.. mode: -*- rst -*-

Arena
=====

:Tag: design.mps.arena
:Author: Pekka P. Pirinen
:Date: 1997-08-11
:Status: incomplete design
:Revision: $Id: //info.ravenbrook.com/project/mps/version/1.113/design/arena.txt#1 $
:Copyright: See `Copyright and License`_.
:Index terms: pair: arena; design


Introduction
------------

_`.intro`: This is the design of the arena structure.

_`.readership`: MPS developers.


Overview
--------

_`.overview`: The arena serves two purposes. It is a structure that is
the top-level state of the MPS, and as such contains a lot of fields
which are considered "global". And it provides raw memory to pools.

An arena belongs to a particular arena class. The class is selected
when the arena is created. Classes encapsulate both policy (such as
how pool placement preferences map into actual placement) and
mechanism (such as where the memory originates: operating system
virtual memory, client provided, or via malloc). Some behaviour
(mostly serving the "top-level datastructure" purpose) is implemented
by generic arena code, and some by arena class code.


Definitions
-----------

_`.def.tract`: Pools request memory from the arena by calling
``ArenaAlloc()``. This returns a block comprising a contiguous sequence
of "tracts". A tract has a specific size (also known as the "arena
alignment", which typically corresponds to the operating system page
size) and all tracts are aligned to that size. "Tract" is also used
for the data structure used to manage tracts.


Requirements
------------

.. note::

    Where do these come from? Need to identify and document the
    sources of requirements so that they are traceable to client
    requirements. Most of these come from the architectural design
    (design.mps.architecture) or the fix function design
    (design.mps.fix). Richard Brooksby, 1995-08-28.

    They were copied from design.mps.arena.vm(1) and edited slightly.
    David Jones, 1999-06-23.


Block management
................

_`.req.fun.block.alloc`: The arena must provide allocation of
contiguous blocks of memory.

_`.req.fun.block.free`: It must also provide freeing of contiguously
allocated blocks owned by a pool - whether or not the block was
allocated via a single request.

_`.req.attr.block.size.min`: The arena must support management of
blocks down to the size of the grain (page) provided by the virtual
mapping interface if a virtual memory interface is being used, or a
comparable size otherwise.

_`.req.attr.block.size.max`: It must also support management of blocks
up to the maximum size allowed by the combination of operating system
and architecture. This is derived from req.dylan.attr.obj.max (at
least).

_`.req.attr.block.align.min`: The alignment of blocks shall not be less
than ``MPS_PF_ALIGN`` for the architecture. This is so that pool
classes can conveniently guarantee pool allocated blocks are aligned
to ``MPS_PF_ALIGN``. (A trivial requirement.)

_`.req.attr.block.grain.max`: The granularity of allocation shall not
be more than the grain size provided by the virtual mapping interface.


Address translation
...................

_`.req.fun.trans`: The arena must provide a translation from any
address to either an indication that the address is not in any tract
(if that is so) or the following data associated with the tract
containing that address:

_`.req.fun.trans.pool`: The pool that allocated the tract.

_`.req.fun.trans.arbitrary`: An arbitrary pointer value that the pool
can associate with the tract at any time.

_`.req.fun.trans.white`: The tracer whiteness information. That is, a
bit for each active trace that indicates whether this tract is white
(contains white objects). This is required so that the "fix" protocol
can run very quickly.

_`.req.attr.trans.time`: The translation shall take no more than @@@@
[something not very large -- drj 1999-06-23]


Iteration protocol
..................

_`.req.iter`: er, there's a tract iteration protocol which is
presumably required for some reason?


Arena partition
...............

_`.req.fun.set`: The arena must provide a method for approximating sets
of addresses.

_`.req.fun.set.time`: The determination of membership shall take no
more than @@@@ [something very small indeed]. (the non-obvious
solution is refsets)


Constraints
...........

_`.req.attr.space.overhead`: req.dylan.attr.space.struct implies that
the arena must limit the space overhead. The arena is not the only
part that introduces an overhead (pool classes being the next most
obvious), so multiple parts must cooperate in order to meet the
ultimate requirements.

_`.req.attr.time.overhead`: Time overhead constraint?

.. note::

    How can there be a time "overhead" on a necessary component? David
    Jones, 1999-06-23.


Architecture
------------

Statics
.......

_`.static`: There is no higher-level data structure than a arena, so in
order to support several arenas, we have to have some static data in
impl.c.arena. See impl.c.arena.static.

_`.static.init`: All the static data items are initialized when the
first arena is created.

_`.static.serial`: ``arenaSerial`` is a static ``Serial``, containing
the serial number of the next arena to be created. The serial of any
existing arena is less than this.

_`.static.ring`: ``arenaRing`` is the sentinel of the ring of arenas.

