In: Communicating Process Architectures 2001
Alan Chalmers, Majid Mirmehdi and Henk Muller (Eds.)
IOS Press, NL, 2001
Pages 305-308
ISBN 1 58603 202 X (IOS Press), 4 274 90467 9 C3000 (Ohmsha), Library of Congress Control Number: 2001094313
In: Concurrent Systems Engineering Series (ISSN 1383-7575)

Eds.: M.R. Jane, J. Hulskamp, P.H. Welch, D. Stiles, T.L.Kunii
Volume 59

Ref.: CPA 2001: http://www.cs.bris.ac.uk/Events/CPA2001/
The presentation: index.html
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CHANnels to deliver memory?
MOBILE structures and ALTing over memory?

Řyvind Teig
Kongsberg Maritime Ship Systems, Ship Control (KMSS-SC)
7005 Trondheim, Norway

Abstract. Memory objects are assigned to processes over a CHANnel like construct. This way one can wait for an object indefinitely, or with timeout in an ALT construct - coexisting with CHANnel inputs. The run-time SYSTEM will handle requests. Alternatively, a user memory handler process may use the underlying SYSTEM and serve other clients. Occam 2 is used as catalyst language.

1 - Introduction

This note was directly inspired by Barnes and Welch, "Mobile Data Types for Communicating Processes" [1], where the concept of copy by ownership moving between concurrent processes is bound into the occam 2 language.

The paper introduces "the basic idea to make dynamic memory allocation not break scheduling independence for parallel (occam-like) systems" (to quote one of the referees). It introduces a way to handle memory allocation failures in these systems. In f.ex. limited memory embedded devices, the idea suggested opens a possibility to dynamically allocate memory, not by a go/no-go malloc, but by treating memory allocation as a blocking synchronisation primitive, which may coexist with channel inputs in ALT statements. The memory "delivered" by the run-time system may be of any type, specified by a system module.

This note tries to "think aloud". It is not part of any ongoing research, contains ideas only, collected by a software engineer working in industry.

Because this paper consists of ideas only, as input from industry to academia or toolmakers, it has not been a goal to suggest usable examples. Also, how the underlying run-time system would function is not discussed. This also goes for the liveliness properties of the run-time system, as well as liveliness properties of occam programs using the concept. So, whether f.ex. processes can deadlock blocking for memory, is not handled. The individual elements in the list below have on purpose been presented in a rather terse form. The good thing is that page count is low!

2 - Notes

Let us just jump on the ideas by playing with imaginative occam examples (this is not occam 2):

  1. MEM MyIntrinsicMem 
    
      CASE
    
        STACK
    
        HEAP
    
        PLACED; address; accessRights
    
        ROM; address
    
        FLASH; address; accessRights
    
        VIRTUAL
    
        EXCEPTION; type -- exceptionally handled
    
    :

    Just like CHAN OF aPROTOCOL, we introduce a similar SYSTEM OF aMEM.
    Above, MyIntrinsicMem is a name defined by the user. In the example MEM, STACK, HEAP etc. are new keywords which are understood by a configurer and run-time system. Maybe their properties, like address and size could be defined above, or maybe a separate configuration language is needed. This probably depends on how often a MEM is used in the program, one time or scattered throughout. 

  1. SYSTEM OF MyIntrinsicMem aMEM:
    
    
    
    MOBILE THINGa buffera: -- A data type with SIZE=512
    
    MOBILE THINGb bufferb: -- A data type with SIZE=64
    
    
    
    THINGc        bufferc: -- Static

    The SYSTEM keyword is  inspired from Modula-2, where features of the "system" were defined within the SYSTEM module, like the size of an integer and how coroutines should be started and synchronized. In Modula-2, this module was built into the compiler "because some of the objects it defines cannot be expressed in the Modula-2 language" [2].
    The MOBILE keyword is taken from [1] and informs us that the data may be dynamically allocated in some way, and ownership of data defined may be passed around.

  1. aMEM[HEAP]             ? buffera     -- blocks for object from HEAP
    
    aMEM[HEAP,STACK]       ? buffera     -- blocks for object from HEAP or STACK
    
    aMEM[PLACED(#1000,RW)] ? dualPortMem -- blocks for object from dual-port memory
    
    aMEM[]                 ? buffera     -- blocks for object from any segment(?)

    The examples above show how different kinds of memory may be assigned for memory objects. In effect, we have a parameterised new operator.

