Chapter 3 Processes

1. Chapter 3Processes Dowon Cho (RTMM Lab) & Jongwon Lee (SE Lab)

2. Index • Threads • Clients • Servers

3. Processes Process Control Block (PCB)

4. Processes (cont.) • Process • Program in execution • OS creates an extended machine multiple virtual CPUs • OS keeps track of all processes (process tables) • Processes are independent of one another • OS protects processes from one another (with h/w support)

5. Processes (cont.) • Process allow the OS to overlap I/O and computation, creating an efficient system • Overhead • Process creation • Context switching • Swapping • All required kernel intervention

6. Threads Thread Concept

7. Threads (cont.) • Threads and Processes • Threads are not protected from each other • Require care from developers • Context switching is cheaper • System call • Process must be blocked • Thread may still proceed

8. Threads (cont.) • Thread is executed entirely in user mode • Cheap to manage threads • Create: setup a stack • Destroy: free up memory • Context switch requires few instructions • Save CPU registers • Execution based on program logic

9. Threads (cont.) • Kernel is aware of and schedules threads • It is expensive to manage threads • It is expensive to context switch • Almost expensive as using processes

10. Threads (cont.) • Light-Weight Processes (LWP) • Support for hybrid (user-level and Kernel) threads • A process contains several LWPs • Developer: creates multi-threaded application • System: Maps threads to LWPs for execution

11. Threads (cont.) Combining kernel-level lightweight processes and user-level threads.

12. Threads (cont.) • Each LWP offers a virtual CPU • LWPs are created by system calls • Thread table is kept in user space • Thread table is shared by all LWPs • LWPs switch context between threads

13. Threads (cont.) • When a thread blocks, LWP schedules another ready thread • Thread context switch is completely done in user-address space • When a thread blocks on a system call, execution mode changes from user to kernel but continues in the context of the current LWP • When current LWP can no longer execute, context is switched to another LWP

14. Threads (cont.) • LWP advantages • Cheap thread management • A blocking system call may not suspend the whole process • LWPs are transparent to the application • LWPs can be easily mapped to different CPUs

15. Clients • Main role of clients • To interact with human user and remote server • Client side software • User Interface (UI) + components for achieving transparent

16. Clients (cont.) • X-window system

17. Clients (cont.) • Goal • Generally used to control bit-mapped terminals • X-kernels • Contains all terminal-specific devices • Offer relative low-level interface for controlling the screen • Capturing events from the keyboard and mouse • 4 major components • X servers: Managing the screen, mouse and keyboard • X client: the application (where the real work gets done) • X protocol: Client/server communication • X library: the programming interface

18. Servers • Design issues • How to organize the server • How client contacts with server • Whether and how a server can be interrupted • Whether a stateful or stateless server

19. Servers (cont.) • How to organize the server • Iterative server (sequential server) • Itself handle the request and return a response to the request. • Concurrent server • It does not handle the request but passes it into threads

20. Servers (cont.) • How client contacts with server • Clients sent the request to the endpoint (port) of the server • Approach 1 • Endpoint is assigned with the well-know service • Approach 2 • Not pre-assigned endpoint • Solution 1: Each server has a special daemon to keep tracks of the current endpoint of each service • Solution 2: using a single super server

21. Servers (cont.) • Client-to-server binding using a daemon as in DCE • Client-to-server binding using a superserver as in UNIX

22. Servers (cont.) • Whether and how a server can be interrupted • Approach 1 • Client suddenly exits the application and restarts immediately • Server thinks that the client had crashed and will tear down the connection • Approach 2 • Send out-of-band data (which is processed by the server before any other client data) • Solution 1 • Out-of-band data is listened by separated endpoint • Solution 2 • Out-of-band data is sent across the same connection with urgent

23. Servers (cont.) • Whether a stateful or stateless server • Stateful server • It maintains information on its clients (ex. File server) • Stateless server • It does not keep information on the state of its client • It can change its own state without having to inform any client

24. Servers (cont.) • Object Server • Tailored for distributed objects support • Provides environment for objects • Not a service by itself • Services are provided by objects • Easy to add services (by adding objects)

25. Servers (cont.) Object server concept Object server Object Object Object Code (methods) Data Object

26. Servers (cont.) • Activation Policies • Decisions on how to invoke objects • All objects are alike • Inflexible • Objects differ and require different policies • Object type • Memory allocation • Threading

27. Servers (cont.) • Each object has its own memory segment • Objects can share memory segments • Security? • Memory resources?

28. Servers (cont.) • One thread in the server • Several threads in the server • Separate thread for each object • Separate thread for each request • Concurrent access?