_`.static.ring.init`: ``arenaRingInit`` is a ``Bool`` showing whether
the ring of arenas has been initialized.

_`.static.ring.lock`: The ring of arenas has to be locked when
traversing the ring, to prevent arenas being added or removed. This is
achieved by using the (non-recursive) global lock facility, provided
by the lock module.

_`.static.check`: The statics are checked each time any arena is
checked.


Arena classes
.............

``typedef mps_arena_s *Arena``

_`.class`: The ``Arena`` data structure is designed to be subclassable
(see design.mps.protocol(0)). Clients can select what arena class
they'd like when instantiating one with ``mps_arena_create()``. The
arguments to ``mps_arena_create()`` are class-dependent.

_`.class.init`: However, the generic ``ArenaInit()`` is called from the
class-specific method, rather than vice versa, because the method is
responsible for allocating the memory for the arena descriptor and the
arena lock in the first place. Likewise, ``ArenaFinish()`` is called
from the finish method.

_`.class.fields`: The ``alignment`` (for tract allocations) and
``zoneShift`` (for computing zone sizes and what zone an address is
in) fields in the arena are the responsibility of the each class, and
are initialized by the ``init`` method. The responsibility for
maintaining the ``commitLimit``, ``spareCommitted``, and
``spareCommitLimit`` fields is shared between the (generic) arena and
the arena class. ``commitLimit`` (see `.commit-limit`_) is changed by
the generic arena code, but arena classes are responsible for ensuring
the semantics. For ``spareCommitted`` and ``spareCommitLimit`` see
`.spare-committed`_ below.

_`.class.abstract`: The basic arena class (``AbstractArenaClass``) is
abstract and must not be instantiated. It provides little useful
behaviour, and exists primarily as the root of the tree of arena
classes. Each concrete class must specialize each of the class method
fields, with the exception of the describe method (which has a trivial
implementation) and the ``extend``, ``retract`` and
``spareCommitExceeded`` methods which have non-callable methods for
the benefit of arena classes which don't implement these features.

_`.class.abstract.null`: The abstract class does not provide dummy
implementations of those methods which must be overridden. Instead
each abstract method is initialized to ``NULL``.


Tracts
......

_`.tract`: The arena allocation function ``ArenaAlloc()`` allocates a
block of memory to pools, of a size which is aligned to the arena
alignment. Each alignment unit (grain) of allocation is represented by
a tract. Tracts are the hook on which the segment module is
implemented. Pools which don't use segments may use tracts for
associating their own data with each allocation grain.

_`.tract.structure`: The tract structure definition looks like this::

    typedef struct TractStruct { /* Tract structure */
      PagePoolUnion pool; /* MUST BE FIRST (design.mps.arena.tract.field.pool) */
      void *p;                    /* pointer for use of owning pool */
      Addr base;                  /* Base address of the tract */
      TraceSet white : TRACE_MAX; /* traces for which tract is white */
      unsigned int hasSeg : 1;    /* does tract have a seg in p?  */
    } TractStruct;

_`.tract.field.pool`: The pool.pool field indicates to which pool the tract
has been allocated (`.req.fun.trans.pool`_). Tracts are only valid
when they are allocated to pools. When tracts are not allocated to
pools, arena classes are free to reuse tract objects in undefined
ways. A standard technique is for arena class implementations to
internally describe the objects as a union type of ``TractStruct`` and
some private representation, and to set the pool field to ``NULL``
when the tract is not allocated. The pool field must come first so
that the private representation can share a common prefix with
``TractStruct``. This permits arena classes to determine from their
private representation whether such an object is allocated or not,
without requiring an extra field.

_`.tract.field.p`: The ``p`` field is used by pools to associate tracts
with other data (`.req.fun.trans.arbitrary`_). It's used by the
segment module to indicate which segment a tract belongs to. If a pool
doesn't use segments it may use the ``p`` field for its own purposes.
This field has the non-specific type ``(void *)`` so that pools can
use it for any purpose.

_`.tract.field.hasSeg`: The ``hasSeg`` bit-field is a Boolean which
indicates whether the ``p`` field is being used by the segment module.
If this field is ``TRUE``, then the value of ``p`` is a ``Seg``.
``hasSeg`` is typed as an ``unsigned int``, rather than a ``Bool``.
This ensures that there won't be sign conversion problems when
converting the bit-field value.

_`.tract.field.base`: The base field contains the base address of the
memory represented by the tract.

_`.tract.field.white`: The white bit-field indicates for which traces
the tract is white (`.req.fun.trans.white`_). This information is also
stored in the segment, but is duplicated here for efficiency during a
call to ``TraceFix`` (see design.mps.trace.fix).

_`.tract.limit`: The limit of the tract's memory may be determined by
adding the arena alignment to the base address.

_`.tract.iteration`: Iteration over tracts is described in
design.mps.arena.tract-iter(0).

