.. mode: -*- rst -*-

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

:Tag: design.mps.root
:Author: Richard Brooksby
:Date: 1995-08-25
:Status: incomplete design
:Revision: $Id: //info.ravenbrook.com/project/mps/version/1.113/design/root.txt#1 $
: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: http://www.ravenbrook.com/consultants/rb/
.. _GDR: http://www.ravenbrook.com/consultants/gdr/


Copyright and License
---------------------

Copyright © 2013-2014 Ravenbrook Limited. All rights reserved.
<http://www.ravenbrook.com/>. This is an open source license. Contact
Ravenbrook for commercial licensing options.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

#. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.

#. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.

#. Redistributions in any form must be accompanied by information on how
   to obtain complete source code for this software and any
   accompanying software that uses this software.  The source code must
   either be included in the distribution or be available for no more than
   the cost of distribution plus a nominal fee, and must be freely
   redistributable under reasonable conditions.  For an executable file,
   complete source code means the source code for all modules it contains.
   It does not include source code for modules or files that typically
   accompany the major components of the operating system on which the
   executable file runs.

**This software is provided by the copyright holders and contributors
"as is" and any express or implied warranties, including, but not
limited to, the implied warranties of merchantability, fitness for a
particular purpose, or non-infringement, are disclaimed.  In no event
shall the copyright holders and contributors be liable for any direct,
indirect, incidental, special, exemplary, or consequential damages
(including, but not limited to, procurement of substitute goods or
services; loss of use, data, or profits; or business interruption)
however caused and on any theory of liability, whether in contract,
strict liability, or tort (including negligence or otherwise) arising in
any way out of the use of this software, even if advised of the
possibility of such damage.**
