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

Memory Pool System Project

                       THE DESIGN OF THE GENERIC SCANNER
                                design.mps.scan
                               incomplete design
                               richard 1995-08-25

    SUMMARIES

    Scanned Summary

    .summary.subset: The summary of reference seens by scan (ss.unfixedSummary) is
    a subset of the summary previously computed (SegSummary).

    There are two reasons that it is not an equality relation:

    1. If the segment has had objects forwarded onto it then its summary will get
    unioned with the summary of the segment that the object was forwarded from.
    This may increase the summary.  The forwarded object of course may have a
    smaller summary (if such a thing were to be computed) and so subsequent
    scanning of the segment may reduce the summary.  (The forwarding process may
    erroneously introduce zones into the destination's summary).

    2. A write barrier hit will set the summary to RefSetUNIV.

    The reason that ss.unfixedSummary is always a subset of the previous summary is
    due to an "optimization" which has not been made in TraceFix.  See
    impl.c.trace.fix.fixed.all.

    Partial Scans

    .clever-summary: With enough cleverness, it's possible to have partial scans of
    condemned segments contribute to the segment summary.  [We had a system which
    nearly worked -- see MMsrc(MMdevel_poolams at 1997/08/14 13:02:55 BST), but it
    did not handle the situation in which a segment was not under the write barrier
    when it was condemned.]

    .clever-summary.acc: Each time we partially scan a segment, we accumulate the
    post-scan summary of the scanned objects into a field in the group, called
    'summarySoFar'.  The post-scan summary is (summary \ white) U fixed.

    .clever-summary.acc.condemn: The cumulative summary is only meaningful while
    the segment is condemned.  Otherwise it is set to RefSetEMPTY (a value which we
    can check).

    .clever-summary.acc.reclaim: Then when we reclaim the segment, we set the
    segment summary to the cumulative summary, as it is a post-scan summary of all
    the scanned objects.

    .clever-summary.acc.other-trace: If the segment is scanned by another trace
    while it is condemned, the cumulative summary must be set to the post-scan
    summary of this scan (otherwise it becomes out-of-date).

    .clever-summary.scan: The scan summary is expected to be a summary of all
    scanned references in the segment.  We don't know this accurately until we've
    scanned everything in the segment.  So we add in the segment summary each time.

    .clever-summary.scan.fix: TraceScan also expects the scan state fixed summary
    to include the post-scan summary of all references which were white.  Since we
    don't scan all white references, we need to add in an approximation to the
    summary of all white references which we didn't scan.  This is the intersection
    of the segment summary and the white summary.

    .clever-summary.wb: If the cumulative summary is smaller than the mutator's
    summary, a write-barrier is needed to prevent the mutator from invalidating it.
     This means that sometimes we'd have to put the segment under the write-barrier
    at condemn [this is not an operation currently available to pool class
    implementations pekka 1998-02-26], which might not be very efficient.

    .clever-summary.method.wb: We need a new pool class method, called when the
    write barrier is hit (or possibly any barrier hit).  The generic method will do
    the usual TraceAccess work, the trivial method will do nothing.

    .clever-summary.acc.wb: When the write barrier is hit, we need to correct the
    cumulative summary to the mutator summary.  This is approximated by setting the
    summary to RefSetUNIV.

## A. References

## B. Document History

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

## C. Copyright and License

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