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Operating Systems 1 K. Salah Module 2.0: Processes and Threads Process Concept Trace of Processes Process Context Context Switching Threads –ULT –KLT.

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Presentation on theme: "Operating Systems 1 K. Salah Module 2.0: Processes and Threads Process Concept Trace of Processes Process Context Context Switching Threads –ULT –KLT."— Presentation transcript:

1 Operating Systems 1 K. Salah Module 2.0: Processes and Threads Process Concept Trace of Processes Process Context Context Switching Threads –ULT –KLT

2 Operating Systems 2 K. Salah Process Also called a task. Useful and Important Concept: Process = program in execution A process is not the same as a program. Program is a passive entity, whereas process is active. Process consists of an executable program, associated data, and execution context. Modern (multiprogramming) operating systems are structured around the concept of a process. Multiprogramming OS supports execution of many concurrent processes. OS issues tend to revolve around management of processes: –How are processes created/destroyed? –How to manage resource requirements of a process during its execution: cpu time, memory, I/O, communication,... ? –How to avoid interference between processes? –How to achieve cooperation and communication between processes?

3 Operating Systems 3 K. Salah Program Creation Program (say, C program) is edited It is compiled into assembly language, which may consist of several modules. Assembly language modules are assembled into machine language. External references (i.e., to procedures and data in another module) are resolved. This is called linking, which creates a load module. Load or image module is stored as a file in file system and may be executed at a later time by loading into memory to be executed.

4 Operating Systems 4 K. Salah Process creation and termination Consider a simple disk operating system (like MS-DOS, typically supports only one process at a time) User types command like “run foo” at Keyboard (I/O device driver for keyboard, screen) Command is parsed by command shell Executable program file (load module) “foo” is located on disk (file system, I/O device driver for disk) Contents are loaded into memory and control transferred ==> process comes alive! (device driver for disk, relocating loader, memory management) During execution, process may call OS to perform I/O: console, disk, printer, etc. (system call interface, I/O device drivers) When process terminates, memory is reclaimed. (memory management)

5 Operating Systems 5 K. Salah Two Types Processes can be described as either: –I/O-bound process –  spends more time doing I/O than computations  many short CPU bursts  Long I/O burst  Ex: vi –CPU-bound process  spends more time doing computations  Heavy number crunching  few very long CPU bursts  Ex: simulation

6 Operating Systems 6 K. Salah Trace of Processes

7 Operating Systems 7 K. Salah Trace of processes (cont.)

8 Operating Systems 8 K. Salah Trace of processes (cont.)

9 Operating Systems 9 K. Salah

10 Operating Systems 10 K. Salah Example If parent chooses to wait until the child executes (but not always the case).

11 Operating Systems 11 K. Salah Multiprogramming/Timesharing Systems They provide interleaved execution of several processes to give an illusion of many simultaneously executing processes. Computers can be a single-processor or multi-processor machine. The OS must keep track of the state for each active process and make sure that the correct information is properly installed when a process is given control of the CPU.

12 Operating Systems 12 K. Salah Multiprogramming (multiple processes) For each process, the O.S. maintains a data structure, called the process control block (PCB). The PCB provides a way of accessing all information relevant to a process: –This data is either contained directly in the PCB, or else the PCB contains pointers to other system tables. Processes (PCBs) are manipulated by two main components of the process subsystem in order to achieve the effects of multiprogramming: –Scheduler: determines the order by which processes will gain access to the CPU. Efficiency and fair-play are issues here. –Dispatcher: actually allocates CPU to process next in line as determined by the scheduler.

13 Operating Systems 13 K. Salah Process Context The context (or image) of a process can be described by –contents of main memory –contents of CPU registers –other info (open files, I/O in progress, etc.) Main memory -- three logically distinct regions of memory: –code region: contains executable code (typically read-only) –data region: storage area for dynamically allocated data structure, e.g., lists, trees (typically heap data structure) –stack region: run-time stack of activation records CPU registers: general registers, PC, SP, PSW, segmentation registers Other info: –open files table, status of ongoing I/O –process status (running, ready, blocked), user id,...

