When nginx will send Connection timed out error?

I am using nginx as proxy server. I got following error once:
upstream timed out (110: Connection timed out) while connecting to

In what situtaion I will get this error? Exaple:

  1. Upstream didn’t accept connection within x time.
    Is there any other case when I can get this error or is it only one case
    when I will get this error?
  2. Saving or reading to tmp file buffer was slow.
  3. Nginx was running out of connection?

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