THE DESIGN OF THE MPS I/O SUBSYSTEM
                             design.mps.io
                           incomplete design
                           richard 1996-08-30

INTRODUCTION

.intro: This document is the design of the MPS I/O Subsystem, a part of the 
plinth.

.readership: This document is intended for MPS developers.


History

.hist.1: Document created from paper notes by Richard Brooksby, 1996-08-30.

.hist.2: Updated with mail.richard.1997-05-30.16-13 and subsequent discussion 
in the Pool Hall at Longstanton.  (See also mail.drj.1997-06-05.15-20.)  
richard 1997-06-10


Background

.bg: This design is partly based on the design of the Internet User Datagram 
Protocol (UDP).  Mainly I used this to make sure I hadn't left out anything 
which we might need.


PURPOSE

.purpose: The purpose of the MPS I/O Subsystem is to provide a means to 
measure, debug, control, and test a memory manager build using the MPS.

.purpose.measure: Measurement consists of emitting data which can be collected 
and analysed in order to improve the attributes of application program, quite 
possibly by adjusting parameters of the memory manager (see overview.mps.usage).

.purpose.control: Control means adjusting the behaviour of the MM dynamically.  
For example, one might want to adjust a parameter in order to observe the 
effect, then transfer that adjustment to the client application later.

.purpose.test: Test output can be used to ensure that the memory manager is 
behaving as expected in response to certain inputs.


REQUIREMENTS


General

.req.fun.non-hosted: The MPM must be a host-independent system.

.req.attr.host: It should be easy for the client to set up the MPM for a 
particular host (such as a washing machine).


Functional

.req.fun.measure: The subsystem must allow the MPS to transmit quantitative 
measurement data to an external tool so that the system can be tuned.

.req.fun.debug: The subsystem must allow the MPS to transmit qualitative 
information about its operation to an external tool so that the system can be 
debugged.

.req.fun.control: The subsystem must allow the MPS to receive control 
information from an external tool so that the system can be adjusted while it 
is running.

.req.dc.env.no-net: The subsystem should operate in environments where there is 
no networking available.

.req.dc.env.no-fs: The subsystem should operate in environments where there is 
no filesystem available.


ARCHITECTURE

.arch.diagram:

 - I/O Architecture Diagram 

.arch.int: The I/O Interface is a C function call interface by which the MPM 
sends and receives "messages" to and from the hosted I/O module.

.arch.module: The modules are part of the MPS but not part of the freestanding 
core system (see design.mps.exec-env).  The I/O module is responsible for 
transmitting those messages to the external tools, and for receiving messages 
from external tools and passing them to the MPM.

.arch.module.example: For example, the "file implementation" might just 
send/write telemetry messages into a file so that they can be received/read 
later by an off-line measurement tool.

.arch.external: The I/O Interface is part of interface to the freestanding core 
system (see design.mps.exec-env).  This is so that the MPS can be deployed in a 
freestanding environment, with a special I/O module.  For example, if the MPS 
is used in a washing machine the I/O module could communicate by writing output 
to the seven-segment display.


Example Configurations

.example.telnet: This shows the I/O Subsystem communicating with a telnet 
client over a TCP/IP connection.  In this case, the I/O Subsystem is 
translating the I/O Interface into an interactive text protocol so that the 
user of the telnet client can talk to the MM.



.example.file: This shows the I/O Subsystem dumping measurement data into a 
file which is later read and analysed.  In this case the I/O Subsystem is 
simply writing out binary in a format which can be decoded.



.example.serial: This shows the I/O Subsystem communicating with a graphical 
analysis tool over a serial link.  This could be useful for a developer who has 
two machines in close proximity and no networking support.

.example.local: In this example the application is talking directly to the I/O 
Subsystem.  This is useful when the application is a reflective development 
environment (such as MLWorks) which wants to observe its own behaviour.


 - MPS I/O Configuration Diagrams 



INTERFACE

.if.msg: The I/O interface is oriented around opaque binary "messages" which 
the I/O module must pass between the MPM and external tools.  The I/O module 
need not understand or interpret the contents of those messages.

.if.msg.opaque: The messages are opaque in order to minimize the dependency of 
the I/O module on the message internals.  It should be possible for clients to 
implement their own I/O modules for unusual environments.  We do not want to 
reveal the internal structure of our data to the clients.  Nor do we want to 
burden them with the details of our protocols.  We'd also like their code to be 
independent of ours, so that we can expand or change the protocols without 
requiring them to modify their modules.

.if.msg.dgram: Neither the MPM nor the external tools should assume that the 
messages will be delivered in finite time, exactly once, or in order.  This 
will allow the I/O modules to be implemented using unreliable transport layers 
such as the Internet User Datagram Protocl (UDP).  It will also give the I/O 
module the freedom to drop information rather than block on a congested 
network, or stop the memory manager when the disk is full, or similar events 
which really shouldn't cause the memory manager to stop working.  The protocols 
we need to implement at the high level can be design to be robust against 
lossage without much difficulty.


I/O Module State

.if.state: The I/O module may have some internal state to preserve.  The I/O 
Interface defines a type for this state, "mps_io_t", a pointer to an incomplete 
structure "mps_io_s".  The I/O module is at liberty to define this structure.

  typedef struct mps_io_s *mps_io_t;


Message Types

.if.type: The I/O module must be able to deliver messages of several different 
types.  It will probably choose to send them to different destinations based on 
their type: telemetry to the measurement tool, debugging output to the 
debugger, etc.

  typedef int mps_io_type_t;
  enum {
    MPS_IO_TYPE_TELEMETRY,
    MPS_IO_TYPE_DEBUG
  };


Limits

.if.message-max: The interface will define an unsigned integral constant 
"MPS_IO_MESSAGE_MAX" which will be the maximum size of messages that the MPM 
will pass to "mps_io_send" (.if.send) and the maximum size it will expect to 
receive from "mps_io_receive".


