17. AWL pool class

17.1. Introduction

.readership: Any MPS developer.

.intro: The AWL (Automatic Weak Linked) pool is used to manage Dylan Weak Tables (see req.dylan.fun.weak). Currently the design is specialised for Dylan Weak Tables, but it could be generalised in the future.

17.2. Requirements

See req.dylan.fun.weak.

See meeting.dylan.1997-02-27(0) where many of the requirements for this pool were first sorted out.

Must satisfy request.dylan.170123.

.req.obj-format: Only objects of a certain format need be supported. This format is a subset of the Dylan Object Format. The pool uses the first slot in the fixed part of an object to store an association. See mail.drj.1997-03-11.12-05.

17.3. Definitions

.def.grain: alignment grain, grain. A grain is a range of addresses where both the base and the limit of the range are aligned and the size of range is equal to the (same) alignment. In this context the alignment is the pool’s alignment (pool->alignment). The grain is the unit of allocation, marking, scanning, etc.

17.4. Overview


.overview.ms: The pool is mark and sweep. .overview.ms.justify: Mark-sweep pools are slightly easier to write (than moving pools), and there are no requirements (yet) that this pool be high performance or moving or anything like that.

.overview.alloc: It is possible to allocate weak or exact objects using the normal reserve/commit AP protocol. .overview.alloc.justify: Allocation of both weak and exact objects is required to implement Dylan Weak Tables. Objects are formatted; the pool uses format A.

.overview.scan: The pool handles the scanning of weak objects specially so that when a weak reference is deleted the corresponding reference in an associated object is deleted. The associated object is determined by using information stored in the object itself (see .req.obj-format).

17.5. Interface

.if.init: The init method takes one extra parameter in the vararg list. This parameter should have type Format and be a format object that describes the format of the objects to be allocated in this pool. The format should support scan and skip methods. There is an additional restriction on the layout of objects, see .req.obj-format.

.if.buffer: The BufferInit() method takes one extra parameter in the vararg list. This parameter should be either RankEXACT or RankWEAK. It determines the rank of the objects allocated using that buffer.

17.6. Data structures

.sig: This signature for this pool will be 0x519bla3l (SIGPooLAWL).

.poolstruct: The class specific pool structure is:

struct AWLStruct {
  PoolStruct poolStruct;
  PoolGenStruct pgenStruct; /* pool generation */
  PoolGen pgen;             /* NULL or pointer to pgenStruct */
  Count succAccesses;       /* number of successive single accesses */
  FindDependentFunction findDependent; /*  to find a dependent object */
  awlStatTotalStruct stats;
  Sig sig;                  /* <code/misc.h#sig> */

.awlseg: The pool defines a segment class AWLSegClass, which is a subclass of MutatorSegClass (see design.mps.seg.over.hierarchy.mutatorseg). All segments allocated by the pool are instances of this class, and are of type AWLSeg, for which the structure is:

struct AWLSegStruct {
  GCSegStruct gcSegStruct;  /* superclass fields must come first */
  BT mark;
  BT scanned;
  BT alloc;
  Count grains;
  Count freeGrains;         /* free grains */
  Count bufferedGrains;     /* grains in buffers */
  Count newGrains;          /* grains allocated since last collection */
  Count oldGrains;          /* grains allocated prior to last collection */
  Count singleAccesses;     /* number of accesses processed singly */
  awlStatSegStruct stats;
  Sig sig;                  /* <code/misc.h#sig> */

.awlseg.bt: The mark, alloc, and scanned fields are bit-tables (see design.mps.bt). Each bit in the table corresponds to a single alignment grain in the pool.

.awlseg.mark: The mark bit table is used to record mark bits during a trace. awlSegWhiten() (see .fun.whiten below) sets all the bits of this table to zero. Fix will read and set bits in this table. Currently there is only one mark bit table. This means that the pool can only be condemned for one trace.

