27. Segment data structure¶
27.2. Overview¶
.over.segments: Segments are the basic units of tracing and shielding. The MPM also uses them as units of scanning and colour, although pool classes may subdivide segments and be able to maintain colour on a finer grain (down to the object level, for example).
.over.objects: The mutator’s objects are stored in segments. Segments are contiguous blocks of memory managed by some pool.
.segments.pool: The arrangement of objects within a segment is determined by the class of the pool which owns the segment. The pool is associated with the segment indirectly via the first tract of the segment.
.over.memory: The relationship between segments and areas of memory is maintained by the segment module. Pools acquire tracts from the arena, and release them back to the arena when they don’t need them any longer. The segment module can associate contiguous tracts owned by the same pool with a segment. The segment module provides the methods SegBase, SegLimit, and SegSize which map a segment onto the addresses of the memory block it represents.
.over.hierarchy: The Segment datastructure is designed to be
subclassable (see design.mps.protocol). The basic segment class
(Seg) supports colour and protection for use by the tracer, as
well as support for a pool ring, and all generic segment functions.
Clients may use Seg directly, but will most probably want to use a
subclass with additional properties.
.over.hierarchy.gcseg: GCSeg is a subclass of Seg which
implements garbage collection, including buffering and the ability to
be linked onto the grey ring. It does not implement hardware barriers,
and so can only be used with software barriers, for example internally
in the MPS.
.over.hierarchy.mutatorseg: MutatorSeg is a subclass of
GCSeg implementing hardware barriers. It is suitable for handing
out to the mutator.
27.3. Data Structure¶
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struct SegStruct *
Seg¶
-
struct GCSegStruct *
GCSeg¶
The implementations are as follows:
typedef struct SegStruct { /* segment structure */
Sig sig; /* <code/misc.h#sig> */
SegClass class; /* segment class structure */
Tract firstTract; /* first tract of segment */
RingStruct poolRing; /* link in list of segs in pool */
Addr limit; /* limit of segment */
unsigned depth : ShieldDepthWIDTH; /* see <code/shield.c#def.depth> */
AccessSet pm : AccessLIMIT; /* protection mode, <code/shield.c> */
AccessSet sm : AccessLIMIT; /* shield mode, <code/shield.c> */
TraceSet grey : TraceLIMIT; /* traces for which seg is grey */
TraceSet white : TraceLIMIT; /* traces for which seg is white */
TraceSet nailed : TraceLIMIT; /* traces for which seg has nailed objects */
RankSet rankSet : RankLIMIT; /* ranks of references in this seg */
} SegStruct;
typedef struct GCSegStruct { /* GC segment structure */
SegStruct segStruct; /* superclass fields must come first */
RingStruct greyRing; /* link in list of grey segs */
RefSet summary; /* summary of references out of seg */
Buffer buffer; /* non-NULL if seg is buffered */
Sig sig; /* design.mps.sig */
} GCSegStruct;
.field.rankSet: The rankSet field represents the set of ranks
of the references in the segment. It is initialized to empty by
SegInit().
.field.rankSet.single: The Tracer only permits one rank per segment [ref?] so this field is either empty or a singleton.
.field.rankSet.empty: An empty rankSet indicates that there are
no references. If there are no references in the segment then it
cannot contain black or grey references.
.field.rankSet.start: If references are stored in the segment then it must be updated, along with the summary (.field.summary.start).
.field.depth: The depth field is used by the Shield
(impl.c.shield) to manage protection of the segment. It is initialized
to zero by SegInit().
.field.sm: The sm field is used by the Shield (impl.c.shield)
to manage protection of the segment. It is initialized to
AccessSetEMPTY by SegInit().
.field.pm: The pm field is used by the Shield (impl.c.shield)
to manage protection of the segment. It is initialized to
AccessSetEMPTY by SegInit(). The field is used by both the
shield and the ANSI fake protection (impl.c.protan).
