Chapter 4.1 Message Passing Communication Prepared by: Karthik V Puttaparthi

OUTLINE Interprocess Communication Message Passing Communication Basic Communication Primitives Message Design Issues Synchronization and Buffering References

INTERPROCESS COMMUNICATION Processes executing concurrently in the operating system may be either independent or cooperating processes. Reasons for providing an environment that allows process cooperation. 1) Information Sharing Several users may be interested in the same piece of information. 2) Computational Speed up Process can be divided into sub tasks to run faster, speed up can be achieved if the computer has multiple processing elements. 3) Modularity Dividing the system functions into separate processes or threads. 4) Convenience Even an individual user may work on many tasks at the same time.

COMMUNICATION MODELS Cooperating processes require IPC mechanism that allow them to exchange data and information. Communication can take place either by Shared memory or Message passing Mechanisms. Shared Memory: 1) Processes can exchange information by reading and writing data to the shared region. 2) Faster than message passing as it can be done at memory speeds when within a computer. 3) System calls are responsible only to establish shared memory regions. Message Passing: Mechanism to allow processes to communicate and synchronize their actions without sharing the same address space and is particularly useful in distributed environment.

Message Passing Communication Messages are collection of data objects and their structures Messages have a header containing system dependent control information and a message body that can be fixed or variable size. When a process interacts with another, two requirements have to be satisfied. Synchronization and Communication. Fixed Length Easy to implement Minimizes processing and storage overhead. Variable Length Requires dynamic memory allocation, so fragmentation could occur.

Basic Communication Primitives Two generic message passing primitives for sending and receiving messages. send (destination, message) receive (source, message) source or dest={ process name, link, mailbox, port} Addressing - Direct and Indirect 1 ) Direct Send/ Receive communication primitives Communication entities can be addressed by process names (global process identifiers) Global Process Identifier can be made unique by concatenating the network host address with the locally generated process id. This scheme implies that only one direct logical communication path exists between any pair of sending and receiving processes. Symmetric Addressing : Both the processes have to explicitly name in the communication primitives. Asymmetric Addressing : Only sender needs to indicate the recipient.

2) Indirect Send/ Receive communication primitives Messages are not sent directly from sender to receiver, but sent to shared data structure. Multiple clients might request services from one of multiple servers. We use mail boxes. Abstraction of a finite size FIFO queue maintained by kernel.

Synchronization and Buffering These are the three typical combinations. 1) Blocking Send, Blocking Receive Both receiver and sender are blocked until the message is delivered. (provides tight synchronization between processes) 2) Non Blocking Send, Blocking Receive Sender can continue the execution after sending a message, the receiver is blocked until message arrives. (most useful combination) 3) Non Blocking Send, Non Blocking Receive Neither party waits.

Message Synchronization Stages Sender source network destination receiver 1 2 message 3 4 request 8 7 ack 6 5 reply Message passing depends on Synchronization at several points. When sending a message to remote destination, the message is passed to sender system kernel which transmits it to communication network. Non blocking Send 1+8 Sender process is released after message has been composed and copied into senders kernel. Blocking Send Sender process is released after message has been transmitted to Network. Reliable Blocking Send Released after message has been received by kernel. Explicit Blocking Send Sender process is released after Message has been received by receiver process. Request & Reply 1-4 service 5-8 Released after message has been processed by the receiver and response returned to the sender.

Message Design Issues Synchronization Blocking vs. Non-blocking Addressing Direct Indirect Message transmission Through value Through reference Format Content Length Fixed Variable Queuing discipline FIFO Priority

The Producer Consumer Problem The producer-consumer problem illustrates the need for synchronization in systems where many processes share a resource. In the problem, two processes share a fixed-size buffer. One process produces information and puts it in the buffer, while the other process consumes information from the buffer. These processes do not take turns accessing the buffer, they both work concurrently. Herein lies the problem. What happens if the producer tries to put an item into a full buffer? What happens if the consumer tries to take an item from an empty buffer?

Producer

Consumer

Pipe & Socket API’s More convenient to the users and to the system if the communication is achieved through a well defined set of standard API’s. Pipe Pipes are implemented with finite size, FIFO byte stream buffer maintained by the kernel. Used by 2 communicating processes, a pipe serves as unidirectional communication link so that one process can write data into tail end of pipe while another process may read from head end of the pipe. Pipe is created by a system call which returns 2 file descriptors, one for reading and another for writing. Pipe concept can be extended to include messages. For unrelated processes, there is need to uniquely identify a pipe since pipe descriptors cannot be shared. So concept of Named pipes. With a unique path name, named pipes can be shared among disjoint processes across different machines with a common file system.

SOCKETS A Socket is a communication end point of a communication link managed by the transport services. It is not feasible to name a communication channel across different domains. A Communication channel can be visualized as a pair of 2 communication endpoints. Sockets have become most popular message passing API. Most recent version of the Windows Socket which is developed by WinSock Standard Group which has 32 companies (including Microsoft) also includes a SSL (Secure Socket Layer) in the specification. The goal of SSL is to provide: Privacy in socket communication by using symmetric cryptographic data encryption. Integrity in socket data by using message integrity check. Authenticity of servers and clients by using asymmetric public key cryptography.

References Operating System Concepts, Silberschatz, Galvin and Gange 2002 Sameer Ajmani ``Automatic Software Upgrades for Distributed Systems'' Ph.D. dissertation, MIT, Sep Sameer Ajmani MIT Message passing information from The University of Edinburgh MPI-2: standards beyond the message-passing model Lusk, E.; Massively Parallel Programming Models, Proceedings. Third Working Conference on Nov Page(s): Digital Object Identifier /MPPM Massively Parallel Programming Models, Proceedings. Third Working Conference on A. N. Bessani, M. Correia, J. S. Fraga, and L. C. Lung. Sharing memory between Byzantine processes using policy-enforced tuple spaces. In Proceedings of the 26th International Conference on Distributed Computing Systems, July 2006 A multithreaded message-passing system for high performance distributed computing applications Park, S.-Y.; Lee, J.; Hariri, S.; Distributed Computing Systems, Proceedings. 18th International Conference on May 1998 Page(s): Digital Object Identifier /ICDCS Distributed Computing Systems, Proceedings. 18th International Conference on A message passing standard for MPP and workstations J. J. Dongarra, S. W. Otto, M. Snir, and D. Walker, CACM, 39(7), 1996, pp A message passing standard for MPP and workstations N. Alon, M. Merrit, O. Reingold, G. Taubenfeld, and R. Wright. Tight bounds for shared memory systems acessed by Byzantine processes. Distributed Computing, 18(2):99–109, 2005

References Lessons for massively parallel applications on message passing computers Fox, G.C.; Compcon Spring '92. Thirty-Seventh IEEE Computer Society International Conference, Digest of Papers Feb Page(s): Digital Object Identifier /CMPCON Compcon Spring '92. Thirty-Seventh IEEE Computer Society International Conference, Digest of Papers. An analysis of message passing systems for distributed memory computers Clematis, A.; Tavani, O.; Parallel and Distributed Processing, Proceedings. Euromicro Workshop on Jan Page(s): Digital Object Identifier /EMPDP Parallel and Distributed Processing, Proceedings. Euromicro Workshop on An analysis of message passing systems for distributed memory computers Clematis, A.; Tavani, O.; Parallel and Distributed Processing, Proceedings. Euromicro Workshop on Jan Page(s): Digital Object Identifier /EMPDP Parallel and Distributed Processing, Proceedings. Euromicro Workshop on

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