20. MRG pool class

20.1. Introduction

.readership: Any MPS developer.

.intro: This is the design of the MRG (Manual Rank Guardian) pool class. The MRG pool class is part of the MPS. The MRG pool class is internal to the MPS (has no client interface) and is used to implement finalization.

.source: Some of the techniques in paper.dbe93 (“Guardians in a Generation-Based Garbage Collector”) were used in this design. Some analysis of this design (including various improvements and some more in-depth justification) is in analysis.mps.poolmrg. That document should be understood before changing this document. It is also helpful to look at design.mps.finalize and design.mps.message.

20.2. Goals

.goal.final: The MRG pool class should support all requirements pertaining to finalization.

20.3. Requirements

.req: We have only one requirement pertaining to finalization:

.req.dylan.fun.finalization: Support the Dylan language-level implementation of finalized objects: objects are registered, and are finalized in random order when they would otherwise have died. Cycles are broken at random places. There is no guarantee of promptness.

.req.general: However, finalization is a very common piece of functionality that is provided by (sophisticated) memory managers, so we can expect other clients to request this sort of functionality.

.anti-req: Is it required that the MRG pool class return unused segments to the arena? MFS, for example, does not do this. MRG will not do this in its initial implementation.

20.4. Terminology

.def.mrg: MRG: The MRG pool class’s identifier will be MRG. This stands for “Manual Rank Guardian”. The pool is manually managed and implements guardians for references of a particular rank (currently just final).

.def.final.ref: final reference: A reference of rank final (see design.mps.type.rank).

.def.final.object: finalizable object: An object is finalizable with respect to a final reference if, since the creation of that reference, there was a point in time when no references to the object of lower (that is, stronger) rank were reachable from a root.

.def.final.object.note: Note that this means an object can be finalizable even if it is now reachable from the root via exact references.

.def.finalize: finalize: To finalize an object is to notify the client that the object is finalizable. The client is presumed to be interested in this information (typically it will apply some method to the object).

.def.guardian: guardian: An object allocated in the MRG Pool. A guardian contains exactly one final reference, and some fields for the pool’s internal use. Guardians are used to implement a finalization mechanism.

20.5. Overview

.over: The MRG pool class is a pool class in the MPS. It is intended to provide the functionality of “finalization”.

.over.internal: The MRG pool class is internal to the MPM: it is not intended to have a client interface. Clients are expected to access the functionality provided by this pool (finalization) using a separate MPS finalization interface (design.mps.finalize).

.over.one-size: The MRG pool class manages objects of a single size, each object containing a single reference of rank final.

.over.one-size.justify: This is all that is necessary to meet our requirements for finalization. Whenever an object is registered for finalization, it is sufficient to create a single reference of rank final to it.

.over.queue: A pool maintains a list of live guardian objects, called (for historical reasons) the “entry” list.

.over.queue.free: The pool also maintains a list of free guardian objects called the “free” list.

.over.queue.exit.not: There used to be an “exit” list, but this is now historical and there shouldn’t be any current references to it.

.over.alloc: When guardians are allocated, they are placed on the entry list. Guardians on the entry list refer to objects that have not yet been shown to be finalizable (either the object has references of lower rank than final to it, or the MPS has not yet got round to determining that the object is finalizable).

.over.message.create: When a guardian is discovered to refer to a finalizable object it is removed from the entry list and becomes a message on the arena’s messages queue.

.over.message.deliver: When the MPS client receives the message the message system arranges for the message to be destroyed and the pool reclaims the storage associated with the guardian/message.

.over.scan: When the pool is scanned at rank final each reference will be fixed. If the reference is to an unmarked object (before the fix), then the object must now be finalizable. In this case the containing guardian will be removed from the entry list and posted as a message.

.over.scan.justify: The scanning process is a crucial step necessary for implementing finalization. It is the means by which the MPS detects that objects are finalizable.

