DESIGN OF THE POOL CLASS INTERFACE
                       design.mps.class-interface
                           incomplete design
                             drj 1996-04-17


INTRODUCTION:

This document describes the interface and protocols between the MPM and the 
pool class implementations.  [This document should be merged into 
design.mps.pool.  pekka 1999-07-20]


Document History

.hist.1: Richard started it.  drj has added comments about how accurate this 
document is at 1997-08-19.


ARCHITECTURE:

Methods

.methods: These methods are provided by pool classes as part of the PoolClass 
object (see impl.h.mpmst.class).  They form the interface which allows the MPM 
to treat pools in a uniform manner.

The following description is based on the definition of the PoolClassStruct 
(impl.h.mpmst.class).

If a class is not required to provide a certain method then they should set the 
appropriate PoolNo* method for that method.  It is not acceptable to use NULL. 
[there are also some PoolTriv* methods -- drj 1997-08-19]

.method.name: The name field should be a short, pithy, cryptic name for the 
pool class. Examples are "AMC", "MV".

The size field is the size of the pool instance structure.  For the Foo 
PoolClass this can reasonably be expected to be sizeof(FooStruct).

The offset field is the offset into the pool instance structure of the generic 
PoolStruct.  Typically this field is called poolStruct, so something like 
offsetof(FooStruct, poolStruct) is typical.  If possible, arrange for this to 
be zero.

The init field is the class's init method.  This method is called via the 
generic function PoolInit, which is in turn called by PoolCreate.  The generic 
function allocates the pool's structure (using the size and offset 
information), initializes the PoolStruct (generic part) then calls the init 
method to do any class-specific initialization.  Typically this means 
initializing the fields in the class instance structure.  If init returns a 
non-OK result code the instance structure will be deallocated and the code 
returned to the caller of PoolInit or Create.  Note that the PoolStruct isn't 
made fully valid until PoolInit returns.

The finish field is the class's finish method.  This method is called via the 
generic function PoolFinish, which is in turn called by PoolDestroy.  It is 
expected to finalise the pool instance structure and release any resources 
allocated to the pool, it is expected to release the memory associated with the 
pool instance structure.  Note that the pool is valid when it is passed to 
finish.  The PoolStruct (generic part) is finished off when the class's finish 
method returns.

The alloc field is the class's allocation method.  This method is called via 
the generic function PoolAlloc.  It is expected to return a pointer to a fresh 
(i.e., not overlapping with any other live object) object of the required 
size.  Failure to allocate should be indicated by returning an appropriate 
Error code, and in such a case, *pReturn should not be updated.  Classes are 
not required to provide this method, but they should provide at least one of 
alloc and bufferCreate.

The free_ field is the class's free method.  This is intended primarily for 
manual style pools.  this method is called via the generic function PoolFree.  
The parameters to this method are required to correspond to a previous 
allocation request (possibly via a buffer).  It is an assertion by the client 
that the indicated object is no longer required and the resources associated 
with it can be recycled.  Pools are not required to provide this method.

The bufferInit field is the class's buffer initialization method.  It is called 
by the generic function BufferCreate, which allocates the buffer descriptor and 
initializes the generic fields.  The pool may optionally adjust these fields or 
fill in extra values when bufferInit is called, but often pools set bufferInit 
to PoolTrivBufferInit because they don't need to do any.  If bufferInit returns 
a non-OK result code the buffer structure is deallocated and the code is 
returned to the called of BufferCreate.  Note that the BufferStruct isn't fully 
valid until BufferCreate returns.

The bufferFinish field is the class's buffer finishing method.  It is called by 
the the generic function BufferDestroy.  The pool is expected to detach the 
buffer from any memory and prepare the buffer for destruction.  The class is 
expected to release the resources associated with the buffer structure, and any 
unreserved memory in the buffer may be recycled.  It is illegal for a buffer to 
be destroyed when there are pending allocations on it (ie an allocation has 
been reserved, but not committed) and this is checked in the generic function.  
This method should be provided if and only if bufferCreate is provided. [there 
is no bufferCreate -- drj 1997-08-19]

