.. mode: -*- rst -*-

The generic scanner
===================

:Tag: design.mps.scan
:Author: Richard Brooksby
:Date: 1995-08-25
:Status: incomplete design
:Revision: $Id$
:Copyright: See `Copyright and License`_.
:Index terms:    pair: generic scanner; design


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 design.mps.trace.fix.fixed.all_.

.. _design.mps.trace.fix.fixed.all: 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.

.. note::

    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) ∪ 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out-of-date).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`: ``traceScanSeg()`` 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, which might not be very
efficient

.. note::

    This is not an operation currently available to pool class
    implementations Pekka P. Pirinen, 1998-02-26.

_`.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``.


Document History
----------------

- 1995-08-25 RB_ Incomplete design.

- 2002-06-07 RB_ Converted from MMInfo database design document.

- 2013-05-22 GDR_ Converted to reStructuredText.

.. _RB: https://www.ravenbrook.com/consultants/rb/
.. _GDR: https://www.ravenbrook.com/consultants/gdr/


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---------------------

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