Path: utzoo!attcan!uunet!mcsun!ukc!cam-cl!scc
From: scc@cl.cam.ac.uk (Stephen Crawley)
Newsgroups: comp.sw.components
Subject: Re: Real-time Garbage Collection
Message-ID: <901@scaup.cl.cam.ac.uk>
Date: 21 Sep 89 01:27:59 GMT
References: <1917@hydra.gatech.EDU> <6506@hubcap.clemson.edu>
Sender: news@cl.cam.ac.uk
Organization: U of Cambridge Comp Lab, UK
Lines: 29

[Sorry about the Re:^n's ... bugs in my news reader]

Bill Wolfe proposes a scheme for getting around the need for garbage 
collection by storing objects in a database, using object expiration 
times to avoid proper garbage collection.  Unfortunately this scheme
doesn't achieve its aim.

Bill says: 

  "Where objects can have an infinite lifetime, a protocol can be devised 
   whereby the database must be checked at least once per some arbitrary 
   time period by all users, which will permit identification numbers 
   to be recycled after one full time period of nonexistence."

Exactly.  In order to check each object that it still refers to, each 
"user" (actually application) must traverse the graph of its objects in 
the database.  This process is exactly equivalent to the mark phase of 
a garbage collection, except that the application is doing it!!!

One other point that Bill Wolfe misses is that the CPU overheads of 
managing objects in this way can be orders of magnitude greater
than the cost of using ordinary pointers.

Don't get me wrong, I've been doing research in this particular area for 
a number of years, and I strongly believe that object databases are the 
way of the future.  But they ain't cheap, and they ain't a substitute
for garbage collection.

-- Steve