2. Arena

2.1. Introduction

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

.readership: MPS developers.

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 */
  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).

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.

2.5.9. 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.

2.5.10. Finalization

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

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.