MPS to Client Message Protocol

This document contains a guide to the MPS to Client Message Protocol, followed by the historical initial design. References, History, Copyright and License are at the end.

Guide

Readership: any MPS developer. Not confidential.

Introduction

Messages are used when the MPS needs to asynchronously send information to the client. Currently, messages are used to signal:

  • finalization ("_finalization" message);
  • trace start ("_gc_start" message);
  • trace end ("_gc" message).

The Originator of the message is some part of the MPS. For example, the finalization system (see poolmrg.c) is the originator of finalization messages, sent to the client when an object is no longer exactly reachable.

The MPS's Message-system (message.c) keeps messages that have been posted and are awaiting collection by the client in an arena-wide queue -- these are "queued messages".

The Client:

  1. periodically calls mps_message_poll() and/or mps_message_get() to get a message from the MPS Message queue;
  2. calls accessor functions (eg. mps_message_finalization_ref()) to read data out of the message;
  3. calls mps_message_discard() when it has finished using the message.

The design is intended to support two alternative models of Client:

  • a client with a central function that Gets all messages, and dispatches them to the various parts of the Client;
  • a client with no such central function, but instead several parts which each Get only the message types they are interested in.

The Message-system

The Message-system does just two things:

  • class-based method dispatch on any valid message (methods: Delete, slot accessors);
  • queue for messages that have been Posted but not yet Got.

Note that message type is now an attribute of a message class: each class implements exactly one type. (But multiple different classes, such as for several different pool classes, may all implement a single common message type).

How to Originate a Message

Any part of the MPS can be a message Originator. Any struct can be a message, as long as it:

  • contains an embedded MessageStruct (used by the Message-system for housekeeping);
  • makes appropriate MessageInit() and MessageFinish() calls (for its embedded MessageStruct);
  • has a Delete() function, to be called when the Client calls mps_message_discard();
  • has appropriate accessor functions.

The Originator must define a message class, which is simply a struct containing the Delete and accessor functions, and a few other bits of housekeeping. The Originator may wish to define a new message type too.

Here's an example of a message. Notice the embedded MessageStruct:

typedef struct TraceStartMessageStruct {
  Sig sig;
  char why[TRACE_START_MESSAGE_WHY_LEN];
  MessageStruct messageStruct;
} TraceStartMessageStruct;

The Originator allocates storage for messages. It is important to consider how to handle the case where this allocation fails because of insufficient free memory. Here are some sensible allocation strategies:

  • Pre-allocate the struct when creating the subject it will refer to. The message is then ready to post at any time. This is ideal for subjects that the client creates explicitly, because failure to allocate the message makes the whole operation fail and report a non-OK result code. This approach only allows a fixed number of messages over the entire lifetime of the subject. (Used for finalization messages).

  • Allocate the struct from control pool (with ControlAlloc) immediately before posting, and free it (with ControlFree) in the Delete method. Must cope with failure to allocate.

  • Allocate the struct from control pool (with ControlAlloc) long in advance of posting. This is needed for messages that are likely to be emitted when memory is low, such as trace start messages. Must cope with failure to allocate. (Used for trace start and trace end messages).

Note: Originators should not re-use message structs. Each message should be dynamically allocated, posted once, queued for as long as the client wants, and then freed when the client discards it. Otherwise the Originator is imposing a limit on the maximum number of ungot (queued) messages, which therefore requires the client to either get the messages 'promptly', or risk losing messages. But typically there is no way for the client to reliably meet this promptness constraint. (Note also that the MPS message system does not distinguish between unsent and received states). See also job001570.

If a message cannot be allocated, the Originator should report an error code if possible. If not possible, for example because the message is about an operation that was not explicitly requested by the Client, then there is a "droppedMessages" counter in the arena. Increment this. It's not great, but what else can you do?

It is good design to make the Client's life as easy as possible by making message behaviour as simple as possible. Ideally, make sure that messages will never be dropped. If there is a set of messages that make sense together (eg trace start and trace end), then ideally make sure that the set is 'all or nothing' -- either all messages will be sent, or all will be dropped. Where logical, add new accessors to an existing message, instead of creating a new message type. All this simplifies the code the Client has to write: Client code that has to rendezvous several messages with one another is tricky to write and tricky to test.

Lifecycle

The normal lifecycle of a Message is:

(alloc Init) unsent (Post) queued (Get) received (Discard Delete Finish free)

The struct is a valid message between Init and Finish. The Originator is permitted to Finish and free an unsent message.

MessagePost() simply links the struct into the message queue (using a RingStruct inside the MessageStruct): no copying occurs. The message is now queued, and MessageOnQueue() returns true. The Originator must not free a queued message.

When the client calls mps_message_get(), the Message-system simply unlinks the message from the queue. The message is now received, and is in use by the client. The Originator must not free or change a received message, because the Client would then have a reference to freed memory, or would see inconsistent data.

