How to prevent running out of connections?

[Rt Ibmer]

[Hello - I sent this last week but did not receive any responses so I am resending it in hopes of getting some information. Thank you!]
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How do I make sure we don't run out of available connections for our nginx 6 box?

For instance when I run stub_status I see that I have x active connections.  I can also see how the keepalive setting in nginx impacts this number.

It seems that under heavy load where connections are coming in at a much faster rate than the keepalive timeout is expiring, that things could get to a point where the number of active connections becomes too great and then there would not be enough connections available for new users.

A few questions please:

1) Is such a situation that I described above possible?

2) In such a situation, would nginx automatically free the idle keepalive connections so that it could have more connections available for use?

3) What nginx and/or Linux settings (I am using Fedora core 8 if that matters) controls what the maximum number of connections is that my box can have active?  How can this be bumped up?

Obviously under some heavy load a box will reach a physical maximum that it is capable of serving.  That is ok and why we have other boxes available.  However I want to understand how this works so that I can keep an eye on the connections and say "Hey that's getting awfully close to our limit - time to get another box up!".  Right now I have no idea how to predict what that limit is.  Thanks!