.awlseg.mark.justify: This is simple, and can be improved later when we want to run more than one trace.

.awlseg.scanned: The scanned bit-table is used to note which objects have been scanned. Scanning (see .fun.scan below) a segment will find objects that are marked but not scanned, scan each object found and set the corresponding bits in the scanned table.

.awlseg.alloc: The alloc bit table is used to record which portions of a segment have been allocated. Ranges of bits in this table are set in awlSegBufferFill() when a buffer is attached to the segment. When a buffer is flushed (that is, awlSegBufferEmpty() is called) from the segment, the bits corresponding to the unused portion at the end of the buffer are reset.

.awlseg.alloc.invariant: A bit is set in the alloc table if and only if the corresponding address is currently being buffered, or the corresponding address lies within the range of an allocated object.

.awlseg.grains: The grains field is the number of grains that fit in the segment. Strictly speaking this is not necessary as it can be computed from SegSize and AWL’s alignment, however, precalculating it and storing it in the segment makes the code simpler by avoiding lots of repeated calculations.

.awlseg.freeGrains: A conservative estimate of the number of free grains in the segment. It is always guaranteed to be greater than or equal to the number of free grains in the segment, hence can be used during allocation to quickly pass over a segment.


Maintained by blah and blah. Unfinished obviously.

17.7. Functions


How will pool collect? It needs an action structure.

17.7.1. External

Res AWLInit(Pool pool, va_list arg)

.fun.init: AWLStruct has four fields, each one needs initializing.

.fun.init.poolstruct: The poolStruct field has already been initialized by generic code (impl.c.pool).

.fun.init.sig: The sig field will be initialized with the signature for this pool.

Res AWLFinish(Pool pool)

.fun.finish: Iterates over all segments in the pool and destroys each segment (by calling SegFree()). Overwrites the sig field in the AWLStruct. Finishing the generic pool structure is done by the generic pool code (impl.c.pool).

.fun.alloc: PoolNoAlloc() will be used, as this class does not implement alloc.

.fun.free: PoolNoFree() will be used, as this class does not implement free.

Res AWLBufferFill(Seg *segReturn, Addr *baseReturn, Pool pool, Buffer buffer, Size size)

.fun.fill: This zips round all the segments applying SegBufferFill() to each segment. awlSegBufferFill() attempts to find a large-enough free range; if it finds one then it may be bigger than the actual request, in which case the remainder can be used to “fill” the rest of the buffer. If no free range can be found in an existing segment then a new segment will be created (which is at least large enough). The range of buffered addresses is marked as allocated in the segment’s alloc table.

Res AWLDescribe(Pool pool, mps_lib_FILE *stream, Count depth)


17.7.2. Internal

Res AWLSegCreate(AWLSeg *awlsegReturn, Size size)

.fun.awlsegcreate: Creates a segment of class AWLSegClass of size at least size.

.fun.awlsegcreate.size.round: size is rounded up to the arena grain size before requesting the segment.

.fun.awlsegcreate.size.round.justify: The arena requires that all segment sizes are rounded up to the arena grain size.

.fun.awlsegcreate.where: The segment is allocated using a generation preference, using the generation number stored in the AWLStruct (the gen field), see .poolstruct.gen above.

Res awlSegInit(Seg seg, Pool pool, Addr base, Size size, ArgList args)

.fun.awlseginit: Init method for AWLSegClass, called for SegAlloc() whenever an AWLSeg is created (see .fun.awlsegcreate above).

.fun.awlseginit.tables: The segment’s mark scanned and alloc tables (see .awlseg.bt above) are allocated and initialised. The segment’s grains field is computed and stored.

void awlSegFinish(Seg seg)

.fun.awlsegfinish: Finish method for AWLSegClass, called from SegFree(). Will free the segment’s tables (see .awlseg.bt).