When the Client calls mps_message_discard(), the Message-system calls through to the Originator-supplied Delete method. The Originator calls Finish and free.

Abnormal lifecycles

If the Client has not enabled messages of this type (with mps_message_type_enable), Posting the message immediately Discards it, which calls the Delete method:

(alloc Init) unsent (Post Discard Delete Finish free) -- client did not enable this message type

Therefore whenever an Originator calls MessagePost(), beware:

  • it must be safe to call the Originator's Delete method;
  • the delete may deallocate and unmap the message, so the Originator must not use its reference to the message any more.

If the client has enabled but never gets the message, it remains on the message queue until ArenaDestroy. Theoretically these messages could accumulate forever until they exhaust memory. This is intentional: the client should not enable a message type and then never get it!

(alloc Init) unsent (Post) queued (ArenaDestroy Discard Delete Finish free) -- client never gets message

Messages can also be dropped from the queue if the client calls mps_message_type_disable() when there are already some messages of that type queued:

(alloc Init) unsent (Post) queued (_type_disable Discard Delete Finish free) -- client disables this message type

If the client simply drops its reference without calling mps_message_discard (this is a client error), the memory will not be reclaimed until ArenaDestroy. This is intentional: the control pool is not garbage-collected:

(alloc Init) unsent (Post) queued (Get) received (ArenaDestroy free) -- client never discards message

Future ideas

Shrinking MessageStruct

(Because every finalised object requires a MessageStruct, so it would be nice to keep it small).

The field Clock postedClock could be implemented and stored by only those message types that use it. A bit messy though.

Initial Design 

                     MPS TO CLIENT MESSAGE PROTOCOL
                           design.mps.message
                             incomplete doc
                             drj 1997-02-13

INTRODUCTION

.readership: Any MPS developer.

.intro: The MCMP provides a means by which clients can receive messages from 
the MPS asynchronously.  Typical messages may be low memory notification (or in 
general low utility), finalization notification, soft-failure notification.  
There is a general assumption that it should not be disastrous for the MPS 
client to ignore messages, but that it is probably in the clients best interest 
to not ignore messages.  The justification for this is that the MPS cannot 
force the MPS client to read and act on messages, so no message should be 
critical [bogus, since we cannot force clients to check error codes either - 
Pekka 1997-09-17].

.contents: This document describes the design of the external and internal 
interfaces and concludes with a sketch of an example design of an internal 
client.  The example is that of implementing finalization using PoolMRG.


REQUIREMENTS

.req: The MPS/Client message protocol will be used for implementing 
finalization (see design.mps.finalize and req.dylan.fun.final).  It will also 
be used for implementing the notification of various conditions (possibly 
req.dylan.prot.consult is relevant here).


INTERFACE


External Interface

.if.queue:

Messages are presented as a single queue per arena.  Various functions are 
provided to inspect the queue and inspect messages in it (see below).


Functions

.if.fun:

The following functions are provided:

.if.fun.poll: Poll.  Sees whether there are any messages pending.

mps_bool_t mps_message_poll(mps_arena_t arena);

Returns 1 only if there is a message on the queue of arena.  Returns 0 
otherwise.

.if.fun.enable: Enable.  Enables the flow of messages of a certain type.

void mps_message_type_enable(mps_arena_t arena, mps_message_type_t type);

Enables the specified message type.  The queue of messages of a arena will 
contain only messages whose types have been enabled.  Initially all message 
types are disabled.  Effectively this function allows the client to declare to 
the MPS what message types the client understands.  The MPS does not generate 
any messages of a type that hasn't been enabled.  This allows the MPS to add 
new message types (in subsequent releases of a memory manager) without 
confusing the client.  The client will only be receiving the messages if they 
have explicitly enabled them (and the client presumably only enables message 
types when they have written the code to handle them).

.if.fun.disable: Disable.  Disables the flow of messages of a certain type.

void mps_message_type_disable(mps_arena_t arena, mps_message_type_t type);

The antidote to mps_message_type_enable.  Disables the specified message type.  
Flushes any existing messages of that type on the queue, and stops any further 
generation of messages of that type. This permits clients to dynamically 
decline interest in a message type, which may help to avoid a memory leak or 
bloated queue when the messages are only required temporarily. 

.if.fun.get: begins a message "transaction".

mps_bool_t mps_message_get(mps_message_t *message_return, mps_arena_t arena, 
mps_message_type_t type);

If there is a message of the specified type on the queue then the first such 
message will be removed from the queue and a handle to it will be returned to 
the client in *messageReturn; in this case the function will return TRUE.  
Otherwise it will return FALSE.  Having obtained a handle on a message in this 
way, the client can use the type-specific accessors to find out about the 
message.  When the client is done with the message the client should call 
mps_message_discard; failure to do so will result in a resource leak.

