Principles and Paradigms Second Edition MAARTEN VAN STEEN. Processes

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1 DISTRIBUTED SYSTEMS Principles and Paradigms Second Edition ANDREW S. TANENBAUM MAARTEN VAN STEEN Chapter 3 Processes 1

2 Processes How the different types of processes play a crucial role in distributed systems. Process : a program in execution. From an O.S. perspective, the management and scheduling of scheduling of processes are perhaps the most important issues to deal with. From an D.S. To efficiently organize client-server er systems, stems convenient to use multithreading. Using the concept of virtualization. The design issue of servers. Moving processes between different machines. Process migration or code migration. 2

3 Threads What a process is. How processes and threads relate. Process Table. Independent Processes. Concurrency Transparency. Switching between CPU Context. Memory Management Unit (MMU). Translation Lookaside Buffer (TLB). Swap Processes. Thread Context. Multithreading. 3

4 Thread Usage in Nondistributed Systems. Several benefits to multithreading gprocesses. In single-thread process, whenever a blocking system call is executed, the process as a whole is blocked. Exploit Parallelism Large Application Designed as a collection of cooperating programs. Large Context Switching. IPC in UNIX Different Parts executed by separate threads. Shared Data. Thread switching user space. Software engineering g -- structure as a collection of cooperating. 4

5 Thread Usage in Nondistributed Systems Figure 3-1. Context switching as the result of IPC. 5

6 Thread Implementation Thread Advantages Package Two approaches Construct a tread library in user mode. Kernel be aware of threads and schedule them. User-level thread library Cheap to Create and Destroy threads. Switching context can be done in few instructions. Disadvantages Invocation of blocking system call will immediately block the entire process. 6

7 Thread Implementation Kernel- High price to pay. level Threads Every thread operation carried out by kernel requiring ii a system call. Switching contexts may now become as expensive as switching process context. Solution: Hybrid form of user-level and kernel-level threads. Lightweight Processes (LWP) Advantages. Drawback. 7

8 Thread Implementation Figure 3-2. Combining kernel-level lightweight processes and user-level threads. 8

9 Threads in Distributed Systems 9

10 Multithreaded Clients Conceal latencies in communication. Initiate communication and do something else. Web Browsers. 10

11 Multithreaded Servers Exploit parallelism to attain high performance. Example file server. Dispatcher. Worker thread. 11

12 Multithreaded Servers (1) Figure 3-3. A multithreaded server organized in a dispatcher/worker model. 12

13 Multithreaded Servers (2) Figure 3-4. Three ways to construct a server. 13

14 Virtualization Single-processor illusion. Rapidly switching between threads Create illusion of parallelism. Leading to what is known as Resource Virtualization. We will discuss Virtualization. How it can be realized. 14

15 The Role of Virtualization in Distributed Systems Every (distributed) computer system offer an interface. Basic instruction set offered by CPU. Virtualization i deals with extending or replacing an existing i interface so as to mimic i the behavior of another system. Most important reason is to run legacy software. Become less of an issue. Then 1990 s became more favored for several reasons: Hardware and low-level system software change fast. Legacy cannot be maintained same pace. Virtualization can help by porting the legacy interface to the new platforms. Networking large Heterogeneous collection of server computers, running different application run each on different Virtual machine. 15

16 The Role of Virtualization in Distributed Systems Figure 3-5. (a) General organization between a program, interface, and system. (b) General organization of virtualizing system A on top of system B. 16

17 Architectures of Virtual Machines (1) Interfaces at different levels An interface between the hardware and software consisting of machine instructions that can be invoked by any program. An interface between the hardware and software, consisting of machine instructions that can be invoked only by privileged programs, such as an operating system. 17

18 Architectures of Virtual Machines (2) Interfaces at different levels An interface consisting of system calls as offered by an operating system. An interface consisting of library calls generally forming what is known as an application programming interface (API). In many cases, the aforementioned system calls are hidden by an API. 18

