DESIGN OF THE MPS ALLOCATION FRAME PROTOCOL
                         design.mps.alloc-frame
                             incomplete doc
                            tony 1998-10-02

INTRODUCTION

.intro: This document explains the design of the support for allocation frames 
in MPS. 

.readership: This document is intended for any MM developer.

.overview: Allocation frames are used for implementing stack pools; each stack 
frame corresponds to an allocation frame.  Allocation frames may also be 
suitable for implementing other sub-pool groupings, such as generations and 
ramp allocation patterns.  .overview.ambition: We now believe this to be a 
design that loses too many advantages of stack allocation for questionable 
gains.  The requirements are almost entirely based on unanalysed anecdote, 
instead of actual clients.  [We plan to supersede this with a stack pool design 
at some point in the future.  pekka 2000-03-09]

.hist.0: Written by Tony 1998-10-02


DEFINITIONS

.def.alloc-frame: An allocation frame is a generic name for a device which 
groups objects together with other objects at allocation time, and which may 
have a perent/child relationship with other allocation frames.


PURPOSE

.purpose.stack-allocation: The allocation frame protocol is intended to support 
efficient memory management for stack allocation, i.e., the allocation of 
objects which have dynamic extent.

.purpose.general: The allocation frame protocol is intended to be sufficiently 
general that it will be useful in supporting other types of nested allocation 
patterns too.  For example, it could be used to for EPVM-style save and 
restore, ramp allocation patterns or generations. 


REQUIREMENTS

Known requirements

.req.stack-alloc: Provide a interface for clients to describe a stack 
allocation pattern, as an alternative to using the control stack.

.req.efficient: Permit an implementation which is comparable in efficiency to 
allocating on the control stack.

.req.ap: Support allocation via allocation points (APs).

.req.format: Support the allocation of formatted objects.

.req.scan: Ensure that objects in allocation frames can participate in garbage 
collection by being scanned.

.req.fix: Ensure that objects in allocation frames can participate in garbage 
collection by accepting Fix requests.

.req.condemn: Ensure that objects in allocation frames can participate in 
garbage collection by being condemned.

.attr.locking: Minimize the synchronization cost for the creation and 
destruction of frames.


Proto-requirements

.proto-req: The following are possible requirements that might be important in 
the future. The design does not necessarily meet all these requirements, but it 
does consider them all. Each requirement either has direct support in the 
framework, or could be supported with future additions to the framework.

.req.parallels: The allocation frame protocol should provide a framework for 
exploiting the parallels between stack extents, generations and "ramps".

.req.pool-destroy: It should be possible to use allocation frames to free all 
objects in a pool without destroying the pool.

.req.epvm: It should be possible to implement EPVM-style save and restore 
operations by creating and destroying allocation frames.

.req.subst: It should be possible to substitute a stack pool with a GC-ed pool 
so that erroneous use of a stack pool can be detected.

.req.format-extensions: It should be possible for stack pools to utilize the 
same format as any other pool, including debugging formats that include 
fenceposting, etc.

.req.mis-nest: Should ensure "mis-nested" stacks are safe.

.req.non-top-level: Should support allocation in the non-top stack extent.

.req.copy-if-necessary: Should ensure that stack pools can support 
"copy-if-necessary" (so that low-level system code can heapify stack objects.)

.req.preserve: When an object is in an allocation frame which is being 
destroyed, it should be possible to preserve that object in the parent frame.

.req.contained: Should allow clients to ask if an object is "contained" in a 
frame. The object is contained in a frame if it is affected when the frame is 
ended.

.req.alloc-with-other:  Should allow clients to allocate an object in the same 
frame as another object.


OVERVIEW

.frame-classes: The protocol supports different types of allocation frames, 
which are represented as "frame classes".  It's up to pools to determine which 
classes of allocation frames they support.  Pools which support more than one 
frame class rely on the client to indicate which class is currently of 
interest.  The client indicates this by means of an operation which stores the 
class in the buffer to which the allocation point is attached.

.frame-handles: Allocation frames are described via abstract "frame handles".  
Pools may choose what the representation of a frame handle should be.  Frame 
handles are static, and the client need not store them in a GC root.

.lightweight-frames: The design includes an extention to the allocation point 
protocol, which permits the creation and destruction of allocation frames 
without the necessity for claiming the arena lock.  Such frames are called 
"lightweight frames".


OPERATIONS

.op.intro: Each operation has both an external (client) interface and an 
internal (MPS) interface.  The external function takes an allocation point as a 
parameter, determines which buffer and pool it belongs to, and calls the 
internal function with the buffer and pool as parameters.

.op.obligatory: The following operations are supported on any allocation point 
which supports allocation frames:-

.operation.push: The PushFrame operation creates a new allocation frame of the 
currently chosen frame class, makes this new frame the current frame, and 
returns a handle for the frame.