.over.message: PoolClassMRG implements a MessageClass (see design.mps.message). All the messages are of one MessageType. This type is MessageTypeFINALIZATION. Messages are created when objects are discovered to be finalizable and destroyed when the MPS client has received the message.

.over.message.justify: Messages provide a means for the MPS to communicate with its client. Notification of finalization is just such a communication. Messages allow the MPS to inform the client of finalization events when it is convenient for the MPS to do so (i.e. not in PageFault context).

.over.manual: Objects in the MRG pool are manually managed.

.over.manual.alloc: They are allocated by ArenaFinalize() when objects are registered for finalization.

.over.manual.free: They are freed when the associated message is destroyed.

.over.manual.justify: The lifetime of a guardian object is very easy to determine so manual memory management is appropriate.

20.6. Protocols

20.6.1. Object Registration

.protocol.register: There is a protocol by which objects can be registered for finalization. This protocol is handled by the arena module on behalf of finalization. see design.mps.finalize.int.finalize.

20.6.2. Finalizer execution

.protocol.finalizer: If an object is proven to be finalizable then a message to this effect will eventually be posted. A client can receive the message, determine what to do about it, and do it. Typically this would involve calling the finalization method for the object, and deleting the message. Once the message is deleted, the object may become recyclable.

20.6.3. Setup / destroy

.protocol.life: An instance of PoolClassMRG is needed in order to support finalization, it is called the “final” pool and is attached to the arena (see design.mps.finalize.int.arena.struct).

.protocol.life.birth: The final pool is created lazily by ArenaFinalize().

.protocol.life.death: The final pool is destroyed during ArenaDestroy().

20.7. Data structures

.guardian: The guardian

.guardian.over: A guardian is an object used to manage the references and other data structures that are used by the pool in order to keep track of which objects are registered for finalization, which ones have been finalized, and so on.

.guardian.state: A guardian can be in one of four states:

.guardian.state.enum: The states are Free, Prefinal, Final, PostFinal (referred to as MRGGuardianFree, etc. in the implementation).

  1. .guardian.state.free: The guardian is free, meaning that it is on the free list for the pool and available for allocation.

  2. .guardian.state.prefinal: The guardian is allocated, and refers to an object that has not yet been discovered to be finalizable. It is on the entry list for the pool.

  3. .guardian.state.final: The guardian is allocated, and refers to an object that has been shown to be finalizable; this state corresponds to the existence of a message.

  4. .guardian.state.postfinal: This state is only used briefly and is entirely internal to the pool; the guardian enters this state just after the associated message has been destroyed (which happens when the client receives the message) and will be freed immediately (whereupon it will enter the Free state). This state is used for checking only (so that MRGFree can check that only guardians in this state are being freed).

.guardian.life-cycle: Guardians go through the following state life-cycle: Free ⟶ Prefinal ⟶ Final ⟶ Postfinal ⟶ Free.

.guardian.two-part: A guardian is a structure consisting abstractly of a link part and a reference part. Concretely, the link part is a LinkPartStruct, and the reference part is a RefPartStruct (which is just a Word). The link part is used by the pool, the reference part forms the object visible to clients of the pool. The reference part is the reference of RankFINAL that refers to objects registered for finalization and is how the MPS detects finalizable objects.

.guardian.two-part.union: The LinkPartStruct is a discriminated union of a RingStruct and a MessageStruct. The RingStruct is used when the guardian is either Free or Prefinal. The MessageStruct is used when the guardian is Final. Neither part of the union is used when the guardian is in the Postfinal state.

.guardian.two-part.justify: This may seem a little profligate with space, but this is okay as we are not required to make finalization extremely space efficient.

.guardian.parts.separate: The two parts will be stored in separate segments.

.guardian.parts.separate.justify: This is so that the data structures the pool uses to manage the objects can be separated from the objects themselves. This avoids the pool having to manipulate data structures that are on shielded segments (analysis.mps.poolmrg.hazard.shield).