.if.fun.discard: ends a message "transaction".

void mps_message_discard(mps_arena_t arena, mps_message_t message);

Indicates to the MPS that the client is done with this message and its 
resources may be reclaimed.

.if.fun.type.any: Determines the type of a message in the queue

mps_bool_t mps_message_queue_type(mps_message_type_t *type_return, mps_arena_t 
arena);

Returns 1 only if there is a message on the queue of arena, and in this case 
updates *type_return to be the type of a message in the queue.  Otherwise 
returns 0.

.if.fun.type: Determines the type of a message (that has already been got).

mps_message_type_t mps_message_type(mps_arena_t arena, mps_message_t message)

Return the type of the message.  Only legal when inside a message transaction 
(i.e. after mps_message_get and before mps_message_discard).  Note that the 
type will be the same as the type that the client passed in the call to 
mps_message_get.


Types of messages

.type: The type governs the "shape" and meaning of the message.

.type.int: Types themselves will just be a scalar quantity, an integer.

.type.semantics: A type indicates the semantics of the message.  
.type.semantics.interpret: The semantics of a message are interpreted by the 
client by calling various accessor methods on the message.  .type.accessor: The 
type of a message governs which accessor methods are legal to apply to the 
message.

.type.example: Some example types:

.type.finalization: There will be a finalization type.  The type is abstractly: 
FinalizationMessage(Ref).

.type.finalization.semantics: A finalization message indicates that an object 
has been discovered to be finalizable (see design.mps.poolmrg.def.final.object 
for a definition of finalizable).  .type.finalization.ref: There is an accessor 
to get the reference of the finalization message (i.e. a reference to the 
object which is finalizable) called mps_message_finalization_ref.  
.type.finalization.ref.scan: Note that the reference returned should be stored 
in scanned memory.


.compatibility:

Compatibility issues

.compatibility.future.type-new: Notice that message of a type that the client 
doesn't understand are not placed on the queue, therefore the MPS can introduce 
new types of message and existing client will still function and will not leak 
resources.  This has been achieved by getting the client to declare the types 
that the client understands (with mps_message_type_enable, .if.fun.enable).

.compatibility.future.type-extend: The information available in a message of a 
given type can be extended by providing more accessor methods.  Old clients 
won't get any of this information but that's okay.


Internal Interface


.message.instance: Messages are instances of Message Classes.  
.message.concrete: Concretely a Message is represented by a MessageStruct.  A 
MessageStruct has the usual signature field (see design.mps.sig).  A 
MessageStruct has a type field which defines its type, a ring node, which is 
used to attach the message to the queue of pending messages, a class field, 
which identifies a MessageClass object.  .message.intent: The intention is that 
a MessageStruct will be embedded in some richer object which contains 
information relevant to that specific type of message.

.message.type:

typedef struct MessageStruct *Message;

.message.struct:

struct MessageStruct {
  Sig sig;
  MessageType type;
  MessageClass class;
  RingStruct node;
} MessageStruct;


.class: A message class is an encapsulation of methods.  It encapsulates 
methods that are applicable to all types of messages (generic) and methods that 
are applicable to messages only of a certain type (type-specific).  
.class.concrete: Concretely a message class is represented by a 
MessageClassStruct (a struct).  Clients of the Message module are expected to 
allocate storage for and initialise the MessageClassStruct.  It is expected 
that such storage will be allocated and initialised statically.

.class.one-type: A message class implements exactly one message type.  
The identifier for this type is stored in the "type" field of the 
MessageClassStruct.  Note that the converse is not true: a single 
message type may be implemented by two (or more) different message 
classes (for example: for two pool classes that require different 
implementations for that message type).

BEGIN OBSOLETE SECTION "class-not-type".
[.class.not-type.not: It used to be the case that message classes and message 
types were completely distinct.  This was never used and was unnecessary.  It 
was removed by making message-type an attribute of the implementing 
message-class, ie. moving the type field out of MessageStruct and into 
MessageClassStruct.  RHSK 2008-11-24.]
.class.not-type: Note that message classes and message types are distinct.  
.class.not-type.why: (see also mail.drj.1997-07-15.10-33(0) from which this is 
derived)  This allows two different implementations (ie classes) of messages 
with the same meaning (ie type).  This may be necessary because the (memory) 
management of the messages may be different in the two implemtations (which is 
bogus).  The case of having one class implement two types is not expected to be 
so useful.  .class.not-type.why.not: It's all pretty feeble justification 
anyway.
END OBSOLETE SECTION "class-not-type"

.class.methods.generic: The generic methods are:

delete - used when the message is destroyed (by the client calling 
mps_message_discard).  The class implementation should finish the message (by 
calling MessageFinish) and storage for the message should be reclaimed (if 
applicable).

