Chapter 4: Multithreaded

Transcription

1 Chapter 4: Multithreaded Programming

2 Chapter 4: Multithreaded Programming Overview Multithreading Models Thread Libraries Threading Issues Operating-System Examples 2009/10/19 2

3 4.1 Overview A thread is a basic unit of CPU utilization; it comprises a thread ID, a program counter, a register set, and a stack. A traditional, or heavyweight process has a single thread of control. It shares with other threads belonging to the same process its code section, data section, and other OS resources, such as open files and signals. In busy WWW server: The server creates a separate thread that would listen for clients requests, when a request was made, creates a thread to service the request. 2009/10/19 3

4 Single and Multithreaded Processes 2009/10/19 4

5 Benefits (1) Responsiveness: Allow a program to continue running even if part of it is blocked or is performing a lengthy operation. Resource sharing: several different threads of activity all within the same address space. Economy: Allocating memory and resources for process creation is costly. In Solaris, creating a process is about 30 times slower than is creating a thread, and context switching is about five times slower. A register set switch is still required, but no memory- management related work is needed. 2009/10/19 5

6 Benefits (2) Scalability(Utilization of multiprocessor architecture): Several thread may be running in parallel on different processors. Of course, multithreading a process may introduce concurrency control problem that requires the use of critical sections or locks. 2009/10/19 6

7 Multithreaded Server Architecture 2009/10/19 7

8 Multicore Programming (1) Placed multiple computing cores on a single chip, where each core appears as a separate processor to OS On a system with a single core, concurrency merely means that the execution of the threads will be interleaved over time On a system with multicore, concurrency means that the threads can run in parallel The trend towards multicore systems has placed pressure on system designers as well as application programmers to make better use of multicore 2009/10/19 8

9 Concurrent Execution on a Single-core System Parallel Execution on a Multi-core System 2009/10/19 9

10 Multicore Programming (2) Multicore systems putting pressure on programmers, challenges include Dividing activities into separate, concurrent tasks Balance the workload of multiple tasks Data splitting into separate cores Data dependency must be carefully examined Testing and debugging are more difficult than single-thread applications 2009/10/19 10

11 User Threads Thread management done by user-level threads library. Fast: All thread creating and scheduling are done in user space without the need for kernel intervention. Any user-level thread performing a blocking system call will cause the entire process to block, even if there are other threads available to run within the applications, if the kernel is single-thread. Examples Pthreads Win32 threads Java threads 2009/10/19 11

12 Kernel Threads Supported and managed directly by the OS Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X 2009/10/19 12

13 4.2 Multithreading Models (1) (Multi-Thread vs. Multi-process) Multiple processes Each is independent and has it own program counter, stack register, and address space. This is useful for unrelated jobs. Multiple processes can perform the same task as well. (e.g., provide data to remote machines in a network file system). Each executes the same code but has it own memory and file resources. Multiple-thread process It is more efficient to have one process containing multiple threads serve the same task. Most Systems Support for both user and kernel threads 2009/10/19 13

14 Multithreading Models (2) Uses fewer resources, including memory, open files and CPU scheduling. Threads are not independent to each other. This structure does not provide protection. Only asingle user can own an individual task with multiple threads. The threads should be designed to assist one another. Threads can create child threads. If one thread is blocked, another thread can run. Threads provide a mechanism that allows sequential processes to make blocking system calls while also achieving parallelism. 2009/10/19 14

15 Many-to-One One-to-One Many-to-Many Multithreading Models (3) 2009/10/19 15

16 Many-to-One Many user-level threads mapped to single kernel thread. Used on systems that do not support kernel threads. Thread management is done in user space, so it is efficient. The entire process will block if a thread makes a blocking system call. Only one thread can access the kernel at a time, multiple threads are unable to run in parallel on multiprocessors. 2009/10/19 16

17 Many-to-One Model 2009/10/19 17

18 One-to-One Each user-level thread maps to a kernel thread. More concurrency Overhead: Creating a thread requires creating the corresponding kernel thread. Examples - Windows XP/NT/2000 -Linux -Solaris 9 and later 2009/10/19 18

