10EC65 Operating Systems - Message Passing

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Operating Systems 10EC65 Unit 8

Message Passing

Reference Book: “Operating Systems - A Concept based Approach” D. M. Dhamdhare, TMH, 3rd Edition, 2010

Shrishail Bhat, AITM Bhatkal

Overview of Message Passing • Message passing is one way in which processes interact with one another

• Process đ?‘ƒđ?‘– sends a message to process đ?‘ƒđ?‘— by executing the statement send (đ?‘ƒđ?‘— ,<message>) • The compiled code of the send statement invokes the library module send • send makes a system call send, with đ?‘ƒđ?‘— and the message as parameters • Execution of the statement receive (đ?‘ƒđ?‘– , msg_area), where msg_area is an area in đ?‘ƒđ?‘— ’s address space, results in a system call receive Shrishail Bhat, AITM Bhatkal

Overview of Message Passing (continued)

The semantics of message passing are as follows: • At a send call by đ?‘ƒđ?‘– , the kernel checks whether process đ?‘ƒđ?‘— is blocked on a receive call for receiving a message from process đ?‘ƒđ?‘– • If so, it copies the message into msg_area and activates đ?‘ƒđ?‘— • If process đ?‘ƒđ?‘— has not already made a receive call, the kernel arranges to deliver the message to it when đ?‘ƒđ?‘— eventually makes a receive call • When process đ?‘ƒđ?‘— receives the message, it interprets the message and takes an appropriate action Shrishail Bhat, AITM Bhatkal

Overview of Message Passing (continued)

• Processes may exist in the same computer or in different computers connected to a network • Uses of message passing: – Client-server paradigm • To communicate between components of a microkernel-based OS and user processes • To provide services such as the print service to processes within an OS • To provide Web-based services to client processes located in other computers

– Backbone of higher-level communication protocols • For communicating between computers or for providing the electronic mail facility

– Parallel and distributed programs • To implement communication between tasks Shrishail Bhat, AITM Bhatkal

Message Passing using Shared Variables

• In principle, message passing can be performed by using shared variables • For example, msg_area in Figure 9.1 could be a shared variable • đ?‘ƒđ?‘– could deposit a value or a message in it and đ?‘ƒđ?‘— could collect it from there • However, this approach is cumbersome because the processes would have to create a shared variable with the correct size and share its name Shrishail Bhat, AITM Bhatkal

Producers – Consumers Problem

• The producers – consumers problem with a single buffer, a single producer process, and a single consumer process can be implemented by message passing

Shrishail Bhat, AITM Bhatkal

Producers – Consumers Problem (continued)

• The solution does not use any shared variables • Instead, process đ?‘ƒđ?‘– , which is the producer process, has a variable called buffer and process đ?‘ƒđ?‘— , which is the consumer process, has a variable called message_area • The producer process produces in buffer and sends the contents of buffer in a message to the consumer • The consumer receives the message in message_area and consumes it from there • The send system call blocks the producer process until the message is delivered to the consumer, and the receive system call blocks the consumer until a message is sent to it Shrishail Bhat, AITM Bhatkal

Issues in Message Passing

• Two important issues in message passing are: – Naming of processes • Names may be explicitly indicated or deduced by the kernel in some manner

– Delivery of messages • Whether sender should be blocked until delivery • What the order is in which messages are delivered to a destination process • How exceptional conditions are handled

Shrishail Bhat, AITM Bhatkal

Direct and Indirect Naming

• In direct naming, sender and receiver processes mention each other’s name – In symmetric naming, both sender and receiver processes specify each other’s name – In asymmetric naming, receiver does not name process from which it wishes to receive a message; kernel gives it a message sent to it by some process

• In indirect naming, processes do not mention each other’s name in send and receive statements Shrishail Bhat, AITM Bhatkal

Blocking and Nonblocking Sends

• A blocking send blocks sender process until message is delivered to destination process – Synchronous message passing – Simplifies design of concurrent processes

• A nonblocking send call permits sender to continue its operation after send call – Asynchronous message passing – Enhances concurrency between sender and receiver

• In both cases, receive is typically blocking • Kernel performs message buffering pending delivery Shrishail Bhat, AITM Bhatkal

