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Memory Pool System Project

                  THE DESIGN OF THE MPS SEGMENT DATA STRUCTURE
                                 design.mps.seg
                               incomplete design
                                 drj 1997-04-03

    INTRODUCTION

    .intro: This document describes the MPS Segment data structure.

    Document History

    .hist.2: The initial draft (replacing various notes in revisions 0 and 1) was
    drafted by Richard Brooksby <richard> on 1997-04-03 as part of editing
    MMsrc!seg.c(MMdevel_action2.1).

    .hist.3: Rewritten to separate segments and tracts, following
    mail.tony.1998-11-02.10-26.  tony 1999-04-16

    OVERVIEW

    .over.segments: Segments are the basic units of tracing and shielding.  The MPM
    also uses them as units of scanning and colour, although pool classes may
    subdivide segments and be able to maintain colour on a finer grain (down to the
    object level, for example).

    .over.objects: The mutator's objects are stored in segments.  Segments are
    contiguous blocks of memory managed by some pool.  .segments.pool: The
    arrangement of objects within a segment is determined by the class of the pool
    which owns the segment. The pool is associated with the segment indirectly via
    the first tract of the segment.

    .over.memory: The relationship between segments and areas of memory is
    maintained by the segment module.  Pools acquire tracts from the arena, and
    release them back to the arena when they don't need them any longer. The
    segment module can associate contiguous tracts owned by the same pool with a
    segment. The segment module provides the methods SegBase, SegLimit, and SegSize
    which map a segment onto the addresses of the memory block it represents.

    .over.hierarchy: The Segment datastructure is designed to be subclassable (see
    design.mps.protocol). The basic segment class (Seg) supports colour and
    protection for use by the tracer, as well as support for a pool ring, and all
    generic segment functions. Clients may use Seg directly, but will most probably
    want to use a subclass with additional properties.

    .over.hierarchy.gcseg: The segment module provides GCSeg - a subclass of Seg
    which has full support for GC including buffering and the ability to be linked
    onto the grey ring.

    DATA STRUCTURE

    typedef struct SegStruct {      /* segment structure */
      Sig sig;                      /* impl.h.misc.sig */
      SegClass class;               /* segment class structure */
      Tract firstTract;             /* first tract of segment */
      RingStruct poolRing;          /* link in list of segs in pool */
      Addr limit;                   /* limit of segment */
      unsigned depth : SHIELD_DEPTH_WIDTH; /* see impl.c.shield.def.depth */
      AccessSet pm : AccessMAX;     /* protection mode, impl.c.shield */
      AccessSet sm : AccessMAX;     /* shield mode, impl.c.shield */
      TraceSet grey : TRACE_MAX;    /* traces for which seg is grey */
      TraceSet white : TRACE_MAX;   /* traces for which seg is white */
      TraceSet nailed : TRACE_MAX;  /* traces for which seg has nailed objects */
      RankSet rankSet : RankMAX;    /* ranks of references in this seg */
    } SegStruct;

    typedef struct GCSegStruct {    /* GC segment structure */
      SegStruct segStruct;          /* superclass fields must come first */
      RingStruct greyRing;          /* link in list of grey segs */
      RefSet summary;               /* summary of references out of seg */
      Buffer buffer;                /* non-NULL if seg is buffered */
      Sig sig;                      /* design.mps.sig */
    } GCSegStruct;

    .field.rankSet: The "rankSet" field represents the set of ranks of the
    references in the segment.  It is initialized to empty by SegInit.
    .field.rankSet.single: The Tracer only permits one rank per segment [ref?] so
    this field is either empty or a singleton.  .field.rankSet.empty: An empty
    rankSet indicates that there are no references.  If there are no references in
    the segment then it cannot contain black or grey references.
    .field.rankSet.start: If references are stored in the segment then it must be
    updated, along with the summary (.field.summary.start).

    .field.depth: The "depth" field is used by the Shield (impl.c.shield) to manage
    protection of the segment.  It is initialized to zero by SegInit.

    .field.sm: The "sm" field is used by the Shield (impl.c.shield) to manage
    protection of the segment.  It is initialized to AccessSetEMPTY by SegInit.