.operation.pop: The PopFrame operation takes a frame handle as a parameter.  
Some pool classes might insist or assume that this is the handle for the 
current frame.  It finds the parent of that frame and makes it the current 
frame.  The operation indicates that all children of the new current frame 
contain objects which are likely to be dead.  The reclaim policy is up to the 
pool; some classes might insist or assume that the objects must be dead, and 
eagerly free them.  Note that this might introduce the possibility of leaving 
dangling pointers elsewhere in the arena.  If so, it's up to the pool to decide 
what to do about this.

.op.optional: The following operations are supported for some allocation 
frames, but not all. Pools may choose to support some or all of these 
operations for certain frame classes. An unsupported operation will return a 
failure value:-

.operation.select: The SelectFrame operation takes a frame handle as a 
parameter and makes that frame the current frame.  It does not indicate that 
any children of the current frame contain objects which are likely to be dead.

.operation.select-addr: The SelectFrameOfAddr operation takes an address as a 
parameter and makes the frame of that address the current frame.  It does not 
indicate that any children of the current frame contain objects which are 
likely to be dead.

.operation.in-frame: The AddrInFrame operation determines whether the supplied 
address is the address of an object allocated in the supplied frame, or any 
child of that frame.

.operation.set: The SetFrameClass operation takes a frame class and an 
allocation point as parameters, and makes that the current frame class for the 
allocation point.  The next PushFrame operation will create a new frame of that 
class.


INTERFACE

External types (used by all client of allocation frames)

.type.client.frame-handle: Frame handles are defined as an abstract type:
        typedef struct mps_frame_s *mps_frame_t;  


External types  (complete set)

.type.client.frame-class: Frame classes are defined as an abstract type:
        typedef struct mps_frame_class_s *mps_frame_class_t;  

.type.client.frame-class.access: Clients access frame classes by means of 
dedicated functions for each frame class. 


External functions (used by all client of allocation frames)

.fn.client.push: The following function is used by clients to invoke the 
PushFrame operation.  For lightweight frames, this might not invoke the 
corresponding internal function:
       mps_res_t mps_ap_frame_push(mps_frame_t *frame_o, mps_ap_t buf);  

.fn.client.pop:  The following function is used by clients to invoke the 
PopFrame operation.  For lightweight frames, this might not invoke the 
corresponding internal function:
       mps_res_t mps_ap_frame_pop(mps_ap_t buf, mps_frame_t frame);


External functions (complete set)

.fn.client.select:  The following function is used by clients to invoke the 
SelectFrame operation:
       mps_res_t mps_ap_frame_select(mps_ap_t buf, mps_frame_t frame);

.fn.client.select-addr:  The following function is used by clients to invoke 
the SelectFrameOfAddr operation:
       mps_res_t mps_ap_frame_select_from_addr(mps_ap_t buf, mps_addr_t addr);

.fn.client.in-frame:  The following function is used by clients to invoke the 
AddrInFrame operation:
       mps_res_t mps_ap_addr_in_frame(mps_bool_t *inframe_o, mps_ap_t buf, 
                                      mps_addr_t *addrref, mps_frame_t frame);  

.fn.client.set: The following function is used by clients to invoke the 
SetFrameClass operation:
       mps_res_t mps_ap_set_frame_class(mps_ap_t buf, mps_frame_class_t 
class);  


.fn.client.stack-frame-class: The following function is used by clients to 
access the frame class used for simple stack allocation:
       mps_frame_class_t mps_alloc_frame_class_stack(void);  


Internal types (used by all implementations of allocation frames)

.type.frame-handle: Frame handles are defined as an abstract type:
       typedef struct AllocFrameStruct *AllocFrame;


Internal types  (complete set)

.type.frame-class: Frame classes are defined as an abstract type:
       typedef struct AllocFrameClassStruct *AllocFrameClass;


Internal functions (used by all implementations of allocation frames)


.fn.push: A pool method of the following type is called (if needed) to invoke 
the PushFrame operation:
       typedef Res (*PoolFramePushMethod)(AllocFrame *frameReturn,
                                          Pool pool, Buffer buf);
 
.fn.pop: A pool method of the following type is called (if needed) to invoke 
the PopFrame operation:
       typedef Res (*PoolFramePopMethod)(Pool pool, Buffer buf,
                                         AllocFrame frame);

Internal functions (complete set)

.fn.select: A pool method of the following type is called to invoke the 
SelectFrame operation:
       typedef Res (*PoolFrameSelectMethod)(Pool pool, Buffer buf,
                                            AllocFrame frame);

.fn.select-addr: A pool method of the following type is called to invoke the 
SelectFrameOfAddr operation:
       typedef Res (*PoolFrameSelectFromAddrMethod)(Pool pool, Buffer buf,
                                                    Addr addr);

.fn.in-frame: A pool method of the following type is called to invoke the 
AddrInFrame operation:
       typedef Res (*PoolAddrInFrameMethod)(Bool *inframeReturn, 
                                            Pool pool, Seg seg, Addr *addrref, 
                                            AllocFrame frame);

.fn.set: A pool method of the following type is called to invoke the 
SetFrameClass operation:
       typedef Res (*PoolSetFrameClassMethod)(Pool pool, Buffer buf, 
                                              AllocFrameClass class);


LIGHTWEIGHT FRAMES 

Overview

.lw-frame.overview: Allocation points provide direct support for lightweight 
frames, and are designed to permit PushFrame and PopFrame operations without 
the need for locking and delegation to the pool method.  Pools can disable this 
mechanism for any allocation point, so that the pool method is always called.  
The pool method will be called whenever synchronization is required for other 
reasons (e.g. the buffer is tripped).