Exceptional Conditions in Message Passing

• To facilitate handling exceptional conditions, send and receive take two additional parameters: – Flags indicate how exceptions should be handled – status_area is for storing code concerning outcome

• Some exceptional conditions: 1. Destination process mentioned in send doesn’t exist 2. Source process does not exist (symmetric naming) 3. send cannot be processed (out of buffer memory) 4. No message exists for process when it makes receive 5. A set of processes become deadlocked when one is blocked on a receive Shrishail Bhat, AITM Bhatkal

Exceptional Conditions in Message Passing (continued) • In cases 1 and 2, the kernel may abort the process that made the send or receive call and set its termination code to describe the exceptional condition • In case 3, the sender process may be blocked until some buffer space becomes available • In case 4, a process may prefer the standard action, which is that the kernel should block the process until a message arrives for it, or it may prefer an action of its own choice, like waiting for a specified amount of time before giving up • The deadlock situation of case 5 is an example of a severe exception. Most operating systems do not handle this particular exception because it incurs the overhead of deadlock detection Shrishail Bhat, AITM Bhatkal

Implementing Message Passing

• When a process đ?‘ƒđ?‘– sends a message to some process đ?‘ƒđ?‘— by using a nonblocking send, the kernel builds an interprocess message control block (IMCB) to store all information needed to deliver the message • The control block contains names of the sender and destination processes, the length of the message, and the text of the message • The control block is allocated a buffer in the kernel area • When process đ?‘ƒđ?‘— makes a receive call, the kernel copies the message from the appropriate IMCB into the message area provided by đ?‘ƒđ?‘— Shrishail Bhat, AITM Bhatkal

Implementing Message Passing (continued)

Shrishail Bhat, AITM Bhatkal

Implementing Message Passing (continued) • Figure 9.4 shows the organization of IMCB lists when blocking sends and FCFS message delivery are used • In symmetric naming, a separate list is used for every pair of communicating processes • When a process đ?‘ƒđ?‘– performs a receive call to receive a message from process đ?‘ƒđ?‘— , the IMCB list for the pair đ?‘ƒđ?‘– –đ?‘ƒđ?‘— is used to deliver the message • In asymmetric naming, a single IMCB list can be maintained per recipient process • When a process performs a receive, the first IMCB in its list is processed to deliver a message Shrishail Bhat, AITM Bhatkal

Delivery of Interprocess Messages

• When a process đ?‘ƒđ?‘– sends a message to process đ?‘ƒđ?‘— , the kernel delivers the message to đ?‘ƒđ?‘— immediately if đ?‘ƒđ?‘— is currently blocked on a receive call for a message from đ?‘ƒđ?‘– , or from any process • After delivering the message, the kernel must also change the state of đ?‘ƒđ?‘— to ready • If process đ?‘ƒđ?‘— has not already performed a receive call, the kernel must arrange to deliver the message when đ?‘ƒđ?‘— performs a receive call later • Thus, message delivery actions occur at both send and receive calls Shrishail Bhat, AITM Bhatkal

Delivery of Interprocess Messages (continued) • The kernel uses an event control block (ECB) to note actions that should be performed when an anticipated event occurs

• The ECB contains three fields: – Description of the anticipated event – Id of the process that awaits the event – An ECB pointer for forming ECB lists Shrishail Bhat, AITM Bhatkal

Delivery of Interprocess Messages (continued)

• When đ?‘ƒđ?‘– makes a send call, the kernel checks whether an ECB exists for the send call by đ?‘ƒđ?‘– , i.e., whether đ?‘ƒđ?‘— had made a receive call and was waiting for đ?‘ƒđ?‘– to send a message • If it is not the case, the kernel knows that the receive call would occur sometime in future, so it creates an ECB for the event “receive from đ?‘ƒđ?‘– by đ?‘ƒđ?‘— â€? and specifies đ?‘ƒđ?‘– as the process that will be affected by the event • Process đ?‘ƒđ?‘– is put into the blocked state and the address of the ECB is put in the event info field of its PCB Shrishail Bhat, AITM Bhatkal

Delivery of Interprocess Messages (continued)