.guardian.assoc: Ref part number n (from the beginning of the segment) in one segment will correspond with link part number n in another segment. The association between the two segments will be managed by the additional fields in pool-specific segment subclasses (see .mrgseg).

.guardian.ref: Guardians that are either Prefinal or Final are live and have valid references (possibly NULL) in their ref parts. Guardians that are free are dead and always have NULL in their ref parts (see .free.overwrite and .scan.free).

.guardian.ref.free: When freeing an object, it is a pointer to the reference part that will be passed (internally in the pool).

.guardian.init: Guardians are initialized when the pool is grown (.alloc.grow). The initial state has the ref part NULL and the link part is attached to the free ring. Freeing an object returns a guardian to its initial state.

.poolstruct: The Pool structure, MRGStruct will have:

  • .poolstruct.entry: the head of the entry list.

  • .poolstruct.free: the head of the free list.

  • .poolstruct.rings: The entry list, the exit list, and the free list will each be implemented as a Ring. Each ring will be maintained using the link part of the guardian.

    .poolstruct.rings.justify: This is because rings are convenient to use and are well tested. It is possible to implement all three lists using a singly linked list, but the saving is certainly not worth making at this stage.

  • .poolstruct.refring: a ring of “ref” segments in use for links or messages (see .mrgseg.ref.mrgring below).

  • .poolstruct.extend: a precalculated extendBy field (see .init.extend). This value is used to determine how large a segment should be requested from the arena for the reference part segment when the pool needs to grow (see .alloc.grow.size).

    .poolstruct.extend.justify: Calculating a reasonable value for this once and remembering it simplifies the allocation (.alloc.grow).

.poolstruct.init: poolstructs are initialized once for each pool instance by MRGInit() (.init). The initial state has all the rings initialized to singleton rings, and the extendBy field initialized to some value (see .init.extend).

.mrgseg: The pool defines two segment subclasses: MRGRefSegClass and MRGLinkSegClass. Segments of the former class will be used to store the ref parts of guardians, segments of the latter will be used to store the link parts of guardians (see .guardian.two-part). Segments are always allocated in pairs, with one of each class, by the function MRGSegPairCreate(). Each segment contains a link to its pair.

.mrgseg.ref: MRGRefSegClass is a subclass of MutatorSegClass. Instances are of type MRGRefSeg, and contain:

.mrgseg.ref.init: A segment is created and initialized once every time the pool is grown (.alloc.grow). The initial state has the segment ring node initialized and attached to the pool’s segment ring, the linkseg field points to the relevant link segment, the grey field is initialized such that the segment is not grey for all traces.

.mrgseg.link: MRGLinkSegClass is a subclass of SegClass. Instances are of type MRGLinkSeg, and contain:

  • .mrgseg.link.refseg: a pointer to the paired ref segment. This may be NULL during initialization, while the pairing is being established.

  • .mrgseg.link.init: The initial state has the linkseg field pointing to the relevant ref segment.

20.8. Functions

Bool MRGCheck(MRG mrg)

.check: Check the signatures, the class, and each field of the MRGStruct. Each field is checked as being appropriate for its type.

.check.justify: There are no non-trivial invariants that can be easily checked.

Res MRGRegister(Pool pool, Ref ref)

.alloc: Add a guardian for ref.

.alloc.grow: If the free list is empty then two new segments are allocated and the free list filled up from them (note that the reference fields of the new guardians will need to be overwritten with NULL, see .free.overwrite)

.alloc.grow.size: The size of the reference part segment will be the pool’s extendBy (.poolstruct.extend) value. The link part segment will be whatever size is necessary to accommodate N link parts, where N is the number of reference parts that fit in the reference part segment.

.alloc.error: If any of the requests for more resource (there are two; one for each of two segments) fail then the successful requests will be retracted and the result code from the failing request will be returned.

.alloc.pop: MRGRegister() pops a ring node off the free list, and add it to the entry list.

