[coyotos-dev] Activations

Christopher Nelson nadiasvertex at gmail.com
Tue May 22 10:02:10 EDT 2007

> Given that the same functionality can be accomplished with a same-space
> "external" handler, and given also that there appears to be no
> performance advantage to activation handlers because of the capability
> register issue, I have two questions:
>   1. Do you still feel that activation handlers are motivated?
>   2. Do you see any examples of real things you want to do that cannot
>      be handled with an external fault handler that shares an address
>      space with the "base" process?

Hmm. Well, having given it some thought, I think that the space sharing
fault handler still has a problem that the activation handler does not: that
being the need to be multiplex.  My concern here is that a single
dispatching fault handler for many processes and/or threads introduces a
single point of failure.  How do threads and processes get their faults?  If
they are posted to a queue for each thread/process, okay, but then the queue
still has to be checked and involves requisite overhead.  If the fault
handler immediately dispatches the fault to the appropriate thread/process
handler, then those handlers could die or hang or otherwise take far too
long causing cascading problems to other threads and processes depending on
that central fault handler.

It also seems like it could induce false sharing between independent threads
and processes, where delays in fault handling by other threads or processes
could cause those to propagate and slow down groups who's processing is
otherwise unrelated.  Of course, this assumes that you are using these
handlers for various types of notification (like memory mapping / etc.)

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