CS452 - Real-Time Programming - Spring 2010

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Lecture 10 - Message Passing

Overview

Message passing combines synchronization and communication in one set of primitives.

• Send, Receive, Reply

int Send( Tid tid, char *message, int mslen, char *reply, int rplen )

• blocks until Reply occurs

int Receive( Tid *tid, char *message, int mslen )

• blocks until message is available

int Reply( Tid tid, char *reply, int rplen )

• does not block
• unblocks task that called Send
• Send and Reply become READY at the same time.

There are two cases

Send before Receive

Send
           ...
           Receive
           ...
           Reply
...        ...

Message copying occurs inside Receive and Reply.

Receive needs to have a list of current senders, the ReceiveQ

Receive before Send

            Receive
...
Send
            ...
            Reply
...         ...

Message copying occurs inside Send and Reply.

Practical Details

• Need to keep around request
• Messages
  • Typed as strings
    • which require formatting and parsing
  • Normally cast from pointers to structs
    • sizeof( ) is useful
    • struct expected by receiver/sender must agree with struct provided by sender/receiver
    • Type-checking by compiler not possible
    • Dynamic type-checking possible
      • First element of struct is its type
      • Even polymorphism is possible
• Message types should be handled in a more modern way
  • type extension
  • would provide a more uniform handling of error returns
• Task states
  1. DEFUNCT is terminal
  2. READY -> ACTIVE
  3. ACTIVE -> SEND_BLOCKED, RECEIVE_BLOCKED, READY, DEFUNCT
  4. SEND_BLOCKED -> READY
  5. RECEIVE_BLOCKED -> REPLY_BLOCKED
  6. REPLY_BLOCKED -> READY
• You can add extra return values beyond those specified

int Send( Tid tid, char *message, int mslen, char *reply, int rplen )

These are pretty self explanatory, except

1. The return value is the number of characters actually placed in the reply-buffer
   • including the \0 if the contents of the reply buffer is a string
2. If something goes wrong, the return value is negative, coded to indicate what went wrong

   What can go wrong

   1. Illegal tid
   2. tid not an existing task

   It's up to Send to check that the reply-buffer was big enough by looking at its return value

   It's not an error if the task to which we Send never Receives

   • Should it be?
3. Parsing argument and reply-buffer is potentially costly and error-prone
   • A type system might be nice
   • But then you would feel compelled to implement run-time type checking

Implementing Send

What's in application space is just stubs.

• checking arguments
• putting arguments in the right place
  • Note that there are five arguments

What the kernel must do

1. Check arguments
   • tid is valid?
2. Change state of sender to RECEIVE_BLOCKED
3. Put sender on the end of the receiver's sendQ

   • active = tail->next
     tail = active

4. If receiver is SEND_BLOCKED, do 3 in receiver

int Receive( Tid *tid, char *message, int msglen )

These are pretty self explanatory, except

1. How is the task id copied form kernel to receiver?
   • That is, where does the pointer point to?
2. What if the buffer wasn't big enough?
3. If several tasks have done Send, which one gets Received first?
4. return value is number of bytes in message, including \0.
   • It seems as though the return value should be the tid. Something is not right.
5. If something goes wrong, the return value is negative, coded to indicate what went wrong

   What can go wrong?

   1. Only part of the message was copied

   It's up to Send to check that the reply-buffer was big enough by looking at its return value

Implementing Receive

What the kernel must do

1. Check arguments
2. Change receiver's state to SEND_BLOCKED
3. Check the sendQ
4. If SENDQ_EMPTY
   1. Exit from kernel after scheduling
5. sendQ is not empty
   1. sender = head( sendQ )

      sendQ = next( sendQ )

   2. copy message from sender to receiver, after checking buffer sizes
   3. change sender's state to REPLY_BLOCKED
   4. change receiver's state to READY
   5. put sender's tid into receiver's argument
   6. put receiver on its readyQ
   7. set up receiver's return value

int Reply( Tid tid, char *reply, int rplen )

These are pretty self explanatory, except

1. The Replyer need not be the Receiver, but must be in contact with the Receiver
   • Why?
2. When all goes well Reply leaves two tasks READY when it completes

Implementing Reply

1. Check arguments
   • sender (tid) must be REPLY_BLOCKED
2. Copy message from replier to sender, checking buffer sizes
3. Put sender on readyQ
4. Set up sender's return value
5. Change sender's state to READY
6. Put replier on readyQ
7. Set up replier's return value

Change replier's state to READY.

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