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  <title>IPC</title>

  <para>Due to the high intertask communication traffic, IPC becomes critical

  subsystem for microkernels, putting high demands on the speed, latency and

  reliability of IPC model and implementation. Although theoretically the use

  of asynchronous messaging system looks promising, it is not often

  implemented because of a problematic implementation of end user

  applications. HelenOS implements fully asynchronous messaging system with a

  special layer providing a user application developer a reasonably

  synchronous multithreaded environment sufficient to develop complex

  protocols.</para>

  <section>

    <title>Kernel Services</title>

    <para>Every message consists of 4 numeric arguments (32-bit and 64-bit on

    the corresponding platforms), from which the first one is considered a

    method number on message receipt and a return value on answer receipt. The

    received message contains identification of the incoming connection, so

    that the receiving application can distinguish the messages between

    different senders. Internally the message contains pointer to the

    originating task and to the source of the communication channel. If the

    message is forwarded, the originating task identifies the recipient of the

    answer, the source channel identifies the connection in case of a hangup

    response.</para>

    <para>Every message must be eventually answered. The system keeps track of

    all messages, so that it can answer them with appropriate error code

    should one of the connection parties fail unexpectedly. To limit buffering

    of the messages in the kernel, every task has a limit on the amount of

    asynchronous messages it can send simultaneously. If the limit is reached,

    the kernel refuses to send any other message, until some active message is

    answered.</para>

    <para>To facilitate kernel-to-user communication, the IPC subsystem

    provides notification messages. The applications can subscribe to a

    notification channel and receive messages directed to this channel. Such

    messages can be freely sent even from interrupt context as they are

    primarily destined to deliver IRQ events to userspace device drivers.

    These messages need not be answered, there is no party that could receive

    such response.</para>

    <section>

      <title>Low Level IPC</title>

      <para>The whole IPC subsystem consists of one-way communication

      channels. Each task has one associated message queue (answerbox). The

      task can call other tasks and connect its phones to their answerboxes,

      send and forward messages through these connections and answer received

      messages. Every sent message is identified by a unique number, so that

      the response can be later matched against it. The message is sent over

      the phone to the target answerbox. The server application periodically

      checks the answerbox and pulls messages from several queues associated

      with it. After completing the requested action, the server sends a reply

      back to the answerbox of the originating task. If a need arises, it is

      possible to <emphasis>forward</emphasis> a received message through any

      of the open phones to another task. This mechanism is used e.g. for

      opening new connections to services via the naming service.</para>

      <para>The answerbox contains four different message queues:</para>

      <itemizedlist>

        <listitem>

          <para>Incoming call queue</para>

        </listitem>

        <listitem>

          <para>Dispatched call queue</para>

        </listitem>

        <listitem>

          <para>Answer queue</para>

        </listitem>

        <listitem>

          <para>Notification queue</para>

        </listitem>

      </itemizedlist>

      <figure float="1">

        <title>Low level IPC</title>

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      <para>The communication between task A, that is connected to task B

      looks as follows: task A sends a message over its phone to the target

      asnwerbox. The message is saved in task B's incoming call queue. When task

      B fetches the message for processing, it is automatically moved into the

      dispatched call queue. After the server decides to answer the message,

      it is removed from dispatched queue and the result is moved into the

      answer queue of task A.</para>

      <para>The arguments contained in the message are completely arbitrary

      and decided by the user. The low level part of kernel IPC fills in

      appropriate error codes if there is an error during communication. It is

      assured that the applications are correctly notified about communication

      state. If a program closes the outgoing connection, the target answerbox

      receives a hangup message. The connection identification is not reused

      until the hangup message is acknowledged and all other pending messages

      are answered.</para>

      <para>Closing an incoming connection is done by responding to any

      incoming message with an EHANGUP error code. The connection is then

      immediately closed. The client connection identification (phone id) is

      not reused, until the client closes its own side of the

      connection ("hangs his phone up").</para>

      <para>When a task dies (whether voluntarily or by being killed), cleanup

      process is started.</para>

      <orderedlist>

        <listitem>

          <para>hangs up all outgoing connections and sends hangup messages to

          all target answerboxes,</para>

        </listitem>

        <listitem>

          <para>disconnects all incoming connections,</para>

        </listitem>

        <listitem>

          <para>disconnects from all notification channels,</para>

        </listitem>

        <listitem>

          <para>answers all unanswered messages from answerbox queues with

          appropriate error code and</para>

        </listitem>

        <listitem>

          <para>waits until all outgoing messages are answered and all

          remaining answerbox queues are empty.</para>

        </listitem>

      </orderedlist>

    </section>

    <section>

      <title>System Call IPC Layer</title>

      <para>On top of this simple protocol the kernel provides special

      services closely related to the inter-process communication. A range of

      method numbers is allocated and protocol is defined for these functions.

