AUTOMATIC WEAK LINKED
design.mps.poolawl
incomplete doc
drj 1997-03-11
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.
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
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.
OVERVIEW
.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).
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.
DATASTRUCTURES
.sig: This signature for this pool will be 0x519bla3l (SIGPooLAWL)
.poolstruct: The class specific pool structure is
struct AWLStruct {
PoolStruct poolStruct;
Format format;
Shift alignShift;
ActionStruct actionStruct;
double lastCollected;
Serial gen;
Sig sig;
}
.poolstruct.format: The format field is used to refer to the object format.
The object format is passed to the pool during pool creation.
.poolstruct.alignshift: The alignShift field is the SizeLog2 of the pool's
alignment. It is computed and initialised when a pool is created. It is used
to compute the number of alignment grains in a segment which is the number of
bits need in the segment's mark and alloc bit table (see .awlseg.bt,
.awlseg.mark, and .awlseg.alloc below). @@ clarify
.poolstruct.actionStruct: Contains an Action which is used to participate in
the collection benefit protocol. See .fun.benefit AWLBenefit below for a
description of the algorithm used for determining when to collect.
.poolstruct.lastCollected: Records the time (using the mutator total allocation
clock, ie that returned by ArenaMutatorAllocSize) of the most recent call to
either AWLInit or AWLTraceBegin for this pool. So this is the time of the
beginning of the last collection of this pool. Actually this isn't true
because the pool can be collected without AWLTraceBegin being called (I think)
as it will get collected by being in the same zone as another pool/generation
that is being collected (which it does arrange to be, see the use of the gen
field in .poolstruct.gen below and .fun.awlsegcreate.where below).
.poolstruct.gen: This part of the mechanism by which the pool arranges to be in
a particular zone and arranges to be collected simultaneously with other
cohorts in the system. gen is the generation that is used in expressing a
generation preference when allocating a segment. The intention is that this
pool will get collected simultaneously with any other segments that are also
allocated using this generation preference (when using the VM arena, generation
preferences get mapped more or less to zones, each generation to a unique set
of zones in the ideal case). Whilst AWL is not generational it is expected
that this mechanism will arrange for it to be collected simultaneously with
some particular generation of AMC.
.poolstruct.gen.1: At the moment the gen field is set for all AWL pools to be 1.
.awlseg: The pool defines a segment class AWLSegClass, which is a subclass of
GCSegClass (see design.mps.seg.over.hierarchy.gcseg). 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;
BT mark;
BT scanned;
BT alloc;
Count grains;
Count free;
Count singleAccesses;
AWLStatSegStruct stats;
Sig sig;
}
.awlseg.bt: The mark, alloc, and scanned fields are bit-tables (see
design.mps.bt). Each bit in the table corresponds to a a single alignment
grain in the pool.
.awlseg.mark: The mark bit table is used to record mark bits during a trace.
Condemn (see .fun.condemn 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 when a
buffer is attached to the segment. When a buffer is flushed (ie AWLBufferEmpty
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 <=> (the corresponding
address is currently being buffered || 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.free: A conservative estimate of the number of free grains in the
segment. It is always guaranteed to be >= 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.
FUNCTIONS
@@ How will pool collect? It needs an action structure.
External
.fun.init:
Res AWLInit(Pool pool, va_list arg);
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.format: The format will be copied from the argument list, checked,
and written into this field.
.fun.init.alignshift: The alignShift will be computed from the pool alignment
and written into this field.
.fun.init.sig: The sig field will be initialized with the signature for this
pool.
.fun.finish:
Res AWLFinish(Pool pool);
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.
.fun.fill:
Res AWLBufferFill(Seg *segReturn, Addr *baseReturn, Pool pool, Buffer buffer,
Size size);
This zips round all the the segments applying AWLSegAlloc to each segment that
has the same rank as the buffer. AWLSegAlloc attempts to find a free range, if
it finds a range 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.
