Docs for RTS option -I

[Bryan Richter]

The original docs are here:
https://downloads.haskell.org/ghc/latest/docs/users_guide/runtime_control.html#rts-flag--I%20%E2%9F%A8seconds%E2%9F%A9

They raised more questions than answers for me, so I took a stab at
rewriting them, and sprinkled in some questions where I need better
understanding. Anyone care to fill in my blanks?

-----------------

Set the amount of idle time which must pass before an idle GC is performed.
Setting -I0 disables the idle GC.

The idle GC only affects the threaded and SMP versions of the RTS (see
-threaded, Options affecting linking). When the idle GC is enabled, a major
GC is automatically performed if the runtime has been idle (i.e., no
Haskell computation has been running) for the specified period of time.

For an interactive application, it is probably a good idea to use the idle
GC, because this will allow finalizers to run and deadlocked threads to be
detected in the idle time when no Haskell computation is happening. [Why is
this a good thing? What happens when the idle GC is disabled?] Also, it
will mean that a GC is less likely to happen when the application is busy,
so application responsiveness may be improved. However, if the amount of
live data in the heap is particularly large, then the idle GC can cause a
significant penalty to responsiveness. [Why? Is it because the idle GC was
delayed by waiting for some idle time, and thus has more work to do?].
Conversely, too small of an interval could adversely affect interactive
responsiveness [How? And how is this worse than having idle GC disabled?
What is the actual behavior when it's disabled, anyway?]

The idle period timer only resets after some activity by a Haskell thread.
Therefore, once an idle GC is triggered, another one won't be scheduled
until more work is performed.

This is an experimental feature. Please let us know if it causes problems
and/or could benefit from further tuning.
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