2. Arena¶
2.2. 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.
2.3. 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.
2.4. 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.
2.4.1. 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.
2.4.2. 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]
2.4.3. Iteration protocol¶
.req.iter: er, there’s a tract iteration protocol which is presumably required for some reason?
2.4.4. 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)
2.4.5. 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.
2.5. Architecture¶
2.5.1. 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.
2.5.2. Arena classes¶
-
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.
2.5.3. 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 */
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: 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).
2.5.4. Control pool¶
.pool: Each arena has a “control pool”,
arena->controlPoolStruct, which is used for allocating MPS control
data structures by calling ControlAlloc().
2.5.5. 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.
2.5.6. 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.
2.5.7. 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.
2.5.8. 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.
2.6. Implementation¶
2.6.1. 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.
2.6.2. 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.
2.6.3. 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.
2.6.4. 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.
2.6.5. 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.
2.6.6. 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.
