0. Course Overview. 8 Architecture models; network architectures: OSI, Internet and LANs; interprocess communication III. Time and Global States

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1 0. Course Overview I. Introduction II. Fundamental Concepts of Distributed Systems 8 Architecture models; network architectures: OSI, Internet and LANs; interprocess communication III. Time and Global States 8 Clocks and concepts of time; Event ordering; Synchronization; Global states IV. Coordination 8 Distributed mutual exclusion; Multicast; Group communication, Byzantine problems (consensus) V. Distribution and Operating Systems 8 Protection mechanisms; Processes and threads; Networked OS; Distributed and Network File Systems (NFSs) VI. Middleware 8 Middleware; Distributed object models; Remote invocation; CORBA; Name and directory services VII. Security 8 Security concepts; Cryptographic algorithms; Digital signatures; Authentication; Secure Sockets (VIII. Distributed Database and Memory Systems) Distributed Systems - Fall 2001 V - 1 Stefan Leue 2002

2 Operating Systems for Distributed Systems Network Operating System 8 one instance running per computer in a network 8 operating system manages local resources as well as access to network infrastucture network file systems rlogin telnet 8 examples Windows (98/NT/ ) Unix (Solaris,...) Linux Distributed Operating System 8 single image system complete transparency for the user where programs run OS has control over all nodes in system 8 not practically in use compatability with existing applications emulations offer very bad performance 8 example Amoeba (Tannenbaum et al.) Distributed Systems - Fall 2001 V - 2 Stefan Leue 2002

3 Operating Systems for Distributed Systems General Architecture in Practical Use 8 network operating system + middleware layer Applications, services Middleware OS: kernel, libraries & servers OS1 Processes, threads, communication,... OS2 Processes, threads, communication,... Platform Computer & network hardware Computer & network hardware Node 1 Node 2 Distributed Systems - Fall 2001 V - 3 Stefan Leue 2002

4 Operating Systems for Distributed Systems Desiderata for Network Operating Systems 8 provide support for middleware layer to work effectively encapsulation iprovide transparent service interface to resources of the computer protection iprotect resources from illigetimate access concurrent processing iusers/clients may share resources and access concurrently Distributed Systems - Fall 2001 V - 4 Stefan Leue 2002

5 Operating Systems for Distributed Systems Process 8 software in execution 8 unit of resource management for operating system execution environment iaddress space ithread synchronization and commnication resources (e.g., semaphores, sockets) icomputing resources (file systems, windows, etc.) threads ischedulable activities attached to processes iarise from the need for concurrent activities sharing resouces within one process * concurrent input/output with problem computation * servers: concurrent processing of client requests, each request handled by one thread 8 processes vs. threads threads are lightweight processes iprocesses expensive to create, threads easier to create and destroy 8 process instantiation one thread will be instantiated as well, may instantiate offsprings Distributed Systems - Fall 2001 V - 5 Stefan Leue 2002

6 Operating Systems for Distributed Systems Process manager Communication manager Thread manager Memory manager Supervisor Core Operating System Functionality 8 process manager maintains processes (creation, termination) 8 thread manager creation, synchronization and scheduling 8 communication manager communication between threads iin different processes ion different computers Distributed Systems - Fall 2001 V - 6 Stefan Leue 2002

7 Operating Systems for Distributed Systems Process manager Communication manager Thread manager Memory manager Supervisor Core Operating System Functionality 8 memory manager management of physical and virtual memory 8 supervisor dispatching of interrupts, system call traps and exceptions control of memory management unit and hardware caches processor and floating point unit register manipulations Distributed Systems - Fall 2001 V - 7 Stefan Leue 2002

8 Protection Mechanisms Protection against illegitimate access 8 clients performing operations need to have right to do so 8 only specified operations may be performed on an object Kernel 8 core part of operating system that has complete access rights to any resource 8 processor modes user supervisor ikernel always executes in supervisor mode some operations are only allowed in supervisor mode 8 kernel sets up address spaces to protect against illegitimate memory accesses collection of ranges of virtual addresses (memory locations) process cannot access memory locations outside it s address space switching between processes entails switching of address spaces imay involve non-negligible amount of work, performance implications Distributed Systems - Fall 2001 V - 8 Stefan Leue 2002

9 Address Spaces 8 regions of memory accessible to threads of that process 8 subdivided into regions lowest address and length read/write/execute permissions for threads in process direction of growth 8 stack for subroutines sometimes one stack region per thread 8 text region to map files into memory 8 shared region regions of virtual memory mapped to identical physical memory for different processes enables inter-process communication 2 N 0 Auxiliary regions Stack Heap Text Distributed Systems - Fall 2001 V - 9 Stefan Leue 2002

10 Process creation 8 choice of target host (only for distributed operating systems) 8 creation of an execution environment contents of newly allocated address space iinitialized empty iinitialized as a (partial) copy of parent s address space * example: Unix fork command: child process shares text region with parent, has own copies of stack and heap (an extension allows choices which regions are shared, and which ones are inherited) icopy-on write (in Mach operating system) * inherited region initially shared * only when one process attempts to write, an interrupt handler will start copying the shared region to a new instance A s page table Shared frame B s page table a) Before write b) After write Distributed Systems - Fall 2001 V - 10 Stefan Leue 2002

