THE DESIGN OF THE MPS ARENA
                            design.mps.arena
                           incomplete design
                            pekka 1997-08-11

INTRODUCTION


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

.readership: MM developers.


Document History

.hist.0: Version 0 is a different document.

.hist.1: First draft written by Pekka P. Pirinen 1997-08-11, based on 
design.mps.space(0) and mail.richard.1997-04-25.11-52(0).

.hist.2: Updated for separation of tracts and segments.  tony 1999-04-16


OVERVIEW

.overview: The arena serves two purposes: A structure that is the top-level 
state of the MPS, and as such contains a lot of fields which are considered 
"global"; Provision of raw memory to pools.

An arena is of a particular arena class, the class is selected when the arena 
is created.  Classes encapsulate both policy (such as how pools placement 
preferences map into actual placement) and mechanism (such as where the memory 
originates: OS VM, client provided, via malloc).  Some behaviour (most in the 
former "top-level datastructure" category) is implemented by generic arena 
code, some by arena class code.  To some extent the arena coordinates placement 
policies between different pools active in the same arena, however this 
functionality is likely to be replaced by something more modular and which does 
a better job: The Locus Manager.


DEFINITIONS

.def.tract: Pools request memory from the arena (using ArenaAlloc) as a block 
comprising a contiguous sequence of units.  The units are known as tracts.  A 
tract has a specific size (the arena alignment, often corresponds to the OS 
page size) and all tracts are aligned to that size.  Also used to mean the 
datastructure used to manage tracts.


REQUIREMENTS

[copied from design.mps.arena.vm(1) and edited slightly -- drj 1999-06-23]

[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 1995-08-28]

These requirements are the responsibility of the class implementations as well 
as the generic arena.  However, some classes (ANSI arena, arenaan.c, in 
particular) are not intended for production use so do not have to meet all the 
speed and space requirements.


Block Management

.req.fun.block.alloc: The Arena Manager 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 Manager must support management of blocks 
down to the size of the grain (page) provided by the virtual mapping interface 
if a VM interface is being used, 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 (defined in "mpstd.h" included via "config.h") 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.  IE a bit for each 
active trace that indicates whether this tract is white (contains white 
objects).  This is required so that the tracer resolve / preserve (aka "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 Manager 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? [how can there be a time 
"overhead" on a necessary component?  drj 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

.class: The Arena datastructure 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, 
spareCommitLimit fields is shared between the (generic) arena and the arena 
class.  commitLimit (see .commit-limit below) 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 an object called 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 as follows:-

typedef struct TractStruct { /* Tract structure */
  Pool 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 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).

.tract.if.tractofaddr: Function TractOfAddr finds the tract corresponding to an 
address in memory. (.req.fun.trans).

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

If addr is an address which has been allocated to some pool, then returns TRUE, 
and sets *tractReturn to the tract corresponding to that address. Otherwise, 
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 (using ControlAlloc()).


Polling

.poll: ArenaPoll is called "often" by other MM 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 (i.e., 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. [clearly it really ought to though, we have a requirement to not run 
out of memory (req.dylan.prot.fail-alloc, req.dylan.prot.consult), emergency 
tracing should not be our only story.  drj 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 (ie 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 symbol.mps.c.mps_arena_spare_committed(0))  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 follows:

.lock.avoid.mps: Firstly we require the MPS not to fault (i.e., 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 (e.g. 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 RefSets.  These are 
the summaries of moved objects since the last ARENA_LD_LENGTH epochs.  If e is 
one of these recent epochs, 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.