patch: Enable continuous monitoring of mnesia overload status

[Ulf Wiger]

http://github.com/uwiger/otp/tree/mnesia_overload

Enable continuous monitoring of mnesia overload status

Mnesia currently issues an event whenever it detects an overload
condition. It recognizes two different types of overload:
- whenever the message queue of mnesia_tm process grows large
- when a log dump interval triggers before the previous dump is done

These events could be used to trigger a load regulation mechanism
to reduce the load until the condition seizes. The missing piece
is that there is no facility to ask mnesia whether the overload
condition still exists.

This patch implements a couple of functions in mnesia_lib that
can be used to sample the overload status. It has been tested
in a load regulator component being developed by Erlang Solutions.

No mnesia test suites have been run, since they are not available.
No documentation has been updated. The functions in mnesia_lib
are at any rate undocumented (as are all other functions in that
module). A decision would have to be made about whether to provide
a documented API on top of these functions.

The internal state record of mnesia_recover has been modified.
For this reason, a code change hook has been provided.


BR,
Ulf W
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