Chapter 9 Message Passing Operating Systems by Dhananjay

Introduction • • • Overview of Message Passing Implementing Message Passing Mailboxes Higher-Level Protocols Using Message Passing Case Studies in Message Passing Operating Systems, by Dhananjay Dhamdhere 2

Overview of Message Passing • Message passing is one way in which processes interact with one another • Processes may exist in the same computer or in different computers connected to a network • Uses of message passing: – Client-server paradigm – Backbone of higher-level communication protocols – Parallel distributed Operating Systems, by and Dhananjay Dhamdhere programs 3

Overview of Message Passing (continued) • 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 Operating Systems, by Dhananjay Dhamdhere 4

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 Operating Systems, by Dhananjay Dhamdhere 5

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 Operating Systems, by Dhananjay Dhamdhere 6

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 Operating Systems, by Dhananjay Dhamdhere 7 blocked on a receive

Implementing Message Passing Operating Systems, by Dhananjay Dhamdhere 8

Delivery of Interprocess Messages • Remember that: – An event control block (ECB) has three fields: • Description of the anticipated event • Id of the process that awaits the event • An ECB pointer forming ECB lists Operating Systems, by Dhananjay Dhamdhere 9

Operating Systems, by Dhananjay Dhamdhere 10

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 Operating Systems, by Dhananjay Dhamdhere 11

Mailboxes (continued) Operating Systems, by Dhananjay Dhamdhere 12

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 – May permit owner to destroy it, transfer ownership, etc. • Use of a mailbox has following advantages: – Anonymity of receiver – Classification of messages • Through use of separate mailboxes for different classes Operating Systems, by Dhananjay Dhamdhere 13

Example: Use of Mailboxes • An airline reservation system: – A set of booking processes – Server wishes to process cancellations before bookings and queries after both of them Operating Systems, by Dhananjay Dhamdhere 14

Higher-Level Protocols Using Message Passing • Several protocols use message passing paradigm to provide diverse services: – Simple mail transfer protocol (SMTP) delivers electronic mail – Remote procedure call (RPC) is a programming language facility for distributed computing • Invokes a part of a program that is located on a different computer – Parallel virtual machine (PVM) and message passing interface (MPI) are message passing standards for parallel programming Operating Systems, by Dhananjay Dhamdhere 15

The Simple Mail Transfer Protocol (SMTP) • SMTP is used to deliver electronic mail to one or more users reliably and efficiently – Uses asymmetric naming – Can deliver mail across a number of interprocess communication environments (IPCEs ) – It is an applications layer protocol: uses TCP / IP • SMTP consists of several simple commands: – MAIL, RCPT, DATA • Typically used with a protocol that provides a mailbox – Internet Message Access Protocol (IMAP) – Post Office Protocol (POP) Operating Systems, by Dhananjay Dhamdhere 16

Remote Procedure Calls • Remote procedure call (RPC): used to invoke a part of a program located in a different computer – Semantics resemble those of a procedure call • call <proc_id> (<message>); – <message> is a list of parameters • Stubs perform marshaling of parameters and convert them to/from machine independent representations – Sun. RPC and OSF/DCE standards, Java RMI Operating Systems, by Dhananjay Dhamdhere 17

Message Passing Standards for Parallel Programming • Parallel program: set of tasks that can be performed in parallel – Executed on a heterogeneous set of computers or on a massively parallel processor (MPP) • Parallel virtual machine (PVM) and message passing interface (MPI) are standards used in coding message passing libraries; provide: – Point-to-point communication between processes – Barrier synchronization between processes – Global operations for scattering (gathering) disjoint portions of data in a message to (from) different Operatingprocesses Systems, by Dhananjay Dhamdhere 18

Case Studies in Message Passing • Message Passing in Unix • Message Passing in Windows Operating Systems, by Dhananjay Dhamdhere 19

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 – 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 Operating Systems, by Dhananjay Dhamdhere 20

Message Passing in Unix (continued) • One common feature: processes can communicate without knowing each other’s identities Operating Systems, by Dhananjay Dhamdhere 21

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 Operating Systems, by Dhananjay Dhamdhere 22

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 Operating Systems, by Dhananjay Dhamdhere 23

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 • Server can set up communication paths with many clients simultaneously – Typically, after connect call, server forks a new process to handle the new connection • Leaves original socket created by server process free to accept more connections • Indirect naming: address (domain) used instead of process ids Operating Systems, by Dhananjay Dhamdhere 24

Message Passing in Windows • Named pipes: reliable bidirectional byte/message mode communication between server and clients – Implemented through the file system interface – Synchronous and asynchronous message passing • Local Procedure Call (LPC) facility performs message passing between processes in same host – Choice of three methods of message passing • Windows socket (Winsock) – Integrated with Windows message passing • RPC: Synchronous and asynchronous Operating Systems, by Dhananjay Dhamdhere 25

Summary • Message passing paradigm realizes exchange of information among processes without using shared memory • Useful in: microkernel-based OSs, client–server computing, higher-level communication protocols, and parallel or distributed programs • Sender/receiver naming: symmetric, asymmetric, indirect (mailbox) • Message passing is employed in higher-level protocols such as SMTP, RPC, PVM, and MPI Operating Systems, by Dhananjay Dhamdhere 26