Idea #1 on integrating RC with GC

[Ola Fosheim Grøstad" <ola.fosheim.grostad+dlang at gmail.com>]

On Wednesday, 5 February 2014 at 12:12:01 UTC, Paulo Pinto wrote:
> Yes, the GC just needs to check roots for already released 
> blocks, if I am not mistaken.

Yes, when they go to non-zero. This is the scheme used in PHPs 
ARC/GC solution, published in this paper:

http://researcher.watson.ibm.com/researcher/files/us-bacon/Bacon01Concurrent.pdf

You push roots that are candidates onto a queue when the counter 
is decreased to nonzero, then you do a concurrent scan when you 
have a set of roots to scan for cycles. So you probably need a 
clever ARC to reduce the scanning.

I think however, that for most programs that use ARC you don't do 
the GC at all. For long lived processes such as servers you might 
run it using heuristics (based on memory headroom or during the 
night). According to the paper above the amount of cyclic garbage 
tends to be low, but varies a great deal. They site another paper 
relating to Inferno that supposedly claims that RC caught 98% of 
the garbage. So the need for cycle collection is rather 
application specific.

Perl has traditionally not caught cycles at all, but then again 
perl programs tends to be short lived.

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There are also some papers on near real time GC, such as Staccato 
and Metronome, on David F. Bacons list:

http://researcher.watson.ibm.com/researcher/view_pubs.php?person=us-bacon&t=1