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

Memory Pool System Project

                     THE DESIGN OF THE LOW-MEMORY RESERVOIR
                              design.mps.reservoir
                               incomplete design
                                tony 1998-07-30

    INTRODUCTION:

    The low-memory reservoir provides client support for implementing handlers for
    low-memory situations which allocate.  The reservoir is implemented inside the
    arena as a pool of unallocatable segments.

    OVERVIEW:

    This is just a placeholder at the moment.

    ARCHITECTURE:

    .adt: The reservoir interface looks (almost) like an abstract data type of type
    Reservoir.  It's not quite abstract because the arena embeds the structure of
    the reservoir (of type ReservoirStruct) into its own structure, for simplicity
    of initialization.

    .align: The reservoir is implemented as a pool of available tracts, along with
    a size and limit which must always be aligned to the arena alignment.  The size
    corresponds to the amount of memory currently maintained in the reservoir.  The
    limit is the maximum amount that it is desired to maintain.

    .wastage: When the reservoir limit is set by the client, the actual limit
    should be increased by an arena alignment amount for every active mutator
    buffer.

    .really-empty: When the reservoir limit is set to 0, assume that the client
    really doesn't have a need for a reservoir at all.  In this case, the client
    won't even want an allowance to be made for wastage in active buffers.

    IMPLEMENTATION:

    .interface: The following functions comprise the interface to the reservoir
    module:

    .interface.check: ReservoirCheck checks the reservoir for consistency:
    extern Bool ReservoirCheck(Reservoir reservoir);

    .interface.init: ReservoirInit initializes the reservoir and its associated
    pool, setting the size and limit to 0:
    extern Res ReservoirInit(Reservoir reservoir, Arena arena);

    .interface.finish: ReservoirFinish de-initializes the reservoir and its
    associated pool:
    extern void ReservoirFinish (Reservoir reservoir);

    .interface.limit: ReservoirLimit returns the limit of the reservoir:
    extern Size ReservoirLimit(Reservoir reservoir);

    .interface.set-limit: ReservoirSetLimit sets the limit of the reservoir, making
    an allowance for wastage in mutator buffers:
    extern void ReservoirSetLimit(Reservoir reservoir, Size size);

    .interface.available: ReservoirAvailable returns the available size of the
    reservoir:
    extern Size ReservoirAvailable(Reservoir reservoir);

    .interface.ensure-full: ReservoirEnsureFull attempts to fill the reservoir with
    memory from the arena, until it is full:
    extern Res ReservoirEnsureFull(Reservoir reservoir);

    .interface.deposit: ReservoirDeposit attempts to fill the reservoir with memory
    in the supplied range, until it is full.  This is called by the arena from
    ArenaFree if the reservoir is not known to be full.  Any memory which is not
    added to the reservoir (because the reservoir is full) is freed via the arena
    class's free method.
    extern void ReservoirDeposit(Reservoir reservoir, Addr base, Size size);

    .interface.withdraw: ReservoirWithdraw attempts to allocate memory of the
    specified size to the specified pool to the reservoir.  If no suitable memory
    can be found it returns ResMEMORY.
    extern Res ReservoirWithdraw(Addr *baseReturn, Tract *baseTractReturn,
                                 Reservoir reservoir, Size size, Pool pool);

    .interface.withdraw.align: Currently, ReservoirWithdraw can only withdraw
    memory in chunks of the size of the arena alignment.  This is because the
    reservoir doesn't attempt to coalesce adjacent memory blocks.  This deficiency
    should be fixed in the future.

    .pool: The memory managed by the reservoir is owned by the reservoir pool.
    This memory is never sub-allocated.  Each tract belonging to the pool is linked
    onto a list.  The head of the list is in the Reservoir object.  Links are
    stored in the TractP fields of each tract object.

## A. References

## B. Document History

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

## C. Copyright and License

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