.. mode: -*- rst -*-

Root manager
============

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


Basics
------

_`.root.def`: The root node of the object graph is the node which
defines whether objects are accessible, and the place from which the
mutator acts to change the graph. In the MPS, a root is an object
which describes part of the root node. The root node is the total of
all the roots attached to the space.

.. note::

    Note that this combines two definitions of root: the accessibility
    is what defines a root for tracing (see analysis.tracer.root.* and
    the mutator action for barriers (see analysis.async-gc.root).
    Pekka P. Pirinen, 1998-03-20.

_`.root.repr`: Functionally, roots are defined by their scanning
functions. Roots *could* be represented as function closures: that is,
a pointer to a C function and some auxiliary fields. The most general
variant of roots is just that. However, for reasons of efficiency,
some special variants are separated out.


Details
-------

Creation
........

_`.create`: A root becomes "active" as soon as it is created.

_`.create.col`: The root inherits its colour from the mutator, since
it can only contain references copied there by the mutator from
somewhere else. If the mutator is grey for a trace when a root is
created then that root will be used to determine accessibility for
that trace. More specifically, the root will be scanned when that
trace flips.

Destruction
...........

_`.destroy`: It's OK to destroy a root at any time, except perhaps
concurrently with scanning it, but that's prevented by the arena lock.
If a root is destroyed the references in it become invalid and
unusable.

Invariants
..........

_`.inv.white`: Roots are never white for any trace, because they
cannot be condemned.

_`.inv.rank`: Roots always have a single rank. A root without ranks
would be a root without references, which would be pointless. The
tracer doesn't support multiple ranks in a single colour.

Scanning
........

_`.method`: Root scanning methods are provided by the client so that
the MPS can locate and scan the root set. See protocol.mps.root for
details.

.. note::

    There are some more notes about root methods in
    meeting.qa.1996-10-16.


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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