``Bool TractOfAddr(Tract *tractReturn, Arena arena, Addr addr)``

_`.tract.if.tractofaddr`: The function ``TractOfAddr()`` finds the tract
corresponding to an address in memory. (See `.req.fun.trans`_.)

If ``addr`` is an address which has been allocated to some pool, then
``TractOfAddr()`` returns ``TRUE``, and sets ``*tractReturn`` to the
tract corresponding to that address. Otherwise, it returns ``FALSE``.
This function is similar to ``TractOfBaseAddr()`` (see
design.mps.arena.tract-iter.if.contig-base) but serves a more general
purpose and is less efficient.

_`.tract.if.TRACT_OF_ADDR`: ``TRACT_OF_ADDR()`` is a macro version of
``TractOfAddr()``. It's provided for efficiency during a call to
``TraceFix()`` (see design.mps.trace.fix.tractofaddr).


Control pool
............

_`.pool`: Each arena has a "control pool",
``arena->controlPoolStruct``, which is used for allocating MPS control
data structures by calling ``ControlAlloc()``.


Polling
.......

_`.poll`: ``ArenaPoll()`` is called "often" by other code (for instance,
on buffer fill or allocation). It is the entry point for doing tracing
work. If the polling clock exceeds a set threshold, and we're not
already doing some tracing work (that is, ``insidePoll`` is not set),
it calls ``TracePoll()`` on all busy traces.

_`.poll.size`: The actual clock is ``arena->fillMutatorSize``. This is
because internal allocation is only significant when copy segments are
being allocated, and we don't want to have the pause times to shrink
because of that. There is no current requirement for the trace rate to
guard against running out of memory.

.. note::

    Clearly it really ought to: we have a requirement to not run out
    of memory (see req.dylan.prot.fail-alloc, req.dylan.prot.consult),
    and emergency tracing should not be our only story. David Jones,
    1999-06-22.

``BufferEmpty()`` is not taken into account, because the splinter will
rarely be useable for allocation and we are wary of the clock running
backward.

_`.poll.clamp`: Polling is disabled when the arena is "clamped", in
which case ``arena->clamped`` is ``TRUE``. Clamping the arena prevents
background tracing work, and further new garbage collections from
starting. Clamping and releasing are implemented by the ``ArenaClamp()``
and ``ArenaRelease()`` methods.

_`.poll.park`: The arena is "parked" by clamping it, then polling until
there are no active traces. This finishes all the active collections
and prevents further collection. Parking is implemented by the
``ArenaPark()`` method.


Commit limit
............

_`.commit-limit`: The arena supports a client configurable "commit
limit" which is a limit on the total amount of committed memory. The
generic arena structure contains a field to hold the value of the
commit limit and the implementation provides two functions for
manipulating it: ``ArenaCommitLimit()`` to read it, and
``ArenaSetCommitLimit()`` to set it. Actually abiding by the contract of
not committing more memory than the commit limit is left up to the
individual arena classes.

_`.commit-limit.err`: When allocation from the arena would otherwise
succeed but cause the MPS to use more committed memory than specified
by the commit limit ``ArenaAlloc()`` should refuse the request and
return ``ResCOMMIT_LIMIT``.

_`.commit-limit.err.multi`: In the case where an ``ArenaAlloc()`` request
cannot be fulfilled for more than one reason including exceeding the
commit limit then class implementations should strive to return a
result code other than ``ResCOMMIT_LIMIT``. That is,
``ResCOMMIT_LIMIT`` should only be returned if the *only* reason for
failing the ``ArenaAlloc()`` request is that the commit limit would be
exceeded. The client documentation allows implementations to be
ambiguous with respect to which result code in returned in such a
situation however.


Spare committed (aka "hysteresis")
..................................

_`.spare-committed`: See ``mps_arena_spare_committed()``. The generic
arena structure contains two fields for the spare committed memory
fund: ``spareCommitted`` records the total number of spare committed
bytes; ``spareCommitLimit`` records the limit (set by the user) on the
amount of spare committed memory. ``spareCommitted`` is modified by
the arena class but its value is used by the generic arena code. There
are two uses: a getter function for this value is provided through the
MPS interface (``mps_arena_spare_commit_limit_set()``), and by the
``SetSpareCommitLimit()`` function to determine whether the amount of
spare committed memory needs to be reduced. ``spareCommitLimit`` is
manipulated by generic arena code, however the associated semantics
are the responsibility of the class. It is the class's responsibility
to ensure that it doesn't use more spare committed bytes than the
value in ``spareCommitLimit``.

_`.spare-commit-limit`: The function ``ArenaSetSpareCommitLimit()`` sets
the ``spareCommitLimit`` field. If the limit is set to a value lower
than the amount of spare committed memory (stored in
``spareCommitted``) then the class specific function
``spareCommitExceeded`` is called.