.class.methods.specific: 

The type specific methods are:

.class.methods.specific.finalization:

Specific to MessageTypeFinalization

finalizationRef - returns a reference to the finalizable object represented by 
this message.

.class.methods.specific.collectionstats:

Specific to MessageTypeCollectionStats

collectionStatsLiveSize - returns the number of bytes (of objects) that were 
condemned but survived.

collectionStatsCondemnedSize - returns the number of bytes condemned in the 
collection.

collectionStatsNotCondemnedSize - returns the the number of bytes (of objects) 
that are subject to a GC policy (ie collectable) but were not condemned in the 
collection.


.class.type:

typedef struct MessageClassStruct *MessageClass;

.class.sig.double: The MessageClassStruct has a signature field at both ends.  
This is so that if the MessageClassStruct changes size (by adding extra methods 
for example) then any static initializers will generate errors from the 
compiler (there will be a type error causes by initialising a non-sig type 
field with a sig) unless the static initializers are changed as well.

.class.struct:

typedef struct MessageClassStruct {
  Sig sig;                      /* design.mps.sig */
  const char *name;             /* Human readable Class name */

  /* generic methods */
  MessageDeleteMethod delete;   /* terminates a message */

  /* methods specific to MessageTypeFinalization */
  MessageFinalizationRefMethod finalizationRef;

  /* methods specific to MessageTypeCollectionStats */
  MessageCollectionStatsLiveSizeMethod collectionStatsLiveSize;
  MessageCollectionStatsCondemnedSizeMethod collectionStatsCondemnedSize;
  MessageCollectionStatsNotCondemnedSizeMethod collectionStatsNotCondemnedSize;

  Sig endSig;                   /* design.mps.message.class.sig.double */
} MessageClassStruct;


.space.queue: The arena structure is augmented with a structure for managing 
for queue of pending messages.  This is a ring in the ArenaStruct.

struct ArenaStruct
{
  ...
  RingStruct messageRing;
  ...
}


Functions

.fun.init:
/* Initializes a message */
void MessageInit(Arena arena, Message message, MessageClass class);

Initializes the MessageStruct pointed to by message.  The caller of this 
function is expected to manage the store for the MessageStruct.

.fun.finish:
/* Finishes a message */
void MessageFinish(Message message);

Finishes the MessageStruct pointed to by message.  The caller of this function 
is expected to manage the store for the MessageStruct.

.fun.post:
/* Places a message on the client accessible queue */
void MessagePost(Arena arena, Message message);

This function places a message on the queue of a arena.  .fun.post.precondition
: Prior to calling the function the node field of the message must be a 
singleton.  After the call to the function the message will be available for 
MPS client to access.  After the call to the function the message fields must 
not be manipulated except from the message's class's method functions (i.e., 
you mustn't poke about with the node field in particular).

.fun.empty:
void MessageEmpty(Arena arena);

Empties the message queue.  This function has the same effect as discarding all 
the messages on the queue.  After calling this function there will be no 
messages on the queue.  .fun.empty.internal-only: This functionality is not 
exposed to clients.  We might want to expose this functionality to our clients 
in the future.




Message Life Cycle

.life: A message will be allocated by a client of the message module, it will 
be initialised by calling MessageInit.  The client will eventually post the 
message on the external queue (in fact most clients will create a message and 
then immediately post it).  The message module may then apply any of the 
methods to the message.  The message module will eventually destroy the message 
by applying the Delete method to it.


EXAMPLES


Finalization

[possibly out of date, see design.mps.finalize and design.mps.poolmrg instead 
-- drj 1997-08-28]

This subsection is a sketch of how PoolMRG will use Messages for finalization 
(see design.mps.poolmrg).

PoolMRG has guardians (see design.mps.poolmrg.guardian), guardians are used to 
manage final references and detect when an object is finalizable.

The link part of a guardian will include a MessageStruct.

The MessageStruct is allocated when the final reference is created (which is 
when the referred to object is registered for finalization).  This avoids 
allocating at the time when the message gets posted (which might be a tricky, 
undesirable, or impossible, time to allocate).

PoolMRG has two queues: the entry queue, and the exit queue.  The entry 
queue will use a ring; the exit queue of MRG will simply be the 
external message queue.

[The evil hack of 'borrowing' MessageStruct's ring node for the entry 
queue was removed from the code ages ago, so I've removed the account 
of it from this text too.  RHSK 2006-12-11]

MRG Message class

del - frees both the link part and the reference part of the guardian.

A. References

B. Document History

2002-06-07 RB Converted from MMInfo database design document.
2006-10-25 RHSK Created guide.
2006-12-11 RHSK More on lifecycle; unmention evil hack in initial design.
2008-12-19 RHSK Simplify and clarify lifecycle. Remove description of and deprecate re-use of messages.