• When process đ?‘ƒđ?‘— makes a receive call before đ?‘ƒđ?‘– makes a send call, an ECB for a “send to đ?‘ƒđ?‘— by đ?‘ƒđ?‘– â€? event is created

• The id of đ?‘ƒđ?‘— is put in the ECB to indicate that the state of đ?‘ƒđ?‘— will be affected when the send event occurs

Shrishail Bhat, AITM Bhatkal

Kernel Actions in Message Passing

Shrishail Bhat, AITM Bhatkal

Kernel Actions in Message Passing (continued) • At a send call, the kernel creates an IMCB even though a sender process is blocked until message delivery • When process đ?‘ƒđ?‘– sends a message to process đ?‘ƒđ?‘— , the kernel first checks whether the send was anticipated, i.e., whether an ECB was created for the send event • It will have happened if process đ?‘ƒđ?‘— has already made a receive call for a message from đ?‘ƒđ?‘– • If this is the case, action S3 immediately delivers the message to đ?‘ƒđ?‘— and changes its state from blocked to ready • The ECB and the IMCB are now destroyed • If an ECB for send does not exist, step S4 creates an ECB for a receive call by process đ?‘ƒđ?‘— , which is now anticipated, blocks the sender process, and enters the IMCB in the IMCB list of process đ?‘ƒđ?‘— Shrishail Bhat, AITM Bhatkal

Kernel Actions in Message Passing (continued)

• At a receive call, if a matching send has already occurred, a message is delivered to process đ?‘ƒđ?‘— and đ?‘ƒđ?‘– is activated

• Otherwise, an ECB is created for a send call and đ?‘ƒđ?‘— is blocked

Shrishail Bhat, AITM Bhatkal

Mailboxes • Mailbox: repository for interprocess messages – Has a unique name – Owner is typically the process that created it

• Indirect naming – Only owner process can receive messages – Any process that knows name of a mailbox can send messages to it

• Kernel may provide fixed set of mailbox names, or it may permit user processes to assign the names – Varying levels of confidentiality Shrishail Bhat, AITM Bhatkal

Mailboxes (continued)

Shrishail Bhat, AITM Bhatkal

Mailboxes (continued) • Process đ?‘ƒđ?‘– creates the mailbox, using the statement create_mailbox

• Process đ?‘ƒđ?‘— sends a message to the mailbox, using the mailbox name in its send statement • If đ?‘ƒđ?‘– has not already executed a receive statement, the kernel would store the message in a buffer

Shrishail Bhat, AITM Bhatkal

Mailboxes (continued) • Kernel may require a process to explicitly “connect” to a mailbox before starting to use it, and to “disconnect” when it finishes using it – It can destroy a mailbox if no process is connected to it

• Alternatively, it may permit the owner of a mailbox to destroy it – It should inform all processes that have “connected” to the mailbox

• The kernel may permit the owner of a mailbox to transfer the ownership to another process Shrishail Bhat, AITM Bhatkal

Advantages of Mailboxes

• Use of a mailbox has following advantages: – Anonymity of receiver • A mailbox relieves the sender process of the need to know the identity of the receiver • If the OS permits the ownership of a mailbox to be changed dynamically, one process can readily take over the service of another

– Classification of messages • Through use of separate mailboxes for different classes • A process may create several mailboxes, and use each mailbox to receive messages of a specific kind Shrishail Bhat, AITM Bhatkal

Use of Mailboxes – An Airline Reservation System

• It consists of a centralized data base and a set of booking processes – Each process represents one booking agent.

Shrishail Bhat, AITM Bhatkal

An Airline Reservation System (continued)

• It uses three mailboxes named enquire, book, and cancel, and expects a booking process to send enquiry, booking, and cancellation messages to these mailboxes, respectively • The server processes all pending cancellation messages before processing a booking request or an enquiry, and performs bookings before enquiries

Shrishail Bhat, AITM Bhatkal

Message Passing in Unix

• Three interprocess communication facilities: – Pipe: • Data transfer facility • Unnamed pipes can be used only by processes that belong to the same process tree, while named pipes can be used by other processes as well.

