[Oz] Oz complaints

[Peter Van Roy]

> 
> > What exactly do you mean when you write about distributed garbage
> > collection? How stable is it?
> 
> Oz supports distributed computations where Oz processes arbirarily
> located in the network may transparently operate on the same
> data-structures, as if they were sharing the same memory store.  Thus,
> a data-structure (a term) may span more than one site.  Distributed
> garbage collection refers to the collection of such data when it is no
> longer referenced by any site.
> 
> Perhaps Peter can elaborate on this issue.

The distributed GC collects all inaccessible data in any distributed
execution except for cross-site cycles of stateful data.  For example,
two stationary objects on different sites that reference each other.  In
practice, this limitation is very easy to live with, e.g., it suffices
for a server that is no longer in touch with a given client to remove the 
reference to that client.

More technically: each site does a copying GC, and between sites there
is a weighted reference counting technique.  The weights are chosen so
that the extra message overhead of GC is practically zero.

You can find more information in the research papers related to Mozart.
For example, check out Section 8.2.1 in "Efficient Logic Variables
for Distributed Computing" in the May 1999 issue of ACM TOPLAS.

The distributed GC is quite stable--we have long-running distributed
applications and we have been using it for some time now (more than
a year).

If you want a complete list of limitations of the Mozart system, I
suggest you check out the document 'Limitations' on the Mozart Web
site, in the Documentation page.

Peter

-- 
Prof. Peter Van Roy
Département d'Ingénierie Informatique
Université catholique de Louvain
B-1348 Louvain-la-Neuve, Belgium

Email: pvr at info.ucl.ac.be
Tel: (+32) (10) 47.83.74
Web: http://www.info.ucl.ac.be/~pvr/
Mozart: http://www.mozart-oz.org


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