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.

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

Architecture

.arch.diagram: I/O Architecture Diagram

[missing 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.

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.

typedef int mps_io_type_t;
enum {
  MPS_IO_TYPE_TELEMETRY,
  MPS_IO_TYPE_DEBUG
};

I/O module implementations

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"

Document History

• 1996-08-30 RB Document created from paper notes.
• 1997-06-10 RB 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.)
• 2002-06-07 RB Converted from MMInfo database design document.
• 2013-05-23 GDR Converted to reStructuredText.

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