.lw-frame.model: Lightweight frames offer direct support for a particular model 
of allocation frame use, whereby the PushFrame operation returns the current 
allocation pointer as a frame handle, and the PopFrame operation causes the 
allocation pointer to be reset to the address of the frame handle.  This model 
should be suitable for simple stack frames, where more advanced operations like 
SelectFrame are not supported.  It may also be suitable for more advanced 
allocation frame models when they are being used simply.  The use of a complex 
operation always involves synchronization via locking, and the pool may disable 
lightweight synchronization temporarily at this time.

State

.lw-frame.states: Allocation points supporting lightweight frames will be in 
one of the following states:-
Valid:        Indicates that PushFrame can be a lightweight operation and need 
not be synchronized
PopPending:   Indicates that there has been a PopFrame operation that the pool 
must respond to
Disabled:     Indicates that the pool has disabled support for lightweight 
operations for this AP
These states are in addition to the state normally held by an AP for allocation 
purposes. An AP will be in the Disabled state at creation.

.lw-frame.transitions: State transitions happen under the following 
circumstances:-
Valid -> PopPending    :  As a result of a client PopFrame operation
Valid -> Disabled      :  At the choice of the pool (e.g. when responding to a 
SelectFrame operation)
PopPending -> Valid    :  At the choice of the pool, when processing a PopFrame
PopPending -> Disabled :  At the choice of the pool, when processing a PopFrame
Disabled -> Valid      :  At the choice of the pool
Disabled -> Popframe   :  Illegal

.lw-frame.state-impl: Each AP contains 3 additional fields to hold this state:

  mps_addr_t frameptr; 
  mps_bool_t enabled; 
  mps_bool_t lwPopPending;

.lw-frame.enabled: The enabled slot holds the following values for each state:
Valid:      TRUE
PopPending: TRUE
Disabled:   FALSE

.lw-frame.frameptr: The frameptr slot holds the following values for each state:

Valid:      NULL
PopPending: Frame handle for most recently popped frame.
Disabled:   NULL

.lw-frame.lwPopPending: The lwPopPending slot holds the following values for 
each state:

Valid:      FALSE
PopPending: TRUE
Disabled:   FALSE


.lw-frame.state-for-gc: It is not necessary for the tracer, format code, pool, 
or any other part of the GC support in MPS to read either of the 2 additional 
AP fields in order to scan a segment which supports a lightweight allocation 
frame.


Synchronization

.lw-frame.sync: The purpose of the design is that mutator may access the state 
of an AP without locking with MPS (via the external functions).  The design 
assumes the normal MPS restriction that an operation on an AP may only be 
performed by a single mutator thread at a time.  Each of the operations on 
allocation frames counts as an operation on an AP.

.lw-frame.sync.pool: Pools are permitted to read or modify the lightweight 
frame state of an AP only in response to an operation on that AP.

.lw-frame.sync.external: The external functions mps_ap_frame_push and 
mps_ap_frame_pop are permitted to read the values of the enabled and frameptr 
fields for the supplied AP without claiming the arena lock.  They are permitted 
to modify the frameptr field iff enabled == FALSE.

.lw-frame.sync.trip: When a buffer trip happens, and the trap wasn't set by MPS 
itself (i.e. it wasn't because of a flip or for logging), then the buffer code 
must check whether the AP has state PopPending.  If it does, the buffer code 
must call the Pool.


Implementation

.lw-frame.push: The external PushFrame operation (mps_ap_frame_push) performs 
the following operations:
   IF (!APIsTrapped(ap) && StateOfFrame(ap) == Valid && ap->init == ap->alloc)
      *frame_o = ap->init;
   ELSE
     WITH_ARENA_LOCK
       PerformInternalPushFrameOperation(...)
     END
   END


.lw-frame.pop: The external PopFrame operation (mps_ap_frame_pop) performs the 
following operations:
   IF (StateOfFrame(ap) != Disabled)
     TrapAP(ap);  /* ensure next allocation or push involves the pool */
     ap->frameptr = frame;
     ap->lwpopPending = TRUE;
   ELSE
     WITH_ARENA_LOCK
       PerformInternalPopFrameOperation(...)
     END
   END

A. References

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