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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: How to get Ada to ?cross the chasm??
Date: Wed, 9 May 2018 09:41:42 +0300
Organization: Tidorum Ltd
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Xref: reader02.eternal-september.org comp.lang.ada:52144
Date: 2018-05-09T09:41:42+03:00
List-Id: <comp.lang.ada>

On 18-05-09 08:02 , Paul Rubin wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>> If referenced object counts are > 1 they are not going to be
>> finalized. So the point stands, no locking is ever required upon
>> finalization.
>
> I'm still perplexed by this.  You have to decrement all those refcounts.
> While this is happening, other threads may also be messing with them.
> You need locks (at least in the form of atomic instructions which work
> by hardware locks, i.e. that are much slower than normal instructions)
> to prevent data races.

But the atomic instructions are usually much faster than a kernel lock.

> When you decrement the counts, some of them might reach zero so their
> objects need freeing (and traversal).  That too can be arbitrarily
> complicated.  Niklas mentions you can possibly do that incrementally,
> but that sounds complicated

As I recall, the method is that if you find that an object's reference 
count becomes zero, but you don't want to spend time *right now* to 
recursively decrement references from that object, you add the object to 
a global list of "garbage objects", and handle it later, as your 
recycling budget allows, or if no other memory is left. Not very 
complex, but one would have to prove that there is enough time to 
recycle objects "in the background" without hampering real-time 
behaviour, which is perhaps not so easy. But real-time GC is more 
complex, I believe.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .