CS 162 Spring 2003 Lecture 3 1/7 CS 162 Operating Systems and Systems Programming Professor: Anthony Joseph Spring 2003 Lecture 3: Concurrency: Processes, Threads, and Address Spaces 3.0 Main point: What are processes? How are they related to threads and address spaces? 3.1 Concurrency 3.1.1 Definitions: Uniprogramming: one process at a time (e.g., MS/DOS, Macintosh) Easier for operating system builder: get rid of problem of concurrency by defining it away. For personal computers, idea was: one user does only one thing at a time. Harder for user: can’t work while waiting for printer Multiprogramming: more than one process at a time (UNIX, OS/2, Windows NT). Note: This is often called multitasking, but multitasking sometimes has other meanings – see below – so not used in this course. 3.1.2 The basic problem of concurrency: • Hardware: single CPU, I/O interrupts. • API: users think they have machine to themselves. OS has to coordinate all the activity on a machine: multiple users, I/O interrupts, etc. How can it keep all these things straight? CS 162 Spring 2003 Lecture 3 2/7 Answer: Decompose hard problem into simpler ones. Instead of dealing with everything going on at once, separate into logical abstractions that we can deal with one at a time. 3.2 Processes The notion of a “process” is a central concept for Operating Systems. Process: Operating system abstraction to represent what is needed to run a single program (this is the traditional UNIX definition) Formally, a process is a sequential stream of execution in its own address space. 3.2.1 Two parts to a (traditional Unix) process: 1. Sequential program execution: the code in the process is executed as a single, sequential stream of execution (no concurrency inside a process). This is known as a thread of control. 2. State Information: everything specific to a particular execution of a program: Encapsulates protection: address space • CPU registers • Main memory (contents of address space) • I/O state (in UNIX this is represented by file descriptors)CS 162 Spring 2003 Lecture 3 3/7 3.2.2 Process =? Program A program is, for example, a set of C statements or commands (vi, ls) main(){ ... } A() { ... }PROGRAMmain(){ ... } A() { ... }PROCESSmainAregisters, PCheapstack 1. More to a process than just a program: • Program is just part of process state. • I run emacs on lecture.txt, you run emacs on homework.java – same program, different processes. 2. Less to a process than a program: • A program can invoke more than one process to get the job done • cc starts up cpp, cc1, cc2, as, ld (each are programs themselves) CS 162 Spring 2003 Lecture 3 4/7 3.3 Multiple Threads of Control The traditional notion of a Process can be extended to allow for additional concurrency: Thread: a sequential execution stream within a process (concurrency) (Sometimes called: a “lightweight” process.). Provides the illusion that each activity (or thread) is running on its own CPU, entirely sequentially. Address space: all the state needed to run a program (literally, all the addresses that can be touched by the program). Provides the illusion that a program is running on its own machine (protection). 3.3.1 Why separate the concept of a thread from that of a process? 1. Discuss the “thread” part of a process (concurrency), separately from the “address space” part of a process (protection). 2. Many situations where you want multiple threads per address space. Question: Why would you want this? Multithreading: a single program made up of a number of different concurrent activities (sometimes called multitasking, as in Ada, just to be confusing!) 3.3.2 Examples of multithreaded programs 1. Embedded systems: elevators, planes, medical systems, wristwatches, etc. Single program, concurrent operations. 2. Most modern OS kernels: internally concurrent because have to deal with concurrent requests by multiple users. But no protection needed within kernel. 3. Database Server: provides access to shared data by potentially many concurrent users. Also has background utility processing that must get done.CS 162 Spring 2003 Lecture 3 5/7 4. Network servers: user applications that get multiple requests concurrently off the network. Again, single program, multiple concurrent operations (examples: file servers, Web server, and airline reservation systems) 5. Parallel programming: split program into multiple threads to make it run faster. This is called multiprocessing. Multiprogramming = multiple jobs or processes Multiprocessing = multiple CPUs Some multiprocessors are in fact uniprogrammed – multiple threads in one address space, but only run one program at a time. 3.3.3 Thread State What state does a thread have? Some state shared by all threads in a process/address space: • Contents of memory (global variables, heap) • I/O state (file system) Some state “private” to each thread – each thread has its own copy • CPU registers (including, program counter) • Execution stack – what is this? Execution stack: where parameters, temporary variables, and return PC are kept, while called procedures are executing (for example, where are A’s variables kept, while B, C are executing?) CS 162 Spring 2003 Lecture 3 6/7 A(int tmp) { if ( tmp< 2) B(); printf(tmp); } B() { C(); } C() { A(2); } A; tmp = 1 B C A; tmp = 2 Execution stack A(1); Threads encapsulate concurrency; address spaces encapsulate protection: Keeps a buggy program from trashing everything else on the system. Address state is passive; thread is active 3.4 Classification Real operating systems have either • One or many address spaces • One or many threads per address space # of address spaces: # of threads per address space: one many One MS/DOS, Macintosh traditional UNIX Many embedded systems JavaOS, Pilot (PC) Mach, OS/2 Windows 95, Windows NT, Solaris, Linux, HP-UX, ... Examples: 1. MS/DOS – one thread, one address space 2. Traditional UNIX – one thread per address space, many address spacesCS 162 Spring 2003 Lecture 3 7/7 3. Mach, Microsoft NT, new UNIX (Linux, Solaris, HPUX) – many threads per address space, many address spaces 4. Embedded systems (Geoworks, VxWorks, JavaOS, etc.). Also, Pilot (the operating system on the first personal computer ever built) – many threads, one
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