  1. aMEM[HEAP] ? bufferc -- blocks for static usage(?)

    It may be a good idea also to let static data come into presence  by this mechanism, an example is shown above. (This may pre-empt the sub-title of this note..) The compiler would be able to see how an object is supposed to come into being.

  1. aMEM[HEAP] ? buffera AFTER time -- ILLEGAL

    Above, it would give up after time if memory did not become available. This is an exception from CHANnel syntax - it cannot not be legal, since we do not have a mechanism to handle timeout failure.

  1. ALT
    
      (NOT needsBuffer) & aCHAN ? someData
    
        ... Process, set needsBuffer
    
      (needsBuffer) & aMEM[HEAP] ? buffera
    
        SEQ
    
          ... Use buffera
    
          ... Send off if appropriate
    
      (needsBuffer) & clock ? AFTER timeout
    
        ...  Handle timeout

    However,  we can always time out in an ALT. This server receives someData on aCHAN, processes it, but needs buffera in order to do some more interesting things. When, or if, buffera has been received, it processes it and sends it on to another process, in which case a new reclaim later on will be recognised by the runtime SYSTEM as a proper request. If it decides not to send off, the runtime SYSTEM will, on next reclaim, see that it already has a buffer, and pass on the privileges it already has.

  1. ALT
    
      aMEM[] ? CASE
    
        STACK; buffera
    
          ...
    
        HEAP; buffera
    
          ...  

    Above, we just want space for buffera, and we do not care from where. If neither STACK nor HEAP is available, we have a STOP situation.

  1. ALT
    
      aMEM[PLACED(#2000,RW)] ? CASE
    
        PLACED; dualPortMem
    
          ... Do this
    
        EXCEPTION; type
    
          .. Handle it 

    Above, we want to serve a dual port memory which resides on a plug-in card. If the card is not present, we receive an EXCEPTION instead. If we decided that we wanted to be signalled whenever a card was inserted, we could just drop the EXCEPTION handling, and the runtime SYSTEM would signal us in due course.

  1. ALT
    
      [2]ab IS [buffera,bufferb]: -- or RETYPES?
    
      PRI ALT i = 0 FOR SIZE ab
    
        aMEM[HEAP] ? ab[i]
    
          SEQ
    
            ... Now we have the largest buffer available of the two
    
            ... Inform client which buffer size I have

    The server  above gets the largest of the two buffers, since buffera, which is largest and has been assigned highest ALT priority, is indexed as [0].

  1. PROC MEMHandler ([]SYSTEM OF MyIntrinsicMem aMEMS)
    
      WHILE TRUE
    
        ALT i=0 FOR SIZE aMEMS
    
          aMEMS[i][] ? CASE
    
            .. Process request and send out access right
    
    :

    Above, we have inserted a local MEMHandler between our processes and SYSTEM. MEMHandler itself is able to communicate with SYSTEM directly. It would perhaps be most natural to let MEMHandler reply over the same SYSTEM "channel" on which the request arrived. By studying the examples above, we understand this has to be so. Our servers request some type of memory (out) and receive some kind of response (in) without specifically using uni-directional mechanisms. An implicit bi-directional scheme is instead suggested.

  1. How this dynamic scheme "plugs into" the occam OO-like suggestion described in [3], and into the fuller MOBILE concept of [1] remains to be seen. All three concepts should break no occam (or extended occam) laws. (Late addition: [3] and an ancestor of [1] are actually present in this very proceeding.)

Acknowledgements

I must thank Peter Welch for sending me a draft version of  [1] half a year ago, and quietly listening on some of the thoughts above at that stage. When I recently asked him how it went with [1], he sent it to me just prior to the PDPTA conference. This gave me time to write this note for the CPA 2001 deadline.

References

[1]
F.R.M. Barnes and P.H.Welch, "Mobile Data Types for Communicating Processes", CSREA Press, June 2001. The 2001 International Conference on Parallel and Distributed Processing Techniques and Applications (PDPTA'2001)
[2]
Logitech M2, Modula-2 Language Reference, Multiscope Inc, Dec.1991, page 273
[3]
Řyvind Teig, "From safe concurrent processes to process-classes? PLUSSING new code by ROLLING out and compile?", submitted to CPA 2001.

Řyvind Teig is Senior Development Engineer at Kongsberg Maritime Ship Systems, Ship Control. He has worked with embedded systems for 25 years, and is especially interested in real-time language issues. See http://home.no.net/oyvteig/ for publications.