[MUD-Dev] Re: atomic functions

J C Lawrence claw at under.engr.sgi.com
Thu May 7 15:57:02 New Zealand Standard Time 1998

On Tue, 5 May 1998 14:52:12 +0200 (MET DST) 
Felix A Croes<felix at xs1.simplex.nl> wrote:

> Jon A. Lambert <jlsysinc at ix.netcom.com> wrote:

>> I don't see how any threads are guaranteed execution in C&C, unless
>> they are evntually escalated to single-threading as per your
>> earlier reference.  How does atomic prevent another thread from
>> accessing and updating objects, unless you single thread/task it?

> What I wrote is wrong -- if N events are executing and the first one
> to complete succeeds, then the ones that complete after that may all
> fail, so after completion of the first event, none of the events
> executing at that moment will succeed.  But in C&C, an event that is
> itself doomed cannot block the progress of another event.  Since an
> event can only be doomed by another event that already completed
> successfully, progress is guaranteed in the system as a whole.  In
> the worst case scenario above, one event would immediately complete
> successfully.

Rephrasing only:

  In the C&C model, the only time an event fails C&C and has to
reschedule is when another event managed to successfully C&C during
its execution.  That's the only way you can get a changed object to
cause the C&C failure.  Or to put it another way, given N executing or 
about to be executing events, at the end of the iteration you are
guaranteed that at least one of them will successfully C&C.

The only reason to degrade the threading model down towards
single-tasking is if you have an event which needs to compleat, but is 
failing due to C&C failures caused by other more sprightly events
"getting in there first".  The classic scenario on-list is Bubba
digging the Panama Canal.

  eg If the "dig panama canal" event were coded to do the whole thing in a
single very long execution, you are it is likely that it would never
C&C due to other shorter/faster events getting in and touching objects 
before the "big event" compleats.  

C&C has guaranteed thru-put, it just doesn't guarantee much more than
a trickle.

Musing: Do you think when 50 years from now someone does a
restrospective or history of the early years of VR...there'll be an
entry under "Bubba"?

BTW: Jon, how do you handle IO for event failures?  Buffer and

J C Lawrence                               Internet: claw at null.net
(Contractor)                               Internet: coder at ibm.net
---------(*)                     Internet: claw at under.engr.sgi.com
...Honourary Member of Clan McFud -- Teamer's Avenging Monolith...

MUD-Dev: Advancing an unrealised future.

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