19 One-to-one Model 2009/10/19 19

20 Many-to-Many Model Allows many user level threads to be mapped to many kernel threads Allows the operating system to create a sufficient number of kernel threads Multiplexes many user-level threads to a smaller or equal number of kernel threads The corresponding kernel threads can run in parallel on a multiprocessor. When a thread performs a blocking call, the kernel can schedule another thread for execution Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package 2009/10/19 20

21 Many-to-Many Model 2009/10/19 21

22 Two-level Model Popular variation on Many-to-Many model Similar to M:M, except that it allows a user thread to be boundto a kernel thread Examples IRIX HP-UX Tru64 UNIX Solaris 8 and earlier 2009/10/19 22

23 Two-level Model 2009/10/19 23

24 4.3 Thread Libraries Thread library provides programmer with API for creating and managing threads Two primary ways of implementing Library entirely in user space Kernel-level library supported by the OS 2009/10/19 24

25 Pthreads May be provided either as user-level or kernellevel A POSIX standard (IEEE c) API for thread creation and synchronization API specifies behavior of the thread library, implementation is up to development of the library Common in UNIX operating systems (Solaris, Linux, Mac OS X) 2009/10/19 25

26 Java Threads Java threads are managed by the JVM Typically implemented using the threads model provided by underlying OS Java threads may be created by: Extending Thread class Implementing the Runnable interface 2009/10/19 26

27 4.4 Threading Issues Semantics of fork() and exec() system calls. Duplicate all the threads or not? Thread cancellation: Asynchronous or deferred Signal handling: Where then should a signal be delivered? Thread pools: Create a number of threads at process startup. Thread specific data: Each thread might need its own copy of certain data. Scheduler activations 2009/10/19 27

28 Semantics of fork() and exec() Does fork()duplicate only the calling thread or all threads? Two versions of fork(): duplicates all threads and another duplicates only the thread that invoked the fork() system call If exec() is called immediately after forking, then duplicating all threads is unnecessary. 2009/10/19 28

29 Thread Cancellation Terminating a thread before it has completed. Two general approaches: Asynchronous cancellationterminates the target thread immediately Deferred cancellationthe target thread to periodically check whether it should be terminate, allowing it an opportunity to terminate itself in an orderly fashion (canceled safely). Cancellation points 2009/10/19 29

30 Signal Handling Signals are used in UNIX systems to notify a process that a particular event has occurred A signal handler is used to process signals Signal is generated by particular event Signal is delivered to a process Signal is handled Delivering Signals: Deliver the signal to the thread to which the signal applies Deliver the signal to every thread in the process Deliver the signal to certain threads in the process Assign a specific thread to receive all signals for the process 2009/10/19 30

31 Thread Pools Create a number of threads at process start and place them in a pool where they await work Advantages: Usually slightly faster to service a request with an existing thread than create a new thread Allows the number of threads in the application(s) to be bound to the size of the pool 2009/10/19 31

32 Thread Specific Data Allows each thread to have its own copy of data Each transaction assigned a unique number in the transaction-processing system Useful when you do not have control over the thread creation process (i.e., when using a thread pool) 2009/10/19 32

33 Scheduler Activations Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application Scheduler activations provide upcalls-a communication mechanism from the kernel to the thread library This communication allows an application to maintain the correct number kernel threads 2009/10/19 33

34 Operating-system Example Explore how threads are implemented in Windows XP and Linux systems. 2009/10/19 34

35 Windows XP Threads Implements the one-to-one mapping Each thread contains A thread id Register set Separate user and kernel stacks Private data storage area The register set, stacks, and private storage area are known as the context of the threads The primary data structures of a thread include: ETHREAD (executive thread block) KTHREAD (kernel thread block) TEB (thread environment block) 2009/10/19 35

36 Windows XP Threads 2009/10/19 36

37 Linux Threads Linux refers to them as tasksrather than threads Thread creation is done through clone()system call clone()allows a child task to share the address space of the parent task (process) 2009/10/19 37

38 End of Chapter 4

39 Homework 4.1, 4.4, 4.6 Due Oct. 28 Project- Matrix Multiplication (multi-thread vs. multi-process) Due Nov /10/19 39