The condemn field is used to condemn a pool.  This method is called via the 
generic function PoolCondemn.  The class is expected to condemn a subset 
(possible the whole set) of objects it manages and participate in a global 
trace to determine liveness.  The class should register the refsig of the 
condemned set with the trace using TraceCondemn.  The class should expect fix 
requests (via the fix method below) during a global trace.  Classes are not 
required to provide this method, it is expected that automatic style classes 
will.  This interface is expected to change in the future. [condemn now takes 
an action and a segment and should condemn the segment (turn it white) if it 
corresponds to the interpretation of the action -- drj 1997-08-19  and is 
called Whiten drj 1998-02-02]

The mark field is used to mark an entire pool.  This method is called via the 
generic function PoolMark.  The class should consider all of its objects, 
except any set that has been condemned in this trace, to be marked, that is 
ready for scanning.  The class should arrange that any appropriate invariants 
are preserved possibly by the Protection interface.  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.  [no longer 
present, grey turns an entire segment grey -- drj 1997-08-19]

The scan field is used to perform scanning.  This method is called via the 
generic function PoolScan.  The class should scan the segment specified.  It 
should scan all the known live (marked, ie those objects on which fix has been 
called) on the segment and accumulate a summary of _all_ the objects on the 
segment.  This means that mark and sweep pools may have to jump through hoops a 
little bit (see design.mps.poolasm.summary for a pedagogical example).  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 reference in them.  
[scan method now takes an extra return parameter which classes should use to 
indicate whether they scanned all objects in segment or not.  Classes should 
return summary only of object they scanned.  Caller of this method (ie 
TraceScan) is responsible for updating summaries correctly when not a total 
scan.  Hence no jumping through hoops required.  drj 1998-01-30]

The fix field is used to perform fixing.  This method is called via the generic 
function TraceFix.  It indicates that the specified reference has been found 
and the class should consider the object live.  There is provision for 
adjusting the value of the reference (to allow for classes that move objects).  
Classes are not required to provide this method, and not doing so indicates 
that the class is not automatic style (ie it does not use global tracing to 
determine liveness).

The reclaim field is used to reclaim memory.  This method is called via the 
generic function PoolReclaim.  It indicates that the trace has fixed all 
references to reachable objects [actually it indicates that any remaining white 
objects have now been proved unreachable, hence are dead].  The class should 
consider objects that have been condemned and not fixed in this trace to be 
dead and may reclaim the resources associated with them.  Classes are not 
required to provide this method. [reclaim is now called on each segment -- drj 
1997-08-19]

The access field is used to indicate client access.  This method is called via 
the generic functions SpaceAccess and PoolAccess.  It indicates that the client 
has attempted to access the specified region, but has been denied and the 
request trapped due to a protection state.  The class should perform any work 
necessary to remove the protection whilst still preserving appropriate 
invariants (typically this will be scanning work).  Classes are not required to 
provide this method, and not doing
so indicates they never protect any memory managed by the pool.  [access is no 
longer present -- drj 1997-08-19]

.method.act: Act is called when the MPM has decided to execute an action that 
the class declared.  The Class should arrange execution of the associated work 
(usually by beginning an incremental trace).

.method.walk: Walk is used by the heap walker.  Walk is only required to be 
implemented by classes which specify the AttrFMT attribute (formatted pools).  
The Walk method should apply the passed in function (along with its closure 
variables (which are also passed in) and the object format) to all _black_ 
objects in the segment.  Padding objects may or may not be included in the walk 
at the classes discretion, in any case in will be the responsibility of the 
client to do something sensible with padding objects.  [what about broken 
hearts?  drj 1998-01-30].

The describe field is used to print out a description of a pool.  This method 
is called via the generic function PoolDescribe.  The class should emit an 
textual description of the pool's contents onto the specified stream.  Each 
line should begin with 2 spaces.  Classes are not required to provide this 
method.


Events

.replay: To work with the allocation replayer (see 
design.mps.telemetry.replayer), the pool has to emit an event for each call to 
an external interface, containing all the parameters passed by the user.  If a 
new event type is required to carry this information, the replayer 
(impl.c.eventrep) must then be extended to recreate the call.  .replay.Init: In 
particular, the Init method should emit a PoolInit<foo> event with all the pool 
parameters.



TEXT:

.alloc.size: The pool class implementation defines the meaning of the "size" 
parameter to the alloc and free methods.  It may not actually correspond to a 
number of bytes of memory.  .alloc.size.align: In particular, the class may 
allow an unaligned size to be passed.