Re: Daily gclist MIME digest V5 #121

On 1/11/2010 6:52 PM, Mingnan Guo wrote:
> Hi all,
>
> I want to say something about the terminology of "Concurrent", "On-The-Fly" and "Full Concurrent".
>
> Many technical papers have used the term "Concurrent" and "On-The-Fly" regarding stack scanning as:
>
> "Concurrent garbage collectors run concurrently with the application and only stop it for a short synchronization phase in the beginning or end of the collection. On-the-fly collectors are special concurrent collectors that never need to stop all threads simultaneously. They stop the application one thread at a time typically for scanning the thread runtime stack"
>
> However, some concurrent GC algorithms do not perform stack scanning at all. As far as I know, there are at least two implementations of that kind of GC. One is HnxGC based on reference counting, another one is not reference counting based. These kinds of GCs are so much different from those "Concurrent" and "On-The-Fly" on stack scanning. So, using a different terminology, such as "Full Concurrent", is more appropriate for these GC algorithms to avoid confusion. For example,
>
> "Full Concurrent garbage collectors run concurrently with the application and behave like usual application threads, which never involuntarily stop other threads except for debug purpose."
>
> Any comment? Thanks,
>
> -Mingnan Guo

In slightly more colloquial English, I would say "Fully Concurrent".

Concerning "Concurrent" versus "On-the-fly", I would agree that the
difference is whether all threads are stopped at once. It is indeed
possible to write collectors where the mutators never need to block
(unless storage is exhausted; with adequate resources, bounds on
mutator allocation rate, etc., it is possible to avoid storage
exhaustion). However, such approaches may need more frequent or
more costly read or write barriers in the mutators. Reference
counting meeds to be augmented with a cycle collector, and in
general you may still need to support compaction in order to
repair fragmentation ....

Best wishes -- Eliot Moss