      These messages are interpreted by the kernel layer and appropriate actions

      are taken depending on the parameters of the message and the answer.</para>

      <para>The kernel provides the following services:</para>

      <itemizedlist>

        <listitem>

          <para>creating new outgoing connection,</para>

        </listitem>

        <listitem>

          <para>creating a callback connection,</para>

        </listitem>

        <listitem>

          <para>sending an address space area,</para>

        </listitem>

        <listitem>

          <para>asking for an address space area.</para>

        </listitem>

      </itemizedlist>

      <para>On startup, every task is automatically connected to a

      <emphasis>naming service task</emphasis>, which provides a switchboard

      functionality. In order to open a new outgoing connection, the client sends a

      <constant>CONNECT_ME_TO</constant> message using any of his phones. If

      the recepient of this message answers with an accepting answer, a new

      connection is created. In itself, this mechanism would allow only

      duplicating existing connection. However, if the message is forwarded,

      the new connection is made to the final recipient.</para>

      <para>In order for a task to be able to forward a message, it

      must have a phone connected to the destination task.

      The destination task establishes such connection by sending the <constant>CONNECT_TO_ME</constant>

      message to the forwarding task. A callback connection is opened afterwards. 

      Every service that wants to receive connections

      has to ask the naming service to create the callback connection via this mechanism.</para>

      <para>Tasks can share their address space areas using IPC messages. The

      two message types - <constant>AS_AREA_SEND</constant> and <constant>AS_AREA_RECV</constant> are used for sending and

      receiving an address space area respectively. The shared area can be accessed

      as soon as the message is acknowledged.</para>

    </section>

  </section>

  <section>

    <title>Userspace View</title>

    <para>The conventional design of the asynchronous API seems to produce

    applications with one event loop and several big switch statements.

    However, by intensive utilization of userspace pseudo threads, it was possible

    to create an environment that is not necessarily restricted to this type

    of event-driven programming and allows for more fluent expression of

    application programs.</para>

    <section>

      <title>Single Point of Entry</title>

      <para>Each task is associated with only one answerbox. If a

      multithreaded application needs to communicate, it must be not only

      able to send a message, but it should be able to retrieve the answer as

      well. If several pseudo threads pull messages from task answerbox, it is a

      matter of coincidence, which thread receives which message. If a particular

      thread needs to wait for a message answer, an idle

      <emphasis>manager</emphasis> pseudo thread is found or a new one is created and

      control is transfered to this manager thread. The manager threads pop

      messages from the answerbox and put them into appropriate queues of

      running threads. If a pseudo thread waiting for a message is not running, the

      control is transferred to it.</para>

      <figure float="1">

        <title>Single point of entry</title>

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      <para>Very similar situation arises when a task decides to send a lot of

      messages and reaches the kernel limit of asynchronous messages. In such

      situation, two remedies are available - the userspace library can either

      cache the message locally and resend the message when some answers

      arrive, or it can block the thread and let it go on only after the

      message is finally sent to the kernel layer. With one exception, HelenOS

      uses the second approach - when the kernel responds that the maximum limit

      of asynchronous messages was reached, the control is transferred to a manager

      pseudo thread. The manager thread then handles incoming replies and, when space

      is available, sends the message to the kernel and resumes the application thread

      execution.</para>

      <para>If a kernel notification is received, the servicing procedure is

      run in the context of the manager pseudo thread. Although it wouldn't be

      impossible to allow recursive calling, it could potentially lead to an

      explosion of manager threads. Thus, the kernel notification procedures

      are not allowed to wait for a message result, they can only answer

      messages and send new ones without waiting for their results. If the

      kernel limit for outgoing messages is reached, the data is automatically

      cached within the application. This behaviour is enforced automatically

      and the decision making is hidden from the developer.</para>

      <figure float="1">

        <title>Single point of entry solution</title>

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    </section>

    <section>

      <title>Ordering Problem</title>

      <para>Unfortunately, the real world is is never so simple. E.g. if a

      server handles incoming requests and as a part of its response sends

      asynchronous messages, it can be easily preempted and another thread may

      start intervening. This can happen even if the application utilizes only

      one userspace thread. Classical synchronization using semaphores is not

      possible as locking on them would block the thread completely so that

      the answer couldn't be ever processed. The IPC framework allows a

      developer to specify, that part of the code should not be preempted by

      any other pseudo thread (except notification handlers) while still being able

      to queue messages belonging to other pseudo threads and regain control when the

      answer arrives.</para>

      <para>This mechanism works transparently in multithreaded environment,

      where additional locking mechanism (futexes) should be used. The IPC

      framework ensures that there will always be enough free userspace threads

      to handle incoming answers and allow the application to run more

      pseudo threads inside the usrspace threads without the danger of

      locking all userspace threads in futexes.</para>

    </section>

    <section>

      <title>The Interface</title>

      <para>The interface was developed to be as simple to use as possible.

      Classical applications simply send messages and occasionally wait for an

      answer and check results. If the number of sent messages is higher than

      the kernel limit, the flow of application is stopped until some answers

      arrive. On the other hand, server applications are expected to work in a

      multithreaded environment.</para>

      <para>The server interface requires the developer to specify a

      <function>connection_thread</function> function. When new connection is

      detected, a new pseudo thread is automatically created and control is

      transferred to this function. The code then decides whether to accept

      the connection and creates a normal event loop. The userspace IPC

      library ensures correct switching between several pseudo threads

      within the kernel environment.</para>

    </section>

  </section>

</chapter>