Bool awlSegBufferFill(Addr *baseReturn, Addr *limitReturn, Seg seg, Size size, RankSet rankSet)

.fun.seg.buffer-fill: Searches for a free block in the segment that is at least size bytes long. The base address of the block is returned in *baseReturn, the limit of the entire free block (which must be at least as large as size and may be bigger) is returned in *limitReturn. The requested size is converted to a number of grains, BTFindResRange() is called to find a run of this length in the alloc bit-table (.awlseg.alloc). The results (if it is successful) from BTFindResRange() are in terms of grains, they are converted back to addresses before returning the relevant values from this function.

void awlSegBufferEmpty(Seg seg, Buffer buffer)

.fun.seg.buffer-empty: Locates the free portion of the buffer, that is the memory between the init and the limit of the buffer and records these locations as being free in the alloc table.

Res awlSegWhiten(Seg seg, Trace trace)

.fun.whiten: The current design only permits each segment to be condemned for one trace (see .awlseg.mark). This function checks that the segment is not white for any trace (seg->white == TraceSetEMPTY). The segment’s mark bit-table is reset, and the whiteness of the seg (seg->white) has the current trace added to it.

void awlSegGreyen(Seg seg, Trace trace)

.fun.grey: If the segment is not white for this trace, the segment’s mark table is set to all 1s and the segment is recorded as being grey.

Res awlSegScan(Bool *totalReturn, Seg seg, ScanState ss)


.fun.scan.overview: The scanner performs a number of passes over the segment, scanning each marked and unscanned (grey) object that is finds.

.fun.scan.overview.finish: It keeps perform a pass over the segment until it is finished.

.fun.scan.overview.finish.condition: A condition for finishing is that no new marks got placed on objects in this segment during the pass.

.fun.scan.overview.finish.approximation: We use an even stronger condition for finishing that assumes that scanning any object may introduce marks onto this segment. It is finished when a pass results in scanning no objects (that is, all objects were either unmarked or both marked and scanned).

.fun.scan.overview.finished-flag: There is a flag called finished which keeps track of whether we should finish or not. We only ever finish at the end of a pass. At the beginning of a pass the flag is set. During a pass if any objects are scanned then the finished flag is reset. At the end of a pass if the finished flag is still set then we are finished. No more passes take place and the function returns.

.fun.scan.pass: A pass consists of a setup phase and a repeated phase.

.fun.scan.pass.buffer: The following assumes that in the general case the segment is buffered; if the segment is not buffered then the actions that mention buffers are not taken (they are unimportant if the segment is not buffered).

.fun.scan.pass.p: The pass uses a cursor called p to progress over the segment. During a pass p will increase from the base address of the segment to the limit address of the segment. When p reaches the limit address of the segment, the pass in complete.

.fun.scan.pass.setup: p initially points to the base address of the segment.

.fun.scan.pass.repeat: The following comprises the repeated phase. The repeated phase is repeated until the pass completion condition is true (that is, p has reached the limit of the segment, see .fun.scan.pass.p above and .fun.scan.pass.repeat.complete below).

.fun.scan.pass.repeat.complete: If p is equal to the segment’s limit then we are done. We proceed to check whether any further passes need to be performed (see .fun.scan.pass.more below).

.fun.scan.pass.repeat.free: If !alloc(p) (the grain is free) then increment p and return to the beginning of the loop.

.fun.scan.pass.repeat.buffer: If p is equal to the buffer’s ScanLimit, as returned by BufferScanLimit(), then set p equal to the buffer’s Limit, as returned by BufferLimit() and return to the beginning of the loop.

.fun.scan.pass.repeat.object-end: The end of the object is located using the format->skip method.

.fun.scan.pass.repeat.object: if mark(p) && !scanned(p) then the object pointed at is marked but not scanned, which means we must scan it, otherwise we must skip it.

.fun.scan.pass.repeat.object.dependent: To scan the object the object we first have to determine if the object has a dependent object (see .req.obj-format).

