5. Pool class interface

5.1. 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]

5.2. Document history

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

.hist.2: Converted from MMInfo database design document. Richard Brooksby, 2002-06-07.

.hist.3: Converted to reStructuredText. Gareth Rees, 2013-03-12.

5.3. Architecture

5.4. 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 it 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 PoolCreate(). 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 (that is, 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. [There is no bufferCreate – gdr 2013-04-14]

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 result code other than ResOK, 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 (that is, 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, but 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, that is, 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 (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 two spaces. Classes are not required to provide this method.

5.5. 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.

5.6. 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.