would the worker share all upstreams and only open a total of 32 regardless of how many upstream blocks were on the website? Also, does the number of keepalive connections have anything to do with the number of cores on a box?

Also, is there any downside to having a large number of upstreams in the http block? I know for "map" block there's no performance degradation since they're only evaluated on demand, but I don't see any kind of documentation regarding how upstreams are handled.

Thanks!