.fun.empty:
void AWLBufferEmpty(Pool pool, Buffer buffer);
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
relevant alloc table. The segment that the buffer is pointing at (which
contains the alloc table that needs to be dinked with) is available via
BufferSeg.
.fun.benefit: The benefit returned is the total amount of mutator allocation
minus the lastRembemberedSize minus 10 Megabytes, so the pool becomes an
increasingly good candidate for collection at a constant (mutator allocation)
rate, crossing the 0 line when there has been 10Mb of allocation since the
(beginning of the) last collection. So it gets collected approximately every
10Mb of allocation. Note that it will also get collected by virtue of being in
the same zone as some AMC generation (assuming there are instantiated AMC
pools), see .poolstruct.gen above.
.fun.condemn:
Res AWLCondemn(Pool pool, Trace trace, Seg seg);
The current design only permits each segment to be condemned for one trace (see
.awlseg.mark). This function checks that the segment is not condemned 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.
.fun.grey:
void AWLGrey(Pool pool, Trace trace, Seg seg);
if the segment is not condemned for this trace the segment's mark table is set
to all 1s and the segment is recorded as being grey.
.fun.scan:
Res AWLScan(ScanState ss, Pool pool, Seg seg);
.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 (ie 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 flags 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 (ie 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 (see
BufferScanLimit), then set p equal to the buffer's Limit (use 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), AWLScan returns.
.fun.scan.pass.more.so: Otherwise (the finished flag is reset) we perform
another pass (see .fun.scan.pass above).
.fun.fix:
Res AWLFix(Pool pool, ScanState ss, Seg seg, Ref *refIO);
ss->wasMarked is set to TRUE (clear compliance with
design.mps.fix.protocol.was-marked.conservative).
If the rank (ss->rank) is RankAMBIG then fix returns immediately unless the
reference is aligned to the pool alignment.
If the rank (ss->rank) is RankAMBIG then fix returns immediately unless the
referenced grain is 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, the remaining actions depend on
whether the rank (ss->rank) is Weak 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 TraceSegGreyen.
Fix returns.
.fun.reclaim:
void AWLReclaim(Pool pool, Trace trace, Seg seg);
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 point to base of segment
while(p < SegLimit(seg) {
if(!alloc(p)) { ++p;continue;}
q = skip(p) (ie q points to just past the object pointed at by p)
if !marked(p) free(p, q); free(p, q) consists of resetting the bits in the
alloc table from p to q-1 inclusive.
p = q
}
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.
.fun.describe:
Res AWLDescribe(Pool pool, mps_lib_FILE *stream);
Internal:
.fun.awlsegcreate:
Res AWLSegCreate(AWLSeg *awlsegReturn, Size size);
Creates a segment of class AWLSegClass of size at least size.
.fun.awlsegcreate.size.round: size is rounded up to an ArenaAlign before
requesting the segment. .fun.awlsegcreate.size.round.justify: The arena
requires that all segment sizes are aligned to the ArenaAlign.
.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.
.fun.awlseginit:
Res awlSegInit(Seg seg, Pool pool, Addr base, Size size,
Bool reservoirPermit, va_list args)
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.
.fun.awlsegfinish:
void awlSegFinish(Seg seg);
Finish method for AWLSegClass, called from SegFree. Will free the segment's
tables (see .awlseg.bt).
.fun.awlsegalloc:
Bool AWLSegAlloc(Addr *baseReturn, Addr *limitReturn, AWLSeg awlseg, AWL awl,
Size size);
Will search 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 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 return 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.
.fun.dependent-object:
Bool AWLDependentObject(Addr *objReturn, Addr parent);
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 2nd word of the object (ie (((Word *)parent)[1])
) is non-NULL. The dependent object is the object referenced by the 2nd 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.
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.
3 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
| 2002-06-07 | RB | Converted from MMInfo database design document. |
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