So, zones are roughly analogous to a JSRuntime from the embedder's POV? Wes
On 18 March 2013 14:19, Bill McCloskey <[email protected]> wrote: > Hi everyone, > > Zones landed over the weekend and it looks like it's going to stick this > time. I saw a few questions this morning on IRC about what the new > invariants are, so I wrote a comment explaining things as best I could (bug > 852229). I'm attaching it in the email. Any input would be welcome. > > /* > * A zone is a collection of compartments. Every compartment belongs to > exactly > * one zone. In Firefox, there is roughly one zone per tab along with a > system > * zone for everything else. Zones mainly serve as boundaries for garbage > * collection. Unlike compartments, they have no special security > properties. > * > * Every GC thing belongs to exactly one zone. GC things from the same zone > but > * different compartments can share an arena (4k page). GC things from > different > * zones cannot be stored in the same arena. The garbage collector is > capable of > * collecting one zone at a time; it cannot collect at the granularity of > * compartments. > * > * GC things are tied to zones and compartments as follows: > * > * - JSObjects belong to a compartment and cannot be shared between > * compartments. If an object needs to point to a JSObject in a different > * compartment, regardless of zone, it must go through a cross-compartment > * wrapper. Each compartment keeps track of its outgoing wrappers in a > table. > * > * - JSStrings do not belong to any particular compartment, but they do > belong > * to a zone. Thus, two different compartments in the same zone can point > to a > * JSString. When a string needs to be wrapped, we copy it if it's in a > * different zone and do nothing if it's in the same zone. Thus, > transferring > * strings within a zone is very efficient. > * > * - Shapes and base shapes belong to a compartment and cannot be shared > between > * compartments. A base shape holds a pointer to its compartment. Shapes > find > * their compartment via their base shape. JSObjects find their > compartment > * via their shape. > * > * - Scripts are also compartment-local and cannot be shared. A script > points to > * its compartment. > * > * - Type objects and IonCode objects belong to a compartment and cannot be > * shared. However, there is no mechanism to obtain their compartments. > * > * A zone remains alive as long as any GC things in the zone are alive. A > * compartment remains alive as long as any JSObjects, scripts, shapes, or > base > * shapes within it are alive. > * > * We always guarantee that a zone has at least one live compartment by > refusing > * to delete the last compartment in a live zone. (This could happen, for > * example, if the conservative scanner marks a string in an otherwise dead > * zone.) > */ > > -Bill > _______________________________________________ > dev-tech-js-engine-internals mailing list > [email protected] > https://lists.mozilla.org/listinfo/dev-tech-js-engine-internals -- Wesley W. Garland Director, Product Development PageMail, Inc. +1 613 542 2787 x 102 _______________________________________________ dev-tech-js-engine-internals mailing list [email protected] https://lists.mozilla.org/listinfo/dev-tech-js-engine-internals
