Started using Google Flex Vms recently and in the logs there are multiple requests to /_ah/background that last ~1 hour each time. The only reference to these I could find is this question which mentions they have to do with background threads but I don't believe that's the case here as:
Any ideas?
/_ah/background is used in flex for AE API calls that are called outside of an incoming request processing context (like threads, async-io, ...).
Even if you don't do that directly log flushing is still done asynchronously (not part of an income request processing).
This is an implementation details and there is a plan to hide it but still find a way to show (in log, trace,...) information about these API calls.