    .field.pm: The "pm" field is used by the Shield (impl.c.shield) to manage
    protection of the segment.  It is initialized to AccessSetEMPTY by SegInit.
    The field is used by both the shield and the ANSI fake protection
    (impl.c.protan).

    .field.black: The "black" field is the set of traces for which there may be
    black objects (i.e. objects containing references, but no references to white
    objects) in the segment.  More precisely, if there is a black object for a
    trace in the segment then that trace will appear in the "black" field.  It is
    initialized to TraceSetEMPTY by SegInit.

    .field.grey: The "grey" field is the set of traces for which there may be grey
    objects (i.e containing references to white objects) in the segment.  More
    precisely, if there is a reference to a white object for a trace in the segment
    then that trace will appear in the "grey" field.  It is initialized to
    TraceSetEMPTY by SegInit.

    .field.white: The "white" field is the set of traces for which there may be
    white objects in the segment.  More precisely, if there is a white object for a
    trace in the segment then that trace will appear in the "white" field.  It is
    initialized to TraceSetEMPTY by SegInit.

    .field.summary: The "summary" field is an approximation to the set of all
    references in the segment.  If there is a reference R in the segment, then
    RefSetIsMember(summary, R) is TRUE.  The summary is initialized to RefSetEMPTY
    by SegInit.  .field.summary.start: If references are stored in the segment then
    it must be updated, along with rankSet (.field.rankSet.start).

    .field.buffer: The "buffer" field is either NULL, or points to the descriptor
    structure of the buffer which is currently allocating in the segment.  The
    field is initialized to NULL by SegInit.  .field.buffer.owner: This buffer must
    belong to the same pool as the segment, because only that pool has the right to
    attach it.

    INTERFACE

    Splitting and Merging

    .split-and-merge: There is support for splitting and merging segments, to give
    pools the flexibility to rearrange their tracts among segments as they see fit.

    .split: Segments may be split with the function SegSplit

    Res SegSplit(Seg *segLoReturn, Seg *segHiReturn, Seg seg, Addr at,
                 Bool withReservoirPermit, ...);

    If successful, segment "seg" is split at address "at", yielding two segments
    which are returned in segLoReturn and segHiReturn for the low and high segments
    respectively. The base of the low segment is the old base of "seg". The limit
    of the low segment is "at". The base of the high segment is "at". This limit of
    the high segment is the old limit of "seg". "seg" is effectively destroyed
    during this operation (actually, it might be reused as one of the returned
    segments). Segment subclasses may make use of the optional arguments; the
    built-in classes do not.

    .split.invariants: The client must ensure some invariants are met before
    calling SegSplit:

    .split.inv.align: "at" must be appropriately aligned to the arena alignment,
    and lie between the base and limit of "seg". Justification: the split segments
    cannot be represented if this is not so.

    .split.inv.buffer: If "seg" is attached to a buffer, the buffered region must
    not include address "at". Justification: the segment module is not in a
    position to know how (or whether) a pool might wish to split a buffer. This
    permits the buffer to remain attached to just one of the returned segments.

    .split.state: Except as noted above, the segments returned have the same
    properties as "seg". I.e. their colour, summary, rankset, nailedness etc. are
    set to the values of "seg".

    .merge: Segments may be merged with the function SegMerge

    Res SegMerge(Seg *mergedSegReturn, Seg segLo, Seg segHi,
                 Bool withReservoirPermit, ...);

    If successful, segments "segLo" and "segHi" are merged together, yielding a
    segment which is returned in mergedSegReturn. "segLo" and "segHi" are
    effectively destroyed during this operation (actually, one of them might be
    reused as the merged segment). Segment subclasses may make use of the optional
    arguments; the built-in classes do not.

    .merge.invariants: The client must ensure some invariants are met before
    calling SegMerge:

    .merge.inv.abut: The limit of "segLo" must be the same as the base of "segHi".
    Justification: the merged segment cannot be represented if this is not so.

    .merge.inv.buffer: One or other of "segLo" and "segHi" may attached to a
    buffer, but not both. Justification: the segment module does not support
    attachment of a single seg to 2 buffers.

