Personal tools

Global nsICycleCollector service

From MozillaWiki

Jump to: navigation, search
Warning sign
Warning: The content of this article may be out of date. The cycle collector was implemented and is part of Firefox 3, but its implementation may differ from the description here.

Contents

Problem overview

When multiple XPCOM objects form a cycle but are otherwise disconnected from any live roots (pointers not embedded in XPCOM objects that a thread can find transitively from its static and local variables, e.g. local nsCOMPtr variables) it is considered a garbage cycle. Currently XPCOM cannot collect garbage cycles. This is a proposal to produce a collector for such cycles.

Common sources of garbage cycles in XPCOM appear to be cycles between javascript objects and browser objects, particularly DOM objects.

Various requirements

Any cycle-collecting algorithm must satisfy a few constraints imposed by the environment.

  • Some objects will never be upgraded to participate in cycle collection. The mechanism must not break if it is applied to only a subset of the objects in the graph.
  • XPCOM objects cannot generally be freed in any consistent fashion, such as by calling "operator delete". Objects should be made to "self destruct" by having all their incoming edges drop.
  • Adding gratuitous new interfaces, vtables, or pointer state is probably too expensive.

A basic cycle collection algorithm

A "concurrent" cycle collection algorithm is presented here: http://citeseer.ist.psu.edu/paz03fly.html

We may wish to implement the simpler, non-concurrent ("stop the world") variant, which avoids using orange and red markers.

A sketch of an implementation in XPCOM

Some notes on the implementation:

  • The implementation requires that each participating XPCOM class implement nsIClassInfo2. The two methods on this interface must be correct: it is better not to implement nsIClassInfo2 than to implement it wrong.
    • unlink(obj) should disconnect all outgoing XPCOM references held by obj.
    • traverse(obj,refcount,childcount,children) should return the refcount for obj, as well as the count and an array of outgoing XPCOM references from obj.
  • An object implementing nsIClassInfo2 should also use the NS_IMPL_CYCLE_COLLECTING_* macros in the nsISupportsImpl header; these implementations hook the appropriate refcount operations to communicate with the global cycle collector.
  • The implementation commandeers a single bit from the refcount stored in nsAutoRefCnt. This is not strictly necessary, but should cut down significantly on pointless traffic to the cycle collector.
  • The global cycle collector service is kept as a singleton pointer in nsAutoRefCnt. This poses potential problems during shutdown. Advice on how to handle this safely would be appreciated.