Ravenbrook / Projects / Memory Pool System / Version 1.111 Product Sources / Design Documents

Memory Pool System Project

                                  FINALIZATION
                              design.mps.finalize
                               incomplete design
                                 drj 1997-02-14

    OVERVIEW:

    Finalization is implemented internally using the Guardian Pool Class
    (design.mps.poolmrg).  Objects can be registered for finalization using an
    interface function (called mps_finalize).  Notification of finalization is
    given to the client via the messaging interface.  PoolClassMRG
    (design.mps.poolmrg) implements a Message Class which implements the
    finalization messages.

    REQUIREMENTS:

    .req: Historically only Dylan had requirements
    for finalization, see req.dylan.fun.final.  Now (2003-02-19) Configura
    have requirements for finalization.  Happily they are very similar.

    ARCHITECTURE:

    External Interface

    .if.register:
    mps_res_t mps_finalize(mps_arena_t arena, mps_addr_t obj);

    increases the number of times that the object located at obj has been
    registered for finalization by one.  The object must have been allocated from
    the arena (space).  Any finalization messages that are created for this object
    will appear on the arena's message queue.  The MPS will attempt to finalize the
    object that number of times.

    .if.deregister:
    void mps_definalize(mps_arena_t arena, mps_addr_t obj);

    mps_definalize reduces the number of times that the object located at obj has
    been registered for finalization by one.  It is an error to definalize that has
    not been registered for finalization.

    .if.deregister.not: At the moment (1997-08-20) mps_definalize is not implemented

    .if.get-ref:
    void mps_message_finalization_ref(mps_addr_t *mps_addr_return,
                                      mps_arena_t mps_arena,
                                      mps_message_t mps_message)

    mps_message_finalization_ref returns the reference to the finalized object
    stored in the finalization message.

    IMPLEMENTATION:

    .int.over: Registering an object for finalization corresponds to allocating a
    reference of rank FINAL to that object.  This reference is allocated in a
    guardian object in a pool of PoolClassMRG (see design.mps.poolmrg).

    .int.arena.struct: The MRG pool used for managing final references is kept in
    the Arena (Space), referred to as the "final pool".  .int.arena.lazy: The pool
    is lazily created, it will not be created until the first object is registered
    for finalization.  .int.arena.flag: There is a flag in the Arena that indicates
    whether the final pool has been created yet or not.

    .int.finalize:

    Res ArenaFinalize(Arena arena, Ref addr)

    .int.finalize.create: Creates the final pool if it has not been created yet.
    .int.finalize.alloc: Allocates a guardian in the final pool.
    .int.finalize.write: Writes a reference to the object into the guardian
    object.  .int.finalize.all: That's all.  .int.finalize.error: if either the
    creation of the pool or the allocation of the object fails then the error will
    be reported back to the caller.  .int.finalize.error.no-unwind: This function
    does not need to do any unwinding in the error cases because the creation of
    the pool is not something that needs to be undone.

    .int.arena-destroy.empty: ArenaDestroy empties the message queue by calling
    MessageEmpty.

    .int.arena-destroy.final-pool: If the final pool has been created then
    ArenaDestroy destroys the final pool.

    .access: mps_message_finalization_ref needs to access the finalization message
    to retrieve the reference and then write it to where the client asks.  This
    must be done carefully, in order to avoid breaking the invariants or creating a
    hidden root.

    .access.invariants: We protect the invariants by using special routines
    ArenaRead and ArenaPoke to read and write the reference.  This works as long as
    there's no write-barrier collection.  [Instead of ArenaPoke, we could put in an
    ArenaWrite that would be identical to ArenaPoke, except for AVERring the
    invariant (or it can just AVER there are no busy traces unflipped).  When we
    get write-barrier collection, we could change it to do the real thing, but in
    the absence of a write-barrier, it's functionally identical to ArenaPoke.
    Pekka 1997-12-09]

## A. References

## B. Document History

2002-06-07 | RB | Converted from MMInfo database design document.

## C. Copyright and License

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