Interface Set-up and Tear-down

.if.create: The MPM will call "mps_io_create" to set up the I/O module.  On 
success, this function should return "MPS_RES_OK".  It may also initialize 
"*mps_io_r" to a "state" value which will be passed to subsequent calls through 
the interface.

  extern mps_res_t mps_io_create(mps_io_t *mps_io_r);

.if.destroy: The MPM will call "mps_io_destroy" to tear down the I/O module, 
after which it guarantees that the state value "mps_io" will not be used 
again.  The "state" parameter is the state previously returned by 
"mps_io_create" (.if.create).

  extern void mps_io_destroy(mps_io_t mps_io);


Message Send and Receive

.if.send: The MPM will call "mps_io_send" when it wishes to send a message to a 
destination.  The "state" parameter is the state previously returned by 
"mps_io_create" (.if.create).  The "type" parameter is the type (.if.type) of 
the message.  The "message" parameter is a pointer to a buffer containing the 
message, and "size" is the length of that message, in bytes.  The I/O module 
must make an effort to deliver the message to the destination, but is not 
expected to guarantee delivery.  The function should return "MPS_RES_IO" only 
if a serious error occurs that should cause the MPM to return with an error to 
the client application.  Failure to deliver the message does not count.  
[Should there be a timeout parameter?  What are the timing constraints?  
mps_io_send shouldn't block.]

  extern mps_res_t mps_io_send(mps_io_t state,
                               mps_io_type_t type,
                               void *message,
                               size_t size);

.if.receive: The MPM will call "mps_io_receive" when it wants to see if a 
message has been sent to it.  The "state" parameter is the state previously 
returned by "mps_io_create" (.if.create).  The "buffer_o" parameter is a 
pointer to a value which should be updated with a pointer to a buffer 
containing the message received.  The "size_o" parameter is a pointer to a 
value which should be updated with the length of the message received.  If 
there is no message ready for receipt, the length returned should be zero.  
(Should we be able to receive truncated messages?  How can this be done neatly?)

  extern mps_res_t mps_io_receive(mps_io_t state,
                                  void **buffer_o,
                                  size_t *size_o);


I/O MODULE IMPLEMENTATIONS


Routeing

The I/O module must decide where to send the various messages.  A file-based 
implementation could put them in different files based on their types.  A 
network-based implementation must decide how to address the messages.  In 
either case, any configuration must either be statically compiled into the 
module, or else read from some external source such as a configuration file.


NOTES

The external tools should be able to reconstruct stuff from partial info.  For 
example, you come across a fragment of an old log containing just a few old 
messages.  What can you do with it?

Here's some completely untested code which might do the job for UDP.

---

#include "mpsio.h"

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <fcntl.h>
#include <errno.h>

typedef struct mps_io_s {
  int sock;
  struct sockaddr_in mine;
  struct sockaddr_in telemetry;
  struct sockaddr_in debugging;
} mps_io_s;

static mps_bool_t inited = 0;
static mps_io_s state;


mps_res_t mps_io_create(mps_io_t *mps_io_o)
{
  int sock, r;
  
  if(inited)
    return MPS_RES_LIMIT;

  state.mine = /* setup somehow from config */;
  state.telemetry = /* setup something from config */;
  state.debugging = /* setup something from config */;

  /* Make a socket through which to communicate. */  
  sock = socket(AF_INET, SOCK_DGRAM, 0);
  if(sock == -1) return MPS_RES_IO;

  /* Set socket to non-blocking mode. */
  r = fcntl(sock, F_SETFL, O_NDELAY);
  if(r == -1) return MPS_RES_IO;

  /* Bind the socket to some UDP port so that we can receive messages. */
  r = bind(sock, (struct sockaddr *)&state.mine, sizeof(state.mine));
  if(r == -1) return MPS_RES_IO;

  state.sock = sock;
  
  inited = 1;

  *mps_io_o = &state;
  return MPS_RES_OK;
}


void mps_io_destroy(mps_io_t mps_io)
{
  assert(mps_io == &state);
  assert(inited);
  
  (void)close(state.sock);

  inited = 0;
}


mps_res_t mps_io_send(mps_io_t mps_io, mps_type_t type,
                      void *message, size_t size)
{
  struct sockaddr *toaddr;

  assert(mps_io == &state);
  assert(inited);

  switch(type) {
    MPS_IO_TYPE_TELEMETRY:
    toaddr = (struct sockaddr *)&state.telemetry;
    break;
    
    MPS_IO_TYPE_DEBUGGING:
    toaddr = (struct sockaddr *)&state.debugging;
    break;
    
    default:
    assert(0);
    return MPS_RES_UNIMPL;
  }

  (void)sendto(state.sock, message, size, 0, toaddr, sizeof(*toaddr));

  return MPS_RES_OK;
}


mps_res_t mps_io_receive(mps_io_t mps_io,
                         void **message_o, size_t **size_o)
{
  int r;
  static char buffer[MPS_IO_MESSAGE_MAX];

  assert(mps_io == &state);
  assert(inited);
  
  r = recvfrom(state.sock, buffer, sizeof(buffer), 0, NULL, NULL);
  if(r == -1)
    switch(errno) {
      /* Ignore interrupted system calls, and failures due to lack */
      /* of resources (they might go away.) */
      case EINTR: case ENOMEM: case ENOSR:
      r = 0;
      break;
      
      default:
      return MPS_RES_IO;
    }

  *message_o = buffer;
  *size_o = r;
  return MPS_RES_OK;
}


ATTACHMENTS
   "O Architecture Diagram"
   "O Configuration Diagrams"