11 Performance considerations: handling server requests 8 processing: 2 ms 8 IO delay (no chaching): 8 ms 8 single thread 10 ms per requests, 100 requests per second 8 two threads (no caching) 8 ms per request, 125 requests per second 8 two threads and caching 75% hit rate mean IO time per request: 0.25 * 8ms = 2ms i500 requests per second increased processing time per request: 2.5 ms i400 requests per second Distributed Systems - Fall 2001 V - 11 Stefan Leue 2002

12 Thread 1 generates results workers T1 Client Thread 2 makes requests to server Receipt & queuing Requests per-connection threads Input-output N threads Server per-object threads I/O remote objects remote objects I/O remote objects a. Thread-per-request b. Thread-per-connection c. Thread-per-object Threads and Servers 8 worker pool pool of server threads serves requests in queue possible to maintain priorities per queue 8 thread per request thread lives only for the duration of request handling maximizes throughput (no queueing) expensive overhead for thread creation and destruction Distributed Systems - Fall 2001 V - 12 Stefan Leue 2002

13 Thread 1 generates results workers T1 Client Thread 2 makes requests to server Receipt & queuing Requests per-connection threads Input-output N threads Server per-object threads I/O remote objects remote objects I/O remote objects a. Thread-per-request b. Thread-per-connection c. Thread-per-object Threads and Servers 8 thread per connection/per object compromise solution no overhead for creation/deletion of threads requests may still block, hence throughput is not maximal Distributed Systems - Fall 2001 V - 13 Stefan Leue 2002

14 Execution environment Thread Address space tables Saved processor registers Communication interfaces, open files Priority and execution state (such as BLOCKED) Semaphores, other synchronization Software interrupt handling information objects List of thread identifiers Execution environment identifier Pages of address space resident in memory; hardware cache entries Threads vs. multiple processes/execution environments 8 creating a new thread is much less expensive than creating new execution environment creating new thread: iallocate region of thread's stack and iset registers and processor status creating new execution environment icreate address space table, communication interfaces inew process starts with "empty" cache, therefore more cache misses than for new thread experiment: new process under Unix 11ms, new thread under Topaz kernel: 1 ms Distributed Systems - Fall 2001 V - 14 Stefan Leue 2002

15 Execution environment Thread Address space tables Saved processor registers Communication interfaces, open files Priority and execution state (such as BLOCKED) Semaphores, other synchronization Software interrupt handling information objects List of thread identifiers Execution environment identifier Pages of address space resident in memory; hardware cache entries Threads vs. multiple processes/execution environments 8 switching between threads more efficient than switching between processes threads ischeduling (deciding which thread to run next) icontext switching (saving processor's register state, loading new register contents) idomain transitions * if new thread is member of a different execution environment * cache misses more severe than in-domain switching iexperimental results * process switch in Unix: 1.8ms, thread switch in Topaz: 0.4 ms Distributed Systems - Fall 2001 V - 15 Stefan Leue 2002

16 Execution environment Thread Address space tables Saved processor registers Communication interfaces, open files Priority and execution state (such as BLOCKED) Semaphores, other synchronization Software interrupt handling information objects List of thread identifiers Execution environment identifier Pages of address space resident in memory; hardware cache entries Threads vs. multiple processes/execution environments 8 Easy sharing of data amongst processes in one execution environment no need for message passing communication via shared memory 8 No protection against malevolent threads one thread can access other thread's data, unless a type-safe programming language is being used Distributed Systems - Fall 2001 V - 16 Stefan Leue 2002

17 Java Thread class Thread(ThreadGroup group, Runnable target, String name) Creates a new thread in the SUSPENDED state, which will belong to group and be identified as name; the thread will execute the run() method of target. setpriority(int newpriority), getpriority() Set and return the thread s priority. run() A thread executes the run() method of its target object, if it has one, and otherwise its own run() method (Thread implements Runnable). start() Change the state of the thread from SUSPENDED to RUNNABLE. sleep(int millisecs) Cause the thread to enter the SUSPENDED state for the specified time. yield() Enter the READY state and invoke the scheduler. destroy() Destroy the thread. Distributed Systems - Fall 2001 V - 17 Stefan Leue 2002

18 Thread Groups 8 every thread belongs to one group, assigned at thread creation time 8 thread groups useful to shield various applications running in parallel on one Java Virtual machine thread in one group may not interrupt thread in another group ie.g., an application may not interrupt the windowing (AWT) thread Java Thread Synchronization 8 each thread s local variables and methods are private to it thread has own stack 8 thread does not have private copies of static (class) variables or object instance variables 8 mutual exclusion via monitor concept abstract data type first implemented in Ada in Java: synchronized keyword iany object can only be accessed through one invokation of any of its synchronized methods ian object can have synchronized and non-synchronized methods example isynchronized addto() and removefrom() methods to serialize requests in worker pool example Distributed Systems - Fall 2001 V - 18 Stefan Leue 2002

Figure 6.1 System layers

Figure 6.1 System layers Applications, services Middleware OS: kernel, libraries & servers OS1 Processes, threads, communication,... OS2 Processes, threads, communication,... Platform Computer & network