29. Servers (cont.) • Object Adapters (wrappers) • A S/W implementation of an activation policy • Generic to support developers • Group objects per policy • Several objects under an adapter • Several adapters under a server

30. Servers (cont.) • Object Adapters (wrappers)

31. Part 4. Code Migration

32. Code Migration • Code Migration = moving processes between machines • Process = 3 segments - Code segment (the code) : is the part that contains the set of instructions that make up the program - Resource segment (references to external resources {files, printers, devices, other processes, etc}) - Execution segment (current execution state of a process {stack, program counter, registers, private data…})

33. Why migrate code? (1) • Main Reason: • Better performance of overall system If processes are moved from heavily-loaded to lightly-loaded machines, overall system performance can be improved.

34. Why migrate code? (2) • Load-Balancing (for multiprocessors) - Move process from over-utilized to under-utilized CPU - Load distribution algorithms by which decisions are made concerning the allocation and redistribution of tasks with respect to a set of processors • Minimizing Communication Costs • Exploiting parallelism - Examples: - client application needs to do many database operations involving large quantities of data => ship part of the client application to the server => send only the results across the network. - Interactive database applications: client needs to fill in forms => ship part of the server application to the client - Searching information in the Web by a small mobile prog. => make some copies of such a prog. => send each off to different sites

35. Why migrate code? (3) Besides improving performance: Flexibility Clients need not have all the S/W pre-installed: download on demand, then dispose. - Remote object stub in Java - Java applets Security: Can you trust any code? (both client & server) Need a standardized protocol for downloading & initializing code Guarantee that downloaded code can be executed

36. Require the protocol for downloading and initializing code Can be executed on the Client’s machine Scenario of Migrating Code • The principle of dynamically configuring a client to communicate to a server. • The client first fetches the necessary software, and then invokes the server.

37. There is no need to start separate process, thereby avoiding Communication at the target machine. Protected against malicious or inadvertent code execution Transfer only the Code segment =>simplicity Transferred program only started from its initial state. e.g.Java applet. Initiated at the machine where the code currently resides or is being executed, Uploading program to compute server. e.g, Search program Require registration & authentication The initiative for code migration is taken by target machine. e.g, Java applet Execution segment can be transferred as well (running process can be stopped & moved to another machine to resume execution). Models for Code Migration • Alternatives for code migration.

38. Unattached resources: can easily moved between different machines, typically (data) files associated only with the program that is to be migrated. Fastened resources: e.g, local databases, complete Websites (moveable but high costs) Fixed resources: local devices Binding by identifier: e.g, uses a URL for Website or IP address for FTP server. Binding by value: e.g, C or Java standard libraries Binding by type: local devices such as monitors, printers, Migration and Local Resources • Actions to be taken with respect to the references to local resources • when migrating code to another machine. Resource-to machine binding Process-to-resource binding GR: Establish a global systemwide reference MV: Move the Resource CP: Copy the value of the resource RB: Rebind process to locally available resource

39. Migration in Heterogeneous Systems • Requirements: - Platform supported: code segment may be recompiled to be executed in new platform - Ensure the execution segment can be represented at each platform properly. Solutions: - Weak mobility: no runtime information needs to be transferred => just recompile the source code - Strong mobility: segment is highly dependent on the platform => migration stack: a copy of the program stack in a machine-independent way.(C, Java)

40. Migration in Heterogeneous Systems Scenario: - Migration can take place only when a next subroutine is called. - When subroutine is called: marshalled data are pushed onto the migration stack along with identifier for the called subroutine & the return label - Then all global program-specific data are also marshalled (current stack & machine-specific data are ignored)

41. Migration in Heterogeneous Systems • The principle of maintaining a migration stack to support migration of an execution segment in a heterogeneous environment end

42. Part 5. Software Agents

43. Software Agents • Definition: • Is as an autonomous process capable ofreactingto, andchangesin, its environment, possibly incollaborationwith users and other agents

44. Software Agents • Taxonomy: Collaborative Agent : an agent that forms part of a multiagent system, in which agents seek to achieve some common goal through collaboration. e.g Cyber-meeting system Mobile Agent : simply an agent having the capability to move between different machines.

45. Software Agents • Taxonomy: Interface Agent : assist an end user in the use of one or more application. Information Agent : manage information from many different sources. ordering, filtering, collating information. e.g e-mail agent

46. Software Agents in Distributed Systems • Some important properties by which different types of agents can be distinguished.

47. Directory Service : describe of agent’s service by attributes Management Component : provides the facilities for creating and deleting agents Agent Communication Channel : take care of of all communication between different agent platform Agent Technology • The general model of an agent platform (adapted from [fipa98-mgt]).

48. Agent Communication Languages • Definition: • Communication between agents takes place by means • of an application-level communication protocol which is • referred to as anagent communication language.

49. Agent Communication Languages • Components of a message

50. Agent Communication Languages • Examples of different message types in the FIPA ACL [fipa98-acl], • giving the purpose of a message, along with the description of the actual message content.