Cooperating Processes

1
Cooperating Processes

• Independent process cannot affect or be affected
  by the execution of another process.
• Cooperating process can affect or be affected by
  the execution of another process
• Advantages of process cooperation
• Information sharing
• Computation speed-up
• Modularity
• Convenience
• Robustness

2
Producer-Consumer Problem

• Paradigm for cooperating processes, producer
  process produces information that is consumed by
  a consumer process.
• unbounded-buffer places no practical limit on the
  size of the buffer.
• bounded-buffer assumes that there is a fixed
  buffer size.

3
Mechanisms for IPC
4
Bounded-Buffer Shared Memory Solution

• Shared data
• define BUFFER_SIZE 10
• typedef struct
• . . .
• item
• item bufferBUFFER_SIZE
• int in 0
• int out 0

5
Bounded-Buffer Shared Memory Solution

• Producer
• while (1)
• while (((in 1) BUFFER_SIZE) out)
• / do nothing /
• bufferin nextProduced
• in (in 1) BUFFER_SIZE
• Consumer
• while (1)
• while (in out)
• / do nothing /
• nextConsumed bufferout
• out (out 1) BUFFER_SIZE
• Can only use BUFFER_SIZE-1 elements

6
Message Passing

• Processes communicate without shared variables
• If P and Q wish to communicate, they need to
• establish a communication link between them
• send(message)
• receive(message)
• Producer - Consumer
• while (1)
• send(consumer,produceNext())
• while (1)
• nextToConsume receive(producer)

7
Message Passing - Link Properties

• How are links established?
• Can a link be associated with more than two
  processes?
• How many links can there be between processes?
• What is the capacity of a link? (buffers?)
• Fixed or variable size messages?
• Is the link simplex or duplex or full duplex?

8
Message Passing - Logical Properties

• Direct or indirect communication (process or
  mailbox)
• Symmetric or asymmetric (names both ways or one
  way)
• Automatic or explicit or no buffering
• Send by copy or send by reference
• Fixed or variable size messages
• Synchronous (blocking) or asynchronous
  (non-blocking)

9
Symmetry of naming
10
Direct Communication

• Symmetric Processes name each explicitly
• send (P, message) send a message to process P
• receive(Q, message) receive a message from
  process Q
• Links are established automatically, on demand.
• A link is associated with exactly two processes.
• Between each pair there exists exactly one link.
• Asymmetric Only sender names the receiver
• send (P, message) send message to process P
• receive(?, message) receive message from anyone
• Asymmetric Only receiver names the sender
• send (?, message) send message to anyone
  (copies?)
• receive(Q, message) receive message from
  process Q

11
Indirect Communication

• Messages are directed and received from mailboxes
  (also referred to as ports).
• Each mailbox has a unique id.
• Processes can communicate only if they share a
  mailbox.
• Operations
• Create a new mailbox (Ownership? Who receives?)
• send(A,message) send a message to mailbox A
• message receive(A) receive a message from A
• Destroy a mailbox (? If owner terminates)
• Properties of communication link
• Link established only if processes share a common
  mailbox
• A link may be associated with many processes.
• Each pair of processes may share several links.

12
Indirect Communication

• Mailbox sharing
• P1, P2, and P3 share mailbox A.
• P1, sends P2 and P3 receive.
• Who gets the message?
• Solutions
• Allow a link to be associated with at most two
  processes.
• Allow only one process at a time to execute a
  receive
• Allow the system to select arbitrarily the
  receiver.
• Sender is notified who the receiver was.
• Atomicity, regardless

13
Synchronization

• Send and receive can be blocking or non-blocking
• Blocking send waits until message is received
• Non-blocking send allows sender to continue
• Sender is not assured of reception - must ack
• Sender
• send(receiver,message)
• receive(receiver,ack_message)
• Receiver
• receive(sender,message)
• send(sender,ack)
• Blocking receive waits until message is available
• Non-blocking receive returns null if no message
• Blocking send and receives forces a rendezvous

14
Buffering

• Queue of messages attached to the link
  implemented in one of three ways.
• Zero capacity 0 messagesSender must wait for
  receiver (must rendezvous).
• Bounded capacity finite length of n
  messagesSender must wait if link full, or
  overwrite
• Unbounded capacity infinite length (right!).

15
Exception Conditions

• Process terminates
• Receiver waiting for a message from terminated
  sender
• Notify or terminate receiver
• Sending to a terminated receiver
• Delete message
• Notify sender?
• Lost messages
• Detect with timeouts - expect an ack within some
  time
• Resend (duplicates)
• Scrambled messages
• Error checking codes