14 Operating Systems 14 K. Salah The Process Control Block (PCB) The PCB typically contains the following types of information: Process status (or state): new, ready to run, user running, kernel running, waiting, halted –Program counter: where in program the process is executing –CPU registers: contents of general-purpose register stack pointer, PSW, index registers –Memory Management info: segment base and limit registers, page table, location of pages on disk, process size –User ID, Group ID, Process ID, Parent PID,... –Event Descriptor: when process is in the “sleep” or waiting state –Scheduling info: process priority, size of CPU quantum, length of current CPU burst

15 Operating Systems 15 K. Salah PCB (cont.) –List of pending signals –Accounting info: process execution time, resource utilization –Real and Effective User IDs: determine various privileges allowed the process such as file access rights –More timers: record time process has spent executing in user and Kernel mode –Array indicating how process wishes to react to signals –System call info: arguments, return value, error field for current system call –Pending I/O operation info: amount of data to transfer, addr in user memory, file offset,... –Current directory and root: file system environment of process –Open file table: records files process has open

16 Operating Systems 16 K. Salah Process States & Transitions Running –User-running –Kernel-running Ready –Ready, suspend –Ready Waiting (or blocked) –Blocked –Blocked, suspend Zooming in Suspend may swap out all or part of the process. Shared regions/segments are not suspended.

17 Operating Systems 17 K. Salah How queues are implemented?

18 Operating Systems 18 K. Salah When to context switch Typically, hardware automatically saves the user PC and PSW when interrupt occurs, and takes new PC from interrupt vector. Interrupt handler may simply perform its function and then return to the same process that was interrupted (restoring the PC and PSW from the stack). Alternatively, may no longer be appropriate to resume execution of process that was running because:  process has used up its current CPU quantum  process has requested I/O and must wait for results  process has asked to be suspended (sleep) for some amount of time  a signal or error requires process be destroyed (killed)  a “higher priority” process should be given the CPU  E.g., pressing ctrl-alt-delete In such a situation, a context switch is performed to install appropriate info for running a new process.

19 Operating Systems 19 K. Salah Mechanics of a Context Switch 1copy contents of CPU registers (general-purpose, SP, PC, PSW, etc.) into a save area in the PCB of running process 2change status of running process from “running” to “waiting” (or “ready”) 3change a system variable running-process to point to the PCB of new process to run 4copy info from register save area in PCB of new process into CPU registers Note:  context switching is pure overhead and should be done as fast as possible  often hardware-assisted - special instructions for steps 1 and 4  determining new process to run accomplished by consulting scheduler queues  step 4 will start execution of new process - known as dispatching.

20 Operating Systems 20 K. Salah MULTIPROGRAMMING Through CONTEXT SWITCHING

21 Operating Systems 21 K. Salah Process Creation Parent process creates children processes, which, in turn create other processes, forming a tree of processes Resource sharing models/types: –Parent and children share all resources –Children share subset of parent’s resources –Parent and child share no resources Execution –Parent and children execute concurrently –Parent waits until children terminate UNIX examples –fork system call creates new process –exec system call used after a fork to replace the process’ memory space with a new program

22 Operating Systems 22 K. Salah C Program Forking Separate Process int main() { Pid_t pid; /* fork another process */ pid = fork(); if (pid < 0) { /* error occurred */ fprintf(stderr, "Fork Failed"); exit(-1); } else if (pid == 0) { /* child process */ execlp("/bin/ls", "ls", NULL); } else { /* parent process */ /* parent will wait for the child to complete */ wait (NULL); printf ("Child Complete"); exit(0); }

23 Operating Systems 23 K. Salah A tree of processes on a typical Solaris

24 Operating Systems 24 K. Salah Introduction to Threads Parallelism at different levels Granularity of parallelism at processor level Coarse-grain – processes Fine-grain -- threads Multitasking OS can do more than one thing concurrently by running more than a single process Processes can do several things concurrently be running more than a single thread. Each thread is a different stream of control that can execute its instructions independently. A program (e.g. Browser) may consist of the following threads: GUI thread I/O thread computation

25 Operating Systems 25 K. Salah Processes and Threads A typical process consists of  a running program  a bundle of resources (file descriptor table, address space) A thread, called a lightweight process, has its own  stack  CPU Registers  state  All the other resources are shared by all threads of that process. These include:  open files  virtual address space (code and data segments).  child processes

26 Operating Systems 26 K. Salah Processes vs. Threads

27 Operating Systems 27 K. Salah Single and Multithreaded Processes

28 Operating Systems 28 K. Salah Single Threaded and Multithreaded Process Models  Thread Control Block contains a register image, thread priority and thread state information.

29 Operating Systems 29 K. Salah Benefits of Threads vs Processes  Takes less time to create a new thread than a process  Less time to terminate a thread than a process  Less time to switch between two threads within the same process  Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel. However, it is necessary to synchronize the activities of various threads so that they do not obtain inconsistent views of the data.