    .merge.inv.similar: "segLo" and "segHi" must be sufficiently similar. Two
    segments are sufficiently similar if they have identical values for each of the
    following fields: class, sm, grey, white, nailed, rankSet. Justification: there
    is no single choice of behaviour for cases where these fields are not
    identical. The pool class must make it's own choices about this if it wishes to
    permit more flexible merging. If so, it should be a simple matter for the pool
    to arrange for the segments to look sufficiently similar before calling
    SegMerge.

    .merge.state: The merged segment will share the same state as "segLo" and
    "segHi" for those fields which are identical (see .merge.inv.similar). The
    summary will be the union of the summaries of "segLo" and "segHi".

    EXTENSIBILITY

    Splitting and Merging

    .method.split: Segment subclasses may extend the support for segment splitting
    by defining their own "split" method.

    Res segSplit(Seg seg, Seg segHi,
                 Addr base, Addr mid, Addr limit,
                 Bool withReservoirPermit, va_list args)

    On entry, "seg" is a segment with region [base,limit), "segHi" is
    uninitialized, "mid" is the address at which the segment is to be split. The
    method is responsible for destructively modifying "seg" and initializing
    "segHi" so that on exit "seg" is a segment with region [base,mid) and segHi is
    a segment with region [mid,limit). Usually a method would only directly modify
    the fields defined for the segment subclass. This might involve allocation,
    which may use the reservoir if "withReservoirPermit" is TRUE.
    .method.split.next: A split method should always call the next method, either
    before or after any class-specific code (see design.mps.protocol
    .overview.next-method).

    .method.merge: Segment subclasses may extend the support for segment merging by
    defining their own "merge" method.

    Res segMerge(Seg seg, Seg segHi,
                 Addr base, Addr mid, Addr limit,
                 Bool withReservoirPermit, va_list args)

    On entry, "seg" is a segment with region [base,mid), "segHi" is a segment with
    region [mid,limit), The method is responsible for destructively modifying "seg"
    and finishing "segHi" so that on exit "seg" is a segment with region
    [base,limit) and segHi is garbage. Usually a method would only modify the
    fields defined for the segment subclass. This might involve allocation, which
    may use the reservoir if "withReservoirPermit" is TRUE. .method.merge.next: A
    merge method should always call the next method, either before or after any
    class-specific code (see design.mps.protocol.overview.next-method).

    .split-merge.shield: Split and merge methods may assume that the segments they
    are manipulating are not in the shield cache. .split-merge.shield.flush: The
    shield cache is flushed before any split or merge methods are invoked.
    .split-merge.shield.re-flush: If a split or merge method performs an operation
    on a segment which might cause the segment to be cached, the method must flush
    the shield cache before returning or calling another split or merge method.

    .split-merge.fail: Split and merge methods might fail, in which case segments
    "seg" and "segHi" must be equivalently valid and configured at exit as they
    were according to the entry conditions. It's simplest if the failure can be
    detected before calling the next method (e.g. by allocating any objects early
    in the method). .split-merge.fail.anti: If it's not possible to detect failure
    before calling the next method, the appropriate anti-method must be used (see
    design.mps.protocol.guide.fail.after-next). Split methods are anti-methods for
    merge methods, and vice-versa. .split-merge.fail.anti.constrain: In general,
    care should be taken when writing split and merge methods to ensure that they
    really are anti-methods for each other. The anti-method must not fail if the
    initial method succeeded. The anti-method should reverse any side effects of
    the initial method, except where it's known to be safe to avoid this (see
    .split-merge.fail.summary for an example of a safe case).
    .split-merge.fail.anti.no: If this isn't possible (it might not be) then the
    methods won't support after-next failure. This fact should be documented, if
    the methods are intended to support further specialization. Note that using
    va_arg with the "args" parameter is sufficient to make it impossible to reverse
    all side effects.

    .split-merge.fail.summary: The segment summary might not be restored exactly
    after a failed merge operation. Each segment would be left with a summary which
    is the union of the original summaries (see .merge.state). This increases the
    conservatism in the summaries, but is otherwise safe.

    .split-merge.unsupported: Segment classes need not support segment merging at
    all. The function SegClassMixInNoSplitMerge is supplied to set the split and
    merge methods to unsupporting methods that will report an error in checking
    varieties.

## A. References

## B. Document History

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

## C. Copyright and License

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