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
| 2002-06-07 | RB | Converted from MMInfo database design document. |
This document is copyright © 1997-2002 Ravenbrook Limited. All rights reserved. This is an open source license. Contact Ravenbrook for commercial licensing options.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
This software is provided by the copyright holders and contributors "as is" and any express or implied warranties, including, but not limited to, the implied warranties of merchantability, fitness for a particular purpose, or non-infringement, are disclaimed. In no event shall the copyright holders and contributors be liable for any direct, indirect, incidental, special, exemplary, or consequential damages (including, but not limited to, procurement of substitute goods or services; loss of use, data, or profits; or business interruption) however caused and on any theory of liability, whether in contract, strict liability, or tort (including negligence or otherwise) arising in any way out of the use of this software, even if advised of the possibility of such damage.
$Id: //info.ravenbrook.com/project/mps/version/1.105/design/alloc-frame/index.html#1 $
Ravenbrook / Projects / Memory Pool System / Master Product Sources / Design Documents