.field.black: The black field is the set of traces for which
there may be black objects (that is, objects containing references,
but no references to white objects) in the segment. More precisely, if
there is a black object for a trace in the segment then that trace
will appear in the black field. It is initialized to
TraceSetEMPTY by SegInit().
.field.grey: The grey field is the set of traces for which
there may be grey objects (i.e containing references to white objects)
in the segment. More precisely, if there is a reference to a white
object for a trace in the segment then that trace will appear in the
grey field. It is initialized to TraceSetEMPTY by SegInit().
.field.white: The white field is the set of traces for which
there may be white objects in the segment. More precisely, if there is
a white object for a trace in the segment then that trace will appear
in the white field. It is initialized to TraceSetEMPTY by
SegInit().
.field.summary: The summary field is an approximation to the
set of all references in the segment. If there is a reference R in
the segment, then RefSetIsMember(summary, R) is TRUE. The
summary is initialized to RefSetEMPTY by SegInit().
.field.summary.start: If references are stored in the segment then
it must be updated, along with rankSet (.field.rankSet.start).
.field.buffer: The buffer field is either NULL, or points
to the descriptor structure of the buffer which is currently
allocating in the segment. The field is initialized to NULL by
SegInit().
.field.buffer.owner: This buffer must belong to the same pool as the segment, because only that pool has the right to attach it.
27.4. Interface¶
27.4.1. Splitting and merging¶
.split-and-merge: There is support for splitting and merging segments, to give pools the flexibility to rearrange their tracts among segments as they see fit.
.split: If successful, segment seg is split at address at,
yielding two segments which are returned in segLoReturn and
segHiReturn for the low and high segments respectively. The base of
the low segment is the old base of seg. The limit of the low
segment is at. The base of the high segment is at. This limit
of the high segment is the old limit of seg. seg is
effectively destroyed during this operation (actually, it might be
reused as one of the returned segments). Segment subclasses may make
use of the optional arguments; the built-in classes do not.
.split.invariants: The client must ensure some invariants are met
before calling SegSplit():
.split.inv.align:
atmust be a multiple of the arena grain size, and lie between the base and limit ofseg. Justification: the split segments cannot be represented if this is not so..split.inv.buffer: If
segis attached to a buffer, the buffered region must not include addressat. Justification: the segment module is not in a position to know how (or whether) a pool might wish to split a buffer. This permits the buffer to remain attached to just one of the returned segments.
.split.state: Except as noted above, the segments returned have the
same properties as seg. That is, their colour, summary, rankset,
nailedness etc. are set to the values of seg.
.merge: If successful, segments segLo and segHi are merged
together, yielding a segment which is returned in mergedSegReturn.
segLo and segHi are effectively destroyed during this
operation (actually, one of them might be reused as the merged
segment). Segment subclasses may make use of the optional arguments;
the built-in classes do not.
.merge.invariants: The client must ensure some invariants are met
before calling SegMerge():
.merge.inv.abut: The limit of
segLomust be the same as the base ofsegHi. Justification: the merged segment cannot be represented if this is not so..merge.inv.buffer: One or other of
segLoandsegHimay be attached to a buffer, but not both. Justification: the segment module does not support attachment of a single seg to 2 buffers..merge.inv.similar:
segLoandsegHimust be sufficiently similar. Two segments are sufficiently similar if they have identical values for each of the following fields:class,grey,white,nailed,rankSet. Justification: There has yet to be a need to implement default behaviour for these cases. Pool classes should arrange for these values to be the same before callingSegMerge().
.merge.state: The merged segment will share the same state as
segLo and segHi for those fields which are identical (see
.merge.inv.similar). The summary will be the union of the summaries
of segLo and segHi.
27.5. Extensibility¶
27.5.1. Allocation¶
-
Bool
(*SegBufferFillMethod)(Addr *baseReturn, Addr *limitReturn, Seg seg, Size size, RankSet rankSet)¶
.method.buffer-fill: Allocate a block in the segment, of at least
size bytes, with the given set of ranks. If successful, update
*baseReturn and *limitReturn to the block and return TRUE.
