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Chapter 3-Process in distributed system.ppt
- 2. Outlines Introduction to Threads Threads in Distributed Systems Clients Servers Code Migration 3/30/2024 2
- 3. Process is a program that is currently being executed on one of the operating system's virtual processors. The management and scheduling of processes are perhaps the most important issues to deal with. Virtualization allows to run concurrently with other applications, but highly independent of the underlying hardware and platforms leads to a high degree of portability. 3/30/2024 3 Definition
- 4. Moreover, virtualization helps in isolating failures caused by errors or security problems. Process migration or code migration, can help in achieving scalability, but can also help to dynamically configure clients and servers. 3/30/2024 4 Definition
- 5. A thread refers to a sequence of instructions that can be executed independently within a process. It is the smallest unit of execution within a program and represents a single flow of control. Thread is building block in distributed systems, lightweight process that can be managed by scheduler. To execute a program, an operating system creates a number of virtual processors, each one for running a different program. 3/30/2024 5
- 6. To keep track of these virtual processors, OS has a process table, containing entries to store CPU register values, memory maps, open files, accounting information. privileges, etc. Multiple processes may be concurrently sharing the same CPU and other hardware resources are made transparent. When a process is created, the operating system must create a complete independent address space. Like a process, a thread executes its own piece of code, independently from other threads. But no attempt is made to achieve a high degree of concurrency transparency if this would result in performance degradation. 3/30/2024 6
- 7. A thread system generally maintains only the minimum information to allow a CPU to be shared by several threads it refers to multithreading. Multithreading leads to high performance. Multithreading is that it becomes possible to exploit parallelism when executing the program on a multiprocessor system. A collection of cooperating programs, each to be executed by a separate process. Mean that a process contain many thread. 3/30/2024 7
- 8. ◦ Process Thread Program is in execution Segmentation of process Not light weight Light weight Isolated/no shared memory Share memory Not share data Share data each other Long time to communication Short time to communication Processes between threads Independent each other Dependant each other 3/30/2024 8
- 9. Threads can provide a convenient means of allowing blocking system calls without blocking the entire process in which the thread is running. This property makes threads particularly attractive to use in distributed systems as it makes it much easier to express communication in the form of maintaining multiple logical connections at the same time. It is good to illustrate this point by taking a closer look at multithreaded clients and servers, respectively. 3/30/2024 9
- 10. To establish a high degree of distribution transparency, distributed systems that operate in WAN may need to conceal long inter-process message propagation times. To hide communication latencies as much as possible, some browsers start displaying data while it is still coming in. Web browser is doing a number of tasks simultaneously. As soon as the main HTML file has been fetched, separate threads can be activated to take care of fetching the other parts. Each thread sets up a separate connection to the server and pulls in the data. 3/30/2024 10
- 11. The main use of multithreading in distributed systems found at the server side. Also makes it much easier to develop servers that exploit parallelism to attain high performance, even on uniprocessor systems. Multithreading for parallelism is even more useful. The file server normally waits for an incoming request for a file operation, subsequently carries out the request, and then sends back the reply. 3/30/2024 11
- 12. Here one thread, the dispatcher, reads incoming requests for a file operation. The requests are sent by clients to a well-known end point for this server. After examining the request, the server chooses an idle (i.e., blocked) worker thread and hands it the request. The worker proceeds by performing a blocking read on the local file system, which may cause the thread to be suspended until the data are fetched from disk. If the thread is suspended, another thread is selected to be executed. 3/30/2024 12
- 13. A multithreaded server organized in a dispatcher/worker mode. 3/30/2024 13
- 14. Three ways to construct a server. 3/30/2024 14
- 15. By rapidly switching between threads and processes, the illusion of parallelism is created(executed simultaneously ). It is impossible in single processor. This separation between having a single CPU and being able to pretend there can be extended to other resources as well, leading to what is known as resource virtualization. The Role of Virtualization in Distributed Systems is virtualization deals with extending or replacing an existing interface so as to mimic the behaviour of another system. 3/30/2024 15
- 16. (a) General organization between a program, interface, and system. (b) General organization of virtualizing system A on top of system B. 3/30/2024 16
- 17. Computer systems generally offer four different types of interfaces, at four different levels The essence of virtualization is to mimic the behaviour of these interfaces 3/30/2024 17
- 18. Virtualization can take place in two different ways. (a) A process virtual machine with multiple instances of (application, runtime) combinations. (b) A virtual machine monitor with multiple instances of (applications, operating system) combinations. 3/30/2024 18
- 19. A major task of client machines is to provide the means for users to interact with remote servers. There are roughly two ways in which this interaction can be supported. First, for each remote service, the client machine will have a separate counterpart that can contact the service over the network. A second solution is to provide direct access to remote services by only offering a convenient user interface. 3/30/2024 19
- 20. In many cases, parts of the processing and data level in a client-server application are executed on the client side as well. A special class is formed by embedded client software, such as for ATMs, cash registers, barcode readers, TV set-top boxes, etc. In these cases, the user interface is a relatively small part of the client software, in contrast to the local processing and communication facilities. Client software comprises components for achieving distribution transparency. Access transparency Location, migration Relocation transparency Replication transparency 3/30/2024 20
- 21. General Design Issues A server is a process implementing a specific service on behalf of a collection of clients. In essence, each server is organized in the same way: it waits for an incoming request from a client and subsequently ensures that the request is taken care of, after which it waits for the next incoming request. A multithreaded server is an example of a concurrent server. The place where clients contact a server in all cases, clients send requests to an end point, also called a port, at the machine where the server is running. 3/30/2024 21
- 22. A server cluster is nothing else but a collection of machines connected through a network, where each machine runs one or more servers. Connected through a LAN, often offering high bandwidth and low latency. Logically organized into three tiers. The general organization of a three-tiered server cluster. 3/30/2024 22
- 23. Situations in which passing programs, sometimes even while they are being executed, simplifies the design of a distributed system. Moving a running process to a different machine is a costly and intricate/complex task. There had better be a good reason for doing so that reason has always been performance. 3/30/2024 23
- 25. Quiz 1. Discus at least two goal of distrbuted system. 2. Define distributed pervasive system and write at least three example. 3/30/2024 25