19 Architectures of Virtual Machines (3) Figure 3-6. Various interfaces offered by computer systems. 19

20 Architectures of Virtual Machines (4) Figure 3-7. (a) A process virtual machine, with multiple instances of (application, runtime) combinations. 20

21 Architectures of Virtual Machines (5) Figure 3-7. (b) A virtual machine monitor, with multiple instances of (applications, operating system) combinations. 21

22 CLIENTS 22

23 Networked User Interfaces (1) Figure 3-8. (a) A networked application with its own protocol. 23

24 Networked User Interfaces (2) Figure 3-8. (b) A general solution to allow access to remote applications 24

25 Example: The X Window System Figure 3-9. The basic organization of the X Window System. 25

26 Client-Side Software for Distribution Transparency Figure Transparent replication of a server using a client-side solution. 26

27 Servers 27

28 General Design Issues (1) Organize Servers. ers Iterative Server. Concurrent Server. How clients contact a server End point port Using daemon. Superserver. How and whether a server can bee interrupted. Abruptly exit break connection. Out-of-band. 28

29 General Design Issues (2) Whether or not the server is stateless t Stateless Server: does not keep information on the state of its clients and can change its own state without having to inform any ckient. Stateful Server: maintains persistent information on its clients. Session state Permanent state. 29

30 General Design Issues (3) Figure (a) Client-to-server to binding using a daemon. 30

31 General Design Issues (4) Figure (b) Client-to-server to binding using a superserver. 31

32 Sever Clusters General Organization: Collection of machines connected through a network. Logically organized into three tires: First tier: consists of a (logical) switch through which clients requests are routed. Second tier: contain servers dedicated to application processing. Third tier: consists of data-processing servers, notably file and database servers. 32

33 Server Clusters (1) Figure The general organization of a three-tiered server cluster. 33

34 Server Clusters (2) Figure The principle of TCP handoff. 34

35 Distributed Servers Figure Route optimization in a distributed server. 35

36 Managing g Server Clusters Example: PlanetLab Figure The basic organization of a PlanetLab node. 36

37 PlanetLab (1) PlanetLab management issues: Nodes belong to different organizations. Each organization should be allowed to specify who is allowed to run applications on their nodes, And restrict resource usage appropriately. Monitoring tools available ailable assume a very specific combination of hardware and software. All tailored to be used within a single organization. Programs from different slices but running on the same node should not interfere with each other. 37

38 PlanetLab (2) Figure The management relationships between various PlanetLab entities. 38

39 PlanetLab (3) Relationships between PlanetLab entities: A node owner puts its node under the regime of a management authority, possibly restricting usage where appropriate. A management authority provides the necessary software to add a node to PlanetLab. A service provider registers itself with a management authority, trusting it to provide wellbehaving nodes. 39

40 PlanetLab (4) Relationships between PlanetLab entities: A service provider contacts t a slice authority to create a slice on a collection of nodes. The slice authority needs to authenticate the service provider. A node owner provides a slice creation service for a slice authority to create slices. It essentially delegates resource management to the slice authority. A management authority delegates the creation of slices to a slice authority. 40

41 Code Migration 41

42 Reasons for Migrating g Code Figure The principle of dynamically configuring a client to communicate to a server. The client first fetches the necessary software, and then invokes the server. 42

43 Models for Code Migration Code Segments. Process consist of three segments: Resource Segments. Execution Segments. 43

44 Models for Code Migration Figure Alternatives for code migration. 44

45 Migration and Local Resources Figure Actions to be taken with respect to the references to local resources when migrating code to another machine. 45

46 Migration in Heterogeneous Systems Three ways to handle migration (which can be combined) Pushing memory pages to the new machine and resending the ones that are later modified during the migration process. Stopping the current virtual machine; migrate memory, and start the new virtual machine. Letting the new virtual machine pull in new pages as needed, that is, let processes start on the new virtual machine immediately and copy memory pages on demand. 46

Code and Process Migration! Motivation!

Code and Process Migration! Motivation How does migration occur? Resource migration Agent-based system Details of process migration Lecture 6, page 1 Motivation! Key reasons: performance and flexibility