Otherwise, return FALSE. The allocated block must be accounted as
buffered (see design.mps.strategy.account.buffered).
.method.buffer-empty: Free the unused part of the buffer to the segment. Account the used part as new (see design.mps.strategy.account.new) and the unused part as free (see design.mps.strategy.account.free).
27.5.2. Garbage collection¶
.method.access: The access method indicates that the client
program attempted to access the address addr, but has been denied
due to a protection fault. The mode indicates whether the client
program was trying to read (AccessREAD) or write (AccessWRITE)
the address. If this can’t be determined, mode is AccessREAD |
AccessWRITE. The segment should perform any work necessary to remove
the protection whilst still preserving appropriate invariants (this
might scanning the region containing addr). Segment classes are
not required to provide this method, and not doing so indicates they
never protect any memory managed by the pool. This method is called
via the generic function SegAccess().
.method.whiten: The whiten method requests that the segment
seg condemn (a subset of, but typically all) its objects for the
trace trace. That is, prepare them for participation in the trace
to determine their liveness. The segment should expect fix requests
(.method.fix) during the trace and a reclaim request
(.method.reclaim) at the end of the trace. Segment
classes that automatically reclaim dead objects must provide this
method, and pools that use these segment classes must additionally set
the AttrGC attribute. This method is called via the generic
function SegWhiten().
.method.grey: The greyen method requires the segment seg to
colour its objects grey for the trace trace (excepting objects
that were already condemned for this trace). That is, make them ready
for scanning by the trace trace. The segment must arrange that any
appropriate invariants are preserved, possibly by using the protection
interface (see design.mps.prot). Segment classes are not required to
provide this method, and not doing so indicates that all instances of
this class will have no fixable or traceable references in them. This
method is called via the generic function SegGreyen().
.method.blacken: The blacken method is called if it is known
that the segment seg cannot refer to the white set for any of the
traces in traceSet. The segment must blacken all its grey objects
for those traces. Segment classes are not required to provide this
method, and not doing so indicates that all instances of this class
will have no fixable or traceable references in them. This method is
called via the generic function SegBlacken().
.method.scan: The scan method scans all the grey objects on the
segment seg, passing the scan state ss to FormatScan. The
segment may additionally accumulate a summary of all its objects. If
it succeeds in accumulating such a summary it must indicate that it
has done so by setting the *totalReturn parameter to TRUE.
Otherwise it must set *totalReturn to FALSE. Segment classes
are not required to provide this method, and not doing so indicates
that all instances of this class will have no fixable or traceable
references in them. This method is called via the generic function
SegScan().
.method.fix: The fix method indicates that the reference
*refIO has been discovered at rank ss->rank by the traces in
ss->traces, and the segment must handle this discovery according
to the fix protocol (design.mps.fix). If the method moves the object,
it must update *refIO to refer to the new location of the object.
If the method determines that the referenced object died (for example,
because the highest-ranking references to the object were weak), it
must update *refIO to NULL. Segment classes that automatically
reclaim dead objects must provide this method, and pools that use
these classes must additionally set the AttrGC attribute. Pool
classes that use segment classes that may move objects must also set
the AttrMOVINGGC attribute. The fix method is on the critical
path (see design.mps.critical-path) and so must be fast. This method
is called via the function TraceFix().
.method.fixEmergency: The fixEmergency method is used to
perform fixing in “emergency” situations. Its specification is
identical to the fix method, but it must complete its work without
allocating memory (perhaps by using some approximation, or by running
more slowly). Segment classes must provide this method if and only if
they provide the fix method. If the fix method does not need
to allocate memory, then it is acceptable for fix and
fixEmergency to be the same.
.method.reclaim: The reclaim method indicates that any
remaining white objects in the segment seg have now been proved
unreachable by the trace trace, and so are dead. The segment
should reclaim the resources associated with the dead objects. Segment
classes are not required to provide this method. If they do, pools
that use them must set the AttrGC attribute. This method is called
via the generic function SegReclaim().
