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.grain: The arena manages memory in units called arena
grains, whose size is returned by the macro ArenaGrainSize().
Memory allocated by ArenaAlloc() is a contiguous sequence of arena
grains, whose base address and size are multiples of the arena grain
size.
.def.tract: A tract is a data structure containing information about a region of address space: which pool it belongs to (if any), for which traces the contents is white, and so on. 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 ranges of address.
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 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 larger of (i) the grain size of the virtual mapping interface (if a virtual memory interface is being used); and (ii) the grain size of the memory protection interface (if protection is used).
Note
On all the operating systems we support, these grain sizes are the same and are equal to the operating system page size. But we want the MPS to remain flexible enough to be ported to operating systems where these are different.
.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.)
2.4.2. Address translation¶
.req.fun.trans: The arena must provide a translation from any address to the following information:
.req.fun.trans.arena: Whether the address is managed by the arena.
.req.fun.trans.pool: Whether the address is managed by a pool within the arena, and if it is, the pool.
.req.fun.trans.arbitrary: If the address is managed by a pool, an arbitrary pointer value that the pool can associate with a group of contiguous addresses at any time.
.req.fun.trans.white: If the address is managed by an automatic pool, the set of traces for which the address is white. This is required so that the second-stage fix protocol can reject non-white addresses quickly. See design.mps.critical-path.
.req.attr.trans.time: The translation shall take no more than @@@@ [something not very large – drj 1999-06-23]
2.4.3. 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)
2.4.4. 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). Clients can select what arena class
they’d like when instantiating one with mps_arena_create_k(). The
arguments to mps_arena_create_k() are class-dependent.
.class.fields: The grainSize (for allocation and freeing) 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.
2.5.3. Chunks¶
.chunk: Each contiguous region of address space managed by the MPS is represented by a chunk.
.chunk.tracts: A chunk contains a table of tracts. See .tract.
.chunk.lookup: Looking of the chunk of an address is the first step in the second-stage fix operation, and so on the critical path. See design.mps.critical-path.
.chunk.tree: For efficient lookup, chunks are stored in a balanced
tree; arena->chunkTree points to the root of the tree. Operations
on this tree must ensure that the tree remains balanced, otherwise
performance degrades badly with many chunks.
.chunk.insert: New chunks are inserted into the tree by calling
ArenaChunkInsert(). This calls TreeInsert(), followed by
TreeBalance() to ensure that the tree is balanced.
.chunk.delete: There is no corresponding function
ArenaChunkDelete(). Instead, deletions from the chunk tree are
carried out by calling TreeToVine(), iterating over the vine
(where deletion is possible, if care is taken) and then calling
TreeBalance() on the remaining tree. The function
TreeTraverseAndDelete() implements this.
.chunk.delete.justify: This is because we don’t have a function that deletes an item from a balanced tree efficiently, and because all functions that delete chunks do so in a loop over the chunks (so the best we can do is O(n) time in any case).
.chunk.delete.tricky: Deleting chunks from the chunk tree is tricky
in the virtual memory arena because vmChunkDestroy() unmaps the
memory containing the chunk, which includes the tree node. So the next
chunk must be looked up before deleting the current chunk. The function
TreeTraverseAndDelete() ensures that this is done.
2.5.4. Tracts¶
.tract.table: The arena maintains tables of tract structures such that every address managed by the arena belongs to exactly one tract.
.tract.size: Each tract covers exactly one arena grain. This is an implementation detail, not a requirement.
.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 */
BOOLFIELD(hasSeg); /* 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. See
design.mps.type.bool.bitfield for why this is declared using the
BOOLFIELD macro.
.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 grain size 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.
2.5.5. Control pool¶
.pool: Each arena has a “control pool”,
arena->controlPoolStruct, which is used for allocating MPS control
data structures by calling ControlAlloc().
2.5.6. 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.
2.5.7. 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.
2.5.8. 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()), and by the
ArenaSetSpareCommitLimit() 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.
2.5.9. Pause time control¶
.pause-time: The generic arena structure contains the field
pauseTime for the maximum time any operation in the arena may take
before returning to the mutator. This value is used by
PolicyPollAgain() to decide whether to do another unit of tracing
work. The MPS interface provides getter (mps_arena_pause_time())
and setter (mps_arena_pause_time_set()) functions.
2.5.10. 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: The historyStruct contains fields used to maintain a
history of garbage collection and in particular object motion in order
to implement location dependency.
.ld.epoch: The epoch is the “current epoch”. This is the number
of “flips” of traces, in which objects might have moved, in the arena
since it was created. From the mutator’s point of view, locations
change atomically at flip.
.ld.history: The history is a circular buffer of
LDHistoryLENGTH elements of type RefSet. These are the
summaries of moved objects since the last LDHistoryLENGTH epochs.
If e is one of these recent epochs, then
history->history[e % LDHistoryLENGTH]
is a summary of (the original locations of) objects moved since epoch
e.
.ld.prehistory: The 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.