.fun.scan.pass.repeat.object.dependent.expose: If it has a dependent object then we must expose the segment that the dependent object is on (only if the dependent object actually points to MPS managed memory) prior to scanning and cover the segment subsequent to scanning.

.fun.scan.pass.repeat.object.dependent.summary: The summary of the dependent segment must be set to RefSetUNIV to reflect the fact that we are allowing it to be written to (and we don’t know what gets written to the segment).

.fun.scan.pass.repeat.object.scan: The object is then scanned by calling the format’s scan method with base and limit set to the beginning and end of the object (.fun.scan.scan.improve.single: A scan1 format method would make it slightly simpler here). Then the finished flag is cleared and the bit in the segment’s scanned table is set.

.fun.scan.pass.repeat.advance: p is advanced past the object and we return to the beginning of the loop.

.fun.scan.pass.more: At the end of a pass the finished flag is examined.

.fun.scan.pass.more.not: If the finished flag is set then we are done (see .fun.scan.overview.finished-flag above), awlSegScan() returns.

.fun.scan.pass.more.so: Otherwise (the finished flag is reset) we perform another pass (see .fun.scan.pass above).

Res awlSegFix(Seg seg, ScanState ss, Ref *refIO)

.fun.fix: If the rank (ss->rank) is RankAMBIG then fix returns immediately unless the reference is in the segment bounds, aligned to the pool alignment, and allocated.

The bit in the marked table corresponding to the referenced grain will be read. If it is already marked then fix returns. Otherwise (the grain is unmarked), ss->wasMarked is set to FALSE (see design.mps.fix.was-marked.not), the remaining actions depend on whether the rank (ss->rank) is RankWEAK or not. If the rank is weak then the reference is adjusted to 0 (see design.mps.weakness) and fix returns. If the rank is something else then the mark bit corresponding to the referenced grain is set, and the segment is greyed using SegSetGrey().

void awlSegReclaim(Seg seg, Trace trace)

.fun.reclaim: This iterates over all allocated objects in the segment and frees objects that are not marked. When this iteration is complete the marked array is completely reset.

p points to base of segment. Then:

while(p < SegLimit(seg) {
  if(!alloc(p)) { ++p;continue; }
  q = skip(p) /* q points to just past the object pointed at by p */
  if !marked(p) free(p, q); /* reset the bits in the alloc table from p to q-1 inclusive. */
  p = q

Finally, reset the entire marked array using BTResRange().

.fun.reclaim.improve.pad: Consider filling free ranges with padding objects. Now reclaim doesn’t need to check that the objects are allocated before skipping them. There may be a corresponding change for scan as well.

Bool AWLDependentObject(Addr *objReturn, Addr parent)

.fun.dependent-object: This function abstracts the association between an object and its linked dependent (see .req.obj-format). It currently assumes that objects are Dylan Object formatted according to design.dylan.container (see analysis.mps.poolawl.dependent.abstract for suggested improvements). An object has a dependent object iff the second word of the object, that is, ((Word *)parent)[1], is non-NULL. The dependent object is the object referenced by the second word and must be a valid object.

This function assumes objects are in Dylan Object Format (see design.dylan.container). It will check that the first word looks like a Dylan wrapper pointer. It will check that the wrapper indicates that the wrapper has a reasonable format (namely at least one fixed field). If the second word is NULL it will return FALSE. If the second word is non-NULL then the contents of it will be assigned to *objReturn, and it will return TRUE.

17.8. Test

  • must create Dylan objects.

  • must create Dylan vectors with at least one fixed field.

  • must allocate weak thingies.

  • must allocate exact tables.

  • must link tables together.

  • must populate tables with junk.

  • some junk must die.

Use an LO pool and an AWL pool. Three buffers. One buffer for the LO pool, one exact buffer for the AWL pool, one weak buffer for the AWL pool.

Initial test will allocate one object from each buffer and then destroy all buffers and pools and exit