.method.walk: The walk method must call the visitor function
f (along with its closure parameters v and s and the
format format) once for each of the black objects in the
segment seg. Padding objects may or may not be included in the
walk, at the segment’s discretion: it is the responsibility of the
client program to handle them. Forwarding objects must not be included
in the walk. Segment classes need not provide this method. This
method is called by the genetic function SegWalk(), which is
called by the heap walker mps_arena_formatted_objects_walk().
.method.flip: Raise the read barrier, if necessary, for a trace that’s about to flip and for which the segment is grey and potentially contains references.
27.5.3. Splitting and merging¶
.method.split: Segment subclasses may extend the support for
segment splitting by defining their own “split” method. On entry,
seg is a segment with region [base,limit), segHi is
uninitialized, mid is the address at which the segment is to be
split. The method is responsible for destructively modifying seg
and initializing segHi so that on exit seg is a segment with
region [base,mid) and segHi is a segment with region
[mid,limit). Usually a method would only directly modify the
fields defined for the segment subclass.
.method.split.next: A split method should always call the next method, either before or after any class-specific code (see design.mps.protocol.overview.next-method).
.method.split.accounting: If seg belongs to a generation in a
chain, then the pool generation accounting must be updated. In the
simple case where the split segments remain in the same generation,
this can be done by calling PoolGenAccountForSegSplit().
.method.merge: Segment subclasses may extend the support for
segment merging by defining their own merge method. On entry,
seg is a segment with region [base,mid), segHi is a
segment with region [mid,limit), The method is responsible for
destructively modifying seg and finishing segHi so that on
exit seg is a segment with region [base,limit) and segHi
is garbage. Usually a method would only modify the fields defined for
the segment subclass.
.method.merge.next: A merge method should always call the next method, either before or after any class-specific code (see design.mps.protocol.overview.next-method).
.method.merge.accounting: If seg belongs to a generation in a
chain, then the pool generation accounting must be updated. In the
simple case where the two segments started in the same generation and
the merged segment remains in that generation, this can be done by
calling PoolGenAccountForSegMerge().
.split-merge.shield: Split and merge methods may assume that the segments they are manipulating are not in the shield queue.
.split-merge.shield.flush: The shield queue is flushed before any split or merge methods are invoked.
.split-merge.shield.re-flush: If a split or merge method performs an operation on a segment which might cause the segment to be queued, the method must flush the shield queue before returning or calling another split or merge method.
.split-merge.fail: Split and merge methods might fail, in which
case segments seg and segHi must be equivalently valid and
configured at exit as they were according to the entry conditions.
It’s simplest if the failure can be detected before calling the next
method (for example, by allocating any objects early in the method).
.split-merge.fail.anti: If it’s not possible to detect failure before calling the next method, the appropriate anti-method must be used (see design.mps.protocol.guide.fail.after-next). Split methods are anti-methods for merge methods, and vice-versa.
.split-merge.fail.anti.constrain: In general, care should be taken when writing split and merge methods to ensure that they really are anti-methods for each other. The anti-method must not fail if the initial method succeeded. The anti-method should reverse any side effects of the initial method, except where it’s known to be safe to avoid this (see .split-merge.fail.summary for an example of a safe case).
.split-merge.fail.anti.no: If this isn’t possible (it might not be)
then the methods won’t support after-next failure. This fact should be
documented, if the methods are intended to support further
specialization. Note that using va_arg with the args parameter is
sufficient to make it impossible to reverse all side effects.
.split-merge.fail.summary: The segment summary might not be restored exactly after a failed merge operation. Each segment would be left with a summary which is the union of the original summaries (see .merge.state). This increases the conservatism in the summaries, but is otherwise safe.
.split-merge.unsupported: Segment classes need not support segment
merging at all. The function SegClassMixInNoSplitMerge() is supplied
to set the split and merge methods to unsupporting methods that will
report an error in checking varieties.