– Message queue: analogous to a mailbox • Used by processes within “Unix system domain” • Access permissions indicate which processes can send or receive messages

– Socket: one end of a communication path • Can be used for setting up communication paths between processes within the Unix system domain and within certain Internet domains

Shrishail Bhat, AITM Bhatkal

Message Passing in Unix (continued)

• One common feature: processes can communicate without knowing each other’s identities

Shrishail Bhat, AITM Bhatkal

Message Passing in Unix: Pipes

• FIFO mechanism for data transfer between processes called reader and writer processes • Usually implemented in file system – But, data put into a pipe can be read only once • Removed from pipe when it is read by a process

• Two kinds of pipes: named and unnamed – Created through the system call pipe – A named pipe has an entry in a directory

• Like a file, but size is limited and kernel treats it as a queue Shrishail Bhat, AITM Bhatkal

Message Passing in Unix: Pipes (continued) • When data is written, it is entered into the pipe by using the write offset, and the write offset is incremented by the number of bytes written • Data written by multiple writers gets mixed up if their writes are interleaved • If a pipe is full, a process wishing to write data into it would be put to sleep • A read operation is performed by using the read offset, and the read offset is incremented by the number of bytes read • A process reading data from a pipe would be put to sleep if the pipe is empty Shrishail Bhat, AITM Bhatkal

Message Passing in Unix: Message Queues • Analogous to a mailbox • Created and owned by one process – Creator specifies access permissions for send/receive

• Size specified at the time of its creation • A message queue is created by a system call msgget (key, flag) where key specifies the name of the message queue and flag indicates some options • Each message consists of a message type, in the form of an integer, and a message text

Shrishail Bhat, AITM Bhatkal

Message Passing in Unix: Message Queues (continued) • Messages are sent and received by using following system calls: msgsnd (msgqid, msg_struct_ ptr, count, flag) msgrcv (msgqid, msg_struct_ ptr, maxcount, type, flag) – The count and flag parameters of a msgsnd call specify the number of bytes in a message and the actions to be taken if sufficient space is not available in the message queue, e.g., whether to block the sender, or return with an error code – msg_struct_ ptr is the address of a structure that contains the type of a message, which is an integer, and the text of the message – maxcount is the maximum length of the message – type indicates the type of the message to be received

Shrishail Bhat, AITM Bhatkal

Message Passing in Unix: Message Queues (continued) • The cancellation, booking, and enquiry messages are assigned the types 1, 2, and 3, respectively • The msgrcv call with type = −4 and flag = “no wait” returns a cancellation message, if one is present • If no cancellation messages are present, it returns a bookings message if present, or an enquiry message

Shrishail Bhat, AITM Bhatkal

Message Passing in Unix: Sockets • A socket is one end of a communication path – Can be used for interprocess communication within the Unix system domain and in the Internet domain

• The client and server processes create a socket each – These two sockets are then connected together to set up a communication path for sending and receiving messages

• Server can set up communication paths with many clients simultaneously • A server creates a socket s using the system call s = socket (domain, type, protocol) • The socket call returns a socket identifier to the process Shrishail Bhat, AITM Bhatkal

Message Passing in Unix: Sockets (continued) • The server process now makes a call bind (s, addr, . . . ), which binds the socket to the address addr • The server performs the system call listen (s, . . . ) to indicate that it is interested in considering some connect calls to its socket s • A client creates a socket by means of a socket call, e.g., cs = socket (. . .), and attempts to connect it to a server’s socket using the system call connect (cs, server_socket_addr, server_socket_addrlen) • The kernel now creates a new socket with the call new_soc = accept (s, client_addr, client_addrlen) • The server uses this socket to implement the client – server communication Shrishail Bhat, AITM Bhatkal

Message Passing in Unix: Sockets (continued) • Typically, after the connect call the server forks a new process to handle the new connection – Leaves the original socket created by the server process free to accept more connections through listen and connect calls

• Communication between a client and a server is implemented through read and write or send and receive calls. • A send call has the format count = send (s, message, message_length, flags) • A socket connection is closed by using the call close (s) or shutdown (s, mode) • Sockets use indirect naming: address (domain) used instead of process ids Shrishail Bhat, AITM Bhatkal

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