30 Operating Systems 30 K. Salah Example I: Web Browser

31 Operating Systems 31 K. Salah Example II: Web Server

32 Operating Systems 32 K. Salah Threads States  Three key states: running, ready, blocked  Generally, it does not make sense to have a suspend state for threads.  Because all threads within the same process share the same address space  Indeed: suspending (ie: swapping) a single thread involves suspending all threads of the same process  Termination of a process, terminates all threads within the process

33 Operating Systems 33 K. Salah User-Level Threads (ULT) The kernel is not aware of the existence of threads All thread management is done by the application by using a thread library Thread switching does not require kernel mode privileges (no mode switch) Scheduling is application specific

34 Operating Systems 34 K. Salah Threads library Contains code for: –creating and destroying threads –passing messages and data between threads –scheduling thread execution –saving and restoring thread contexts Three primary thread libraries: – POSIX Pthreads. The P stands for POSIX and run on unix, linux, and MS Windows. –Cthreads – Win32 threads – Java threads

35 Operating Systems 35 K. Salah Kernel activity for ULTs The kernel is not aware of thread activity but it is still managing process activity When a thread makes a system call, the whole process will be blocked but for the thread library that thread is still in the running state So thread states are independent of process states

36 Operating Systems 36 K. Salah Advantages and inconveniences of ULT Advantages –Thread switching does not involve the kernel: no mode switching  thread_yield() –Strong sharing of data with little blocking  No need for shared memory system calls  Excel sheets share a lot other than files –Scheduling can be application specific: choose the best algorithm.  Run a garbage collection thread at convenient points –ULTs can run on any OS. Only needs a thread library  Portable Inconveniences –Most system calls are blocking and the kernel blocks processes. So all threads within the process will be unable to run –The kernel can only assign processes to processors. Two threads within the same process cannot run simultaneously on two processors For theads that run for too long (1 sec), preemption is done using signals or alarms (e.g., ualarm). However this requires a lot more overhead in switching. Signal delivery by kernel to process is very complex. Kernel checks for signal at termination of phase interrupts, if one is pending: save context of process, K-U to handle signal, U-K to restore context of process, K-U to resume process.

37 Operating Systems 37 K. Salah Improving blocking with ULT -- Advanced Use nonblocking I/O system calls –Returns quickly without need to complete the full I/O operation Use asynchronous I/O system calls –Setup a callback function and returns quick –When I/O is completed a function is called (part of signal handling) Identify blocking system calls, and place a jacket or wrapper around them –Needs to modify API or system call library –If we know it will block, defer the thread and let other threads run first

38 Operating Systems 38 K. Salah Kernel-Level Threads (KLT) All thread management is done by kernel No thread library but an API (I.e. system calls) to the kernel thread facility Kernel maintains context information for the process and the threads Switching between threads requires the kernel Scheduling on a thread basis Examples –Windows XP/2000 –Solaris –Linux –Tru64 UNIX –Mac OS X

39 Operating Systems 39 K. Salah Kernel Multithreading Models Many-to-One One-to-One Many-to-Many

40 Operating Systems 40 K. Salah Advantages and inconveniences of KLT Advantages –the kernel can simultaneously schedule many threads of the same process on many processors –blocking is done on a thread level –kernel routines can be multithreaded Inconveniences –thread switching within the same process involves the kernel. We have 2 mode switches per thread switch: user to kernel and kernel to user. –this results in a significant slow down due to:  Interrupt overhead due to mode switch  Updates to TCB info  Cache pollution and flushing to Process tables and page tables

41 Operating Systems 41 K. Salah Combined ULT/KLT Approaches Thread creation done in the user space Bulk of scheduling and synchronization of threads done in the user space The programmer may adjust the number of KLTs May combine the best of both approaches Examples: –Solaris prior to version 9 –Windows NT/2000 with the ThreadFiber package

42 Operating Systems 42 K. Salah Solaris: versatility We can use ULTs when logical parallelism does not need to be supported by hardware parallelism (we save mode switching) –Ex: Multiple windows but only one is active at any one time –Excel sheet (sheet1, sheet2, etc) –Power point –Word processor It is wise to have 2 KLTs under this situation. So if one window is blocked when making a system call, use the other KLT to run the other selected window). If the windows are doing a lot of blocking, use more KLTs. –Reason is efficiency  ULTs can be created, blocked, destroyed, without involving the kernel  Efficiency in terms of memory and data structure allocated in kernel space  Minimizing cache pollution and flushing If threads may block then we can specify two or more LWPs (or KLTs) to avoid blocking the whole application

43 Operating Systems 43 K. Salah Further Readings What is the difference between RPC and RMI? What is meant by marshalling parameters? What is the idea behind a thread pool? What is hyperthreading? Answer this: –If you have CPU –bound application, when does it make sense to use ULTs for them as opposed to KLTs?  Example is a parallel array computation where you divide the rows of its arrays among different threads –Answer:  use ULTs to minimize switching with uniprocessor  Use KLTs for more concurrency with SMP


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