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From: George Neuner <gneuner2@comcast.net>
Newsgroups: comp.arch
Subject: Re: Tonight's tradeoff
Date: Sun, 10 Mar 2024 14:29:52 -0400
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On Fri, 8 Mar 2024 20:26:08 -0500, Robert Finch <robfi680@gmail.com>
wrote:

>I plan on having garbage collection as part of the OS. There is a shared 
>hardware-card table involved. 

What kind?  
[Actually "kind" is the wrong word because any non-toy, real world GC
will need to employ a combination of techniques.  So the question
really should be "in what major class is your GC"?]

Problem is - whatever you choose - it will be wrong and have bad
performance for some important class of GC'd applications.


>So, I guess that would disallow user 
>garbage collectors using untouchable pages. The MMU could be faked out 
>using a VM, so I have read.

Yes, a VM can emulate MMU operation, but currently that requires using
a hypervisor - a heavyweight solution that also requires a guest OS to
run the program.

There are a number of light(er) weight VMs for running programs in
managed environments [which include GC] ... but all of them have to
run under an OS and are at its mercy.

GCs that use no-access pages are not rare, and they are just one class
of MMU assisted GC systems.  There are a number of ways a collector
can leverage the MMU to help.