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JavaScript:SpiderMonkey:GC Futures: Difference between revisions
JavaScript:SpiderMonkey:GC Futures (view source)
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== JS GC Futures == | == JS GC Futures == | ||
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| Compartmentalize Gecko | | Compartmentalize Gecko | ||
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| MT wrappers | | MT wrappers | ||
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| Lock-free allocation and slot access | | Lock-free allocation and slot access | ||
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'''GCSubheaps''' — Factor GC-related code into a class, js::GCHeap ({{bug|556324}}). Carve out a second class, GCSubheap, so that a single GCHeap can have several GCSubheaps, each of which handles its own set of VM pages from which individual GC things may be allocated. Give each compartment its own GCSubheap. Allocate every object, double, and string from the GCSubheap for the compartment where it will live. | '''GCSubheaps''' — Factor GC-related code into a class, js::GCHeap ({{bug|556324}}). Carve out a second class, GCSubheap, so that a single GCHeap can have several GCSubheaps, each of which handles its own set of VM pages from which individual GC things may be allocated. Give each compartment its own GCSubheap. Allocate every object, double, and string from the GCSubheap for the compartment where it will live. | ||
'''MT wrappers''' — Implement an automatically-spreading membrane of proxy objects that allow one thread to access objects from another compartment that is running in another thread. There is some risk here because it's unclear how this should work regarding objects on the scope chain (global objects, Call and Block objects). See {{bug|558866}} comments 1-4. | '''MT wrappers''' — Implement an automatically-spreading membrane of proxy objects that allow one thread to access objects from another compartment that is running in another thread. There is some risk here because it's unclear how this should work regarding objects on the scope chain (global objects, Call and Block objects). See {{bug|558866}} comments 1-4. See also {{bug|566951}}, which has a prototype patch that addresses the scope chain issue. | ||
'''Lock-free allocation and slot access''' — Remove locking from allocation paths. Remove scope locking everywhere. ({{bug|558866}}) | '''Lock-free allocation and slot access''' — Remove locking from allocation paths. Remove scope locking everywhere. ({{bug|558866}}) | ||
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'''GC roots''' will be per-compartment. This means <code>JS_AddGCRootRT</code> will add a root to the default compartment. This behavior is a bit unexpected. To help embeddings get this right, GC should assert that each gc thing pointed to by a root is in the expected compartment. | '''GC roots''' will be per-compartment. This means <code>JS_AddGCRootRT</code> will add a root to the default compartment. This behavior is a bit unexpected. To help embeddings get this right, GC should assert that each gc thing pointed to by a root is in the expected compartment. | ||
''' | '''API functions related to the GC heap.''' Several API functions do something that involves the GC heap: <code>JS_GC</code> and friends; <code>JS_SetGCCallback</code>; <code>JS_SetGCParameter</code>; <code>JS_TraceChildren</code> and friends; <code>JS_DumpHeap</code>; <code>JS_SetGCZeal</code>. These will need to have a mode for collecting/walking the entire heap and a separate mode where they apply to just one compartment. TBD. | ||
'''Wrapper API.''' Since the cross-compartment reference from a wrapper to the wrappee is so special, we will need API for it. TBD. | '''Wrapper API.''' Since the cross-compartment reference from a wrapper to the wrappee is so special, we will need API for it. TBD. | ||
== Emerging Invariants == | |||
This section describes invariants and rules which have emerged during initial development of the conservative GC and the compartments code. They are not likely to change, but still may. | |||
* The C stack is not scanned for GC roots when there are no contexts (suspended or otherwise) in requests on a given thread | |||
* When doing a single-compartment GC, only the current thread's stack is scanned (unless there are no contexts in requests on that thread) | |||
* A context's compartment is equal to JS_GetScopeChain(cx)->getCompartment. A NULL scope chain indicates the default compartment. | |||
* Corollary: All non-default compartments have at least a global object. | |||
* Only one thread per compartment may be in a request at any given time | |||