Message Passing Concepts and Synchronization Applications

Message Passing Concepts and Synchronization Applications CS 350 – Fall 2002, 10/23/02, 9/17/03

Message Passing (Reference: Stallings, “Operating Systems”, 4th ed., section 5.6)

Below are some of the key ideas on this topic.

Fundamental requirements for concurrent processing – cooperative or interactive processing: Synchronization and communication. We saw these elements in the use of semaphores in conjunction with shared memory. A message passing capability combines both of these elements into a single facility. In addition message passing has the further advantage that it lends itself to distributed processing over a networked on set of different physical processors, as well as for uniprocessor applications. The former is more difficult with semaphore and other techniques due to the problems of managing interrupts across multiple interconnected machines.

Basic message passing primitives: send( destination, message) receive( source, message)

In general destination is where the message it to be sent to, and source is where the receiver wants to receive a message from.

“message” is the message sent (or a reference to it) when sending, and a place (or a reference to it) to put the message when receiving.

Design characteristics of message passing system is given by Stallings, table 5.4, page 242 (repeated below):

Chapters 4 and 7 (Silberschatz, 6th ed.) - Message Passing - Fall 2002, 10/23/02 page 1 Synchronization characteristics:

Because of the “causal” relationship between send and receive, message passing has natural or implicit synchronization characteristics.

Send blocking: Either the sending process is blocked until the message is received, or it is not (or also, in some cases, when a message queue is at maximum (UNIX)).

Receive blocking: there are two possibilities: - If a message has been sent and is available, the message is received and the receiver continues execution.

- If there is no message waiting, then either (a) the process is blocked until a message arrives, or (b) the process continuers to execute abandoning the receive attempt.

Blocking/unblocking send and receive can occur in any combination, for example:

- Blocking send and blocking receive – very tight synchronization (rendezvous)

- Nonblocking send, blocking receive – most common.

- Nonblocking send, nonblocking receive – loosest synchronization. Chapters 4 and 7 (Silberschatz, 6th ed.) - Message Passing - Fall 2002, 10/23/02 page 2 A more in-depth discussion of advantages and disadvantages and other variations of these schemes is given on page 243 of Stallings book.

Addressing Characteristics

Send primitive: specify which process is to receive the message. Receive primitive: indicate the process source of a message to be received.

Direct addressing:

Send primitive has a parameter identifying the intended destination process. Receive primitive: - Explicit direct addressing: specifically designate a sending process a message is expected from, the process must know in advance from which process a message is expected – used in cooperative processing. - Implicit direct addressing: The “source” parameter is a “return variable”/pointer indicating where the message came from; the sender fills this in with its identification. No need for the receiver to know in advance which process to expect a message from.

Indirect addressing:

Messages not sent directly to a receiver, but sent to a shared data structure or queue sometimes called a mail box. This decouples the sender and receiver for more flexibility. See Stallings fig. 5.24 for various indirect addressing schemes (repeated below):

Chapters 4 and 7 (Silberschatz, 6th ed.) - Message Passing - Fall 2002, 10/23/02 page 3 Relationship between sender and receiver can be: One-to-one: Allows a private communication link to be set up between two processes. Many-to-one: the bottom part of fig. 5.24 – a client/server model. The requesting clients are on the left and the server is on the right. Mail box is associated with the remote server service and is known as a “port” (see discussion on sockets). One-to-many: provides a broadcast capability

The association of processes to mailboxes can be static or dynamic. In static association (typically one-to-one), mailboxes are permanently assigned to a process until the process is destroyed. In dynamic associations, (typically many senders), mailboxes assigned and removed dynamically (via “connect”/disconnect” primitives).

Mailbox ownership: Generally ownership is assigned to the creating process. When this process is destroyed, the mailbox is destroyed. Sometimes, ownership is by the OS, which in this case, an explicit command is required to destroy the mailbox. Message format: Typically has a header and a body. The header is made up of message type, Destination ID, Source ID, message length, and control information. The body is the message content. Queuing discipline: The queuing discipline is typically FIFO, but message message priorities may be used based on message type or designated by sender. Also a scheme may allow a receiver to inspect a queue and select the message to receive next. Chapters 4 and 7 (Silberschatz, 6th ed.) - Message Passing - Fall 2002, 10/23/02 page 4 Mutual Exclusion: Mutual exclusion could be achieved by message passing. Here is how it is done:

We assume the use of blocking receive and nonblocking sends. A set of concurrent processes share a mailbox which we call mutex because of the obvious analogous to a semaphore mutex. This mailbox is initialized to contain a single message with null content. A process wishing to enter its critical section first attempts to receive this message. If the mailbox is empty, this process blocks. Once a process has acquired the message, it performs its critical section and then places the message back into the mailbox. Thus, the message serves as a token that is passed from process to process. Fig. 5.26 from Stallings shows how message passing accomplishes mutual exclusion (below):

Fig 5.26 assumes that more that if one process performs the receive operation concurrently:  If there is a message, it is delivered to only one process and the others are blocked, or

 If the message queue is empty, all processes are blocked. When a message is available, only one blocked process is activated and takes the message.

Chapters 4 and 7 (Silberschatz, 6th ed.) - Message Passing - Fall 2002, 10/23/02 page 5 The bounded buffer – producer/consumer problem using message passing pseudocode (below from Stallings fig. 5.27):

We use the mutual exclusion power of message passing. Compare this with the semaphore solution in fig. 5.16. This solution capitalizes on the ability of message passing to both pass data in addition to provide mutual exclusion. Two mailboxes are used. As the producer generates data, it is sent as a message to the mailbox mayconsume. As long as there is at least one message in that mailbox, the consumer can consume. Thus mayconsume serves as the buffer (contains data) ; the data in the buffer are organized as a queue of messages. The buffer has a fixed size given by capacity. Initially the mailbox mayproduce is filled with some number of NULL messages equal to the capacity of the buffer. The number of messages in mayproduce shrinks with each production and grows with each consumption.

Chapters 4 and 7 (Silberschatz, 6th ed.) - Message Passing - Fall 2002, 10/23/02 page 6 Note the synchronization aspect is achieved, in addition to keeping track of the full and empty buffer. The two examples given here demonstrate tht the properties of both mutex and counting semaphores are modeled using message passing!

Chapters 4 and 7 (Silberschatz, 6th ed.) - Message Passing - Fall 2002, 10/23/02 page 7