Locks
.....

_`.lock.ring`: ``ArenaAccess()`` is called when we fault on a barrier.
The first thing it does is claim the non-recursive global lock to
protect the arena ring (see design.mps.lock(0)).

_`.lock.arena`: After the arena ring lock is claimed, ``ArenaEnter()`` is
called on one or more arenas. This claims the lock for that arena.
When the correct arena is identified or we run out of arenas, the lock
on the ring is released.

_`.lock.avoid`: Deadlocking is avoided as described below:

_`.lock.avoid.mps`: Firstly we require the MPS not to fault (that is,
when any of these locks are held by a thread, that thread does not
fault).

_`.lock.avoid.thread`: Secondly, we require that in a multi-threaded
system, memory fault handlers do not suspend threads (although the
faulting thread will, of course, wait for the fault handler to
finish).

_`.lock.avoid.conflict`: Thirdly, we avoid conflicting deadlock between
the arena and global locks by ensuring we never claim the arena lock
when the recursive global lock is already held, and we never claim the
binary global lock when the arena lock is held.


Location dependencies
.....................

_`.ld`: Location dependencies use fields in the arena to maintain a
history of summaries of moved objects, and to keep a notion of time,
so that the staleness of location dependency can be determined.


Finalization
............

_`.final`: There is a pool which is optionally (and dynamically)
instantiated to implement finalization. The fields ``finalPool`` and
``isFinalPool`` are used.


Implementation
--------------


Tract cache
...........

_`.tract.cache`: When tracts are allocated to pools by ``ArenaAlloc()``,
the first tract of the block and it's base address are cached in arena
fields ``lastTract`` and ``lastTractBase``. The function
``TractOfBaseAddr()`` (see design.mps.arena.tract-iter.if.block-base(0))
checks against these cached values and only calls the class method on
a cache miss. This optimizes for the common case where a pool
allocates a block and then iterates over all its tracts (for example,
to attach them to a segment).

_`.tract.uncache`: When blocks of memory are freed by pools,
``ArenaFree()`` checks to see if the cached value for the most recently
allocated tract (see `.tract.cache`_) is being freed. If so, the cache
is invalid, and must be reset. The ``lastTract`` and ``lastTractBase``
fields are set to ``NULL``.


Control pool
............

_`.pool.init`: The control pool is initialized by a call to
``PoolInit()`` during ``ArenaCreate()``.

_`.pool.ready`: All the other fields in the arena are made checkable
before calling ``PoolInit()``, so ``PoolInit()`` can call
``ArenaCheck(arena)``. The pool itself is, of course, not checkable,
so we have a field ``arena->poolReady``, which is false until after
the return from ``PoolInit()``. ``ArenaCheck()`` only checks the pool if
``poolReady``.


Traces
......

_`.trace`: ``arena->trace[ti]`` is valid if and only if
``TraceSetIsMember(arena->busyTraces, ti)``.

_`.trace.create`: Since the arena created by ``ArenaCreate()`` has
``arena->busyTraces = TraceSetEMPTY``, none of the traces are
meaningful.

_`.trace.invalid`: Invalid traces have signature ``SigInvalid``, which
can be checked.


Polling
.......

_`.poll.fields`: There are three fields of a arena used for polling:
``pollThreshold``, ``insidePoll``, and ``clamped`` (see above).
``pollThreshold`` is the threshold for the next poll: it is set at the
end of ``ArenaPoll()`` to the current polling time plus
``ARENA_POLL_MAX``.


Location dependencies
.....................

_`.ld.epoch`: ``arena->epoch`` is the "current epoch". This is the
number of 'flips' of traces in the arena since the arena was created.
From the mutator's point of view locations change atomically at flip.

_`.ld.history`: ``arena->history`` is an array of ``ARENA_LD_LENGTH``
elements of type ``RefSet``. These are the summaries of moved objects
since the last ``ARENA_LD_LENGTH`` epochs. If ``e`` is one of these
recent epochs, then ::

    arena->history[e % ARENA_LD_LENGTH]

is a summary of (the original locations of) objects moved since epoch
``e``.

_`.ld.prehistory`: ``arena->prehistory`` is a ``RefSet`` summarizing
the original locations of all objects ever moved. When considering
whether a really old location dependency is stale, it is compared with
this summary.


Roots
.....

_`.root-ring`: The arena holds a member of a ring of roots in the
arena. It holds an incremental serial which is the serial of the next
root.


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

- 1997-08-11 Pekka P. Pirinen. First draft, based on
  design.mps.space(0) and mail.richard.1997-04-25.11-52(0).

- 1999-04-16 Tony Mann. Updated for separation of tracts and segments.

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

- 2013-03-11 GDR_ Converted to reStructuredText.

- 2014-02-17 RB_ Updated first field of tract structure.

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


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

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