Res MRGDeregister(Pool pool, Ref obj)

.free: Remove the guardian from the message queue and add it to the free list.

.free.push: The guardian will simply be added to the front of the free list (that is, no keeping the free list in address order or anything like that).

.free.inadequate: No attempt will be made to return unused free segments to the arena (although see analysis.mps.poolmrg.improve.free.* for suggestions).

.free.overwrite: MRGDeregister() also writes over the reference with NULL. .free.overwrite.justify: This is so that when the segment is subsequently scanned (.scan.free), the reference that used to be in the object is not accidentally fixed.

Res MRGInit(Pool pool, ArgList args)

.init: Initializes the entry list, the free ring, the ref ring, and the extendBy field.

.init.extend: The extendBy field is initialized to the arena grain size.

.init.extend.justify: This is adequate as the pool is not expected to grow very quickly.

void MRGFinish(Pool pool)

.finish: Iterate over all the segments, returning all the segments to the arena.

Res mrgRefSegScan(Bool *totalReturn, Pool pool, Seg seg, ScanState ss)

.scan: mrgRefSegScan() scans a segment of guardians.

.scan.trivial: Scan will do nothing (that is, return immediately) if the tracing rank is anything other than final.


This optimization is missing. impl.c.trace.scan.conservative is not a problem because there are no faults on these segs, because there are no references into them. But that’s why TraceScan() can’t do it. Pekka P. Pirinen, 1997-09-19.

.scan.trivial.justify: If the rank is lower than final then scanning is detrimental, it will only delay finalization. If the rank is higher than final there is nothing to do, the pool only contains final references.

.scan.guardians: mrgRefSegScan() will iterate over all guardians in the segment. Every guardian’s reference will be fixed (.scan.free: note that guardians that are on the free list have NULL in their reference part).

.scan.wasold: If the object referred to had not been fixed previously (that is, was unmarked) then the object is not referenced by a reference of a lower rank (than RankFINAL) and hence is finalizable.

.scan.finalize: The guardian will be finalized. This entails moving the guardian from state Prefinal to Final; it is removed from the entry list and initialized as a message and posted on the arena’s message queue.

.scan.finalize.idempotent: In fact this will only happen if the guardian has not already been finalized (which is determined by examining the state of the guardian).

.scan.unordered: Because scanning occurs a segment at a time, the order in which objects are finalized is “random” (it cannot be predicted by considering only the references between objects registered for finalization). See analysis.mps.poolmrg.improve.semantics for how this can be improved.

.scan.unordered.justify: Unordered finalization is all that is required.

See analysis.mps.poolmrg.improve.scan.nomove for a suggested improvement that avoids redundant unlinking and relinking.

Res MRGDescribe(Pool pool, mps_lib_FILE *stream, Count depth)

.describe: Describes an MRG pool. Iterates along each of the entry and exit lists and prints the guardians in each. The location of the guardian and the value of the reference in it will be printed out. Provided for debugging only.

20.9. Transgressions

.trans.no-finish: The MRG pool does not trouble itself to tidy up its internal rings properly when being destroyed.

.trans.free-seg: No attempt is made to release free segments to the arena. A suggested strategy for this is as follows:

  • Add a count of free guardians to each segment, and maintain it in appropriate places.

  • Add a free segment ring to the pool.

  • In mrgRefSegScan(), if the segment is entirely free, don’t scan it, but instead detach its links from the free ring, and move the segment to the free segment ring.

  • At some appropriate point, such as the end of MRGAlloc(), destroy free segments.

  • In MRGAlloc(), if there are no free guardians, check the free segment ring before creating a new pair of segments. Note that this algorithm would give some slight measure of segment hysteresis. It is not the place of the pool to support general segment hysteresis.

20.10. Future

.future.array: In future, for speed or simplicity, this pool could be rewritten to use an array. See mail.gavinm.1997-09-04.13-08.

20.11. Tests


This section is utterly out of date. Pekka P. Pirinen, 1997-09-19.

.test: The test impl.c.finalcv is similar to the weakness test (see design.mps.weakness, impl.c.weakcv).

20.11.1. Functionality

This is the functionality to be tested:

  • .fun.alloc: Can allocate objects.

  • .fun.free: Can free objects that were allocated.

  • .prot.write: Can write a reference into an allocated object.

  • .prot.read: Can read the reference from an allocated object.

  • .promise.faithful: A reference stored in an allocated object will continue to refer to the same object.

  • .promise.live: A reference stored in an allocated object will preserve the object referred to.

  • .promise.unreachable: Any objects referred to in finalization messages are not (at the time of reading the message) reachable via a chain of ambiguous or exact references. (we will not be able to test this at first as there is no messaging interface)

  • .promise.try: The pool will make a “good faith” effort to finalize objects that are not reachable via a chain of ambiguous or exact references.

20.11.2. Attributes

The following attributes will be tested:

20.11.3. Implementation

The test will simply allocate a number of objects in the AMC pool and finalize each one, throwing away the reference to the objects. Churn.

.test.mpm: The test will use the MPM interface (impl.h.mpm).

.test.mpm.justify: This is because it is not intended to provide an MPS interface to this pool directly, and the MPS interface to finalization has not been written yet (impl.h.mps).

.test.mpm.change: Later on it may use the MPS interface, in which case, where the following text refers to allocating objects in the MRG pool it will need adjusting.

.test.two-pools: The test will use two pools, an AMC pool, and an MRG pool.

.test.alloc: A number of objects will be allocated in the MRG pool.

.test.free: They will then be freed. This will test .fun.alloc and .fun.free, although not very much.

.test.rw.a: An object, “A”, will be allocated in the AMC pool, a reference to it will be kept in a root.

.test.rw.alloc: A number of objects will be allocated in the MRG pool.

.test.rw.write: A reference to “A” will be written into each object.

.test.rw.read: The reference in each object will be read and checked to see if it refers to “A”.

.test.rw.free: All the objects will be freed.

.test.rw.drop: The reference to “A” will be dropped. This will test .prot.write and .prot.read.

.test.promise.fl.alloc: A number of objects will be allocated in the AMC pool.

.test.promise.fl.tag: Each object will be tagged uniquely.

.test.promise.fl.refer: a reference to it will be stored in an object allocated in the MRG pool.

.test.promise.fl.churn: A large amount of garbage will be allocated in the AMC pool. Regularly, whilst this garbage is being allocated, a check will be performed that all the objects allocated in the MRG pool refer to valid objects and that they still refer to the same objects. All objects from the MRG pool will then be freed (thus dropping all references to the AMC objects). This will test .promise.faithful and .promise.live.

.test.promise.ut.alloc: A number of objects will be allocated in the AMC pool.

.test.promise.ut.refer: Each object will be referred to by a root and also referred to by an object allocated in the MRG pool.

.test.promise.ut.drop: References to a random selection of the objects from the AMC pool will be deleted from the root.

.test.promise.ut.churn: A large amount of garbage will be allocated in the AMC pool.

.test.promise.ut.message: The message interface will be used to receive finalization messages.

.test.promise.ut.final.check: For each finalization message received it will check that the object referenced in the message is not referred to in the root.

.test.promise.ut.nofinal.check: After some amount of garbage has been allocated it will check to see if any objects are not in the root and haven’t been finalized. This will test .promise.unreachable and .promise.try.

20.12. Notes

.access.inadequate: SegAccess() will scan segments at RankEXACT`. Really it should be scanned at whatever the minimum rank of all grey segments is (the trace rank phase), however there is no way to find this out. As a consequence we will sometimes scan pages at RankEXACT when the pages could have been scanned at RankFINAL. This means that finalization of some objects may sometimes get delayed.