Saurav Karmakar

Chapter 4: Threads  Overview  Multithreading Models  Thread Libraries  Threading Issues  Operating System Examples  Windows XP Threads  Linux Threads

Objectives  To introduce the notion of a thread — a fundamental unit of CPU utilization that forms the basis of multithreaded computer systems  To discuss the APIs for the Pthreads, Win32, and Java thread libraries  To examine issues related to multithreaded programming

Threads  A thread is a single sequence stream within in a process.  Processes are used to group resources together and threads are the entities scheduled for execution on the CPU.  Sometimes called lightweight processes.  A thread has or consists of thread id, a program counter (PC), a register set, and a stack space.  Threads are not independent of one other like processes

Single and Multithreaded Processes

Multithreaded Server Architecture When some similar tasks to be served by a server should it create a process or a thread?

Benefits  Responsiveness  Resource Sharing  Economy  Scalability

Process VsThreads  Similarities Threads share CPU and only one thread active (running) at a time. Threads within a processes execute sequentially. Thread can create children. If one thread is blocked, another thread can run.  Differences Threads are not independent of one another. All threads can access every address in the task. Threads are designed to assist one other. Note that processes might or might not assist one another because processes may originate from different users.

Concurrent Execution on a Single-core System Parallel Execution on a Multicore System

Multicore Programming  Multicore systems putting pressure on programmers, challenges include Dividing activities Balance Data splitting Data dependency Testing and debugging

User Threads  Thread management done by user-level threads library  Thread switching does not need to call operating system and to cause interrupt to the kernel.  The kernel knows nothing about user-level threads and manages them as if they were single-threaded processes.  Three primary thread libraries: POSIX Pthreads Win32 threads Java threads

Threads in User Space  Each process needs a thread table Similar to a process table, but tracks only: ○ Program Counter ○ Stack Pointer ○ Registers ○ State  Table managed by the run-time system

Kernel Threads  Supported by the Kernel  The kernel has a thread table that keeps track of all threads in the system.  Operating Systems kernel provides system call to create and manage threads.  Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X

Threads in Kernel Space  Put the concept of the thread in the kernel There is no need of a run-time system  The kernel has a thread table  Threads make calls to the kernel for thread creation/termination  Kernel holds each thread’s registers, state, etc...

Hybrid Implementations  Trys to get to advantage of both the designs User and kernel threads are implemented where they will be more efficient Disadvantage : Complicated design by far

Advantage-Disadvantage Factors  Representaion  Management  Speed and Efficiency  Scheduler Decesion  Overheads  Application

Multithreading Models  Describes the relationship between user threads and kernel threads  Three general types : Many-to-One One-to-One Many-to-Many

Many-to-One  Many user-level threads mapped to single kernel thread  Thread management – in user space  Entire process gets blocked if one blocks.  Only one thread can use kernel at a time  Examples: Solaris Green Threads GNU Portable Threads

One-to-One  Each user-level thread maps to a kernel thread  Better in concurrency  Only drawback is creating one of this kind requires corresponding kernel thread creation  Examples Windows NT/XP/2000 Linux Solaris 9 and later

Many-to-Many Model  Allows many user level threads to be mapped to many (smaller or equal #) kernel threads (# depends on application/machine)  Allows the operating system to create a sufficient number of kernel threads  On a blocking call kernel can schedule another thread  Example : Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

Two-level Model  Similar to M:M, except that it allows a user thread to be bound to kernel thread  Examples IRIX HP-UX Tru64 UNIX Solaris 8 and earlier

Threads Vs Multiprocesses  Advantages : Context Switching Sharing  Disadvantages : Blocking Security

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

Pthreads  May be provided either as user-level or kernel-level  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)

Win32 Thread Program

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

Threading Issues  Semantics of fork() and exec() system calls  Thread cancellation of target thread Asynchronous or deferred  Signal handling  Thread pools  Thread-specific data  Scheduler activations

Semantics of fork() and exec()  Does fork() duplicate only the calling thread or all threads?  What about exec() ?  What type of fork() is better ?

Thread Cancellation  Terminating a thread before it has finished  The thread to be cancelled is Target Thread  Two general approaches: Asynchronous cancellation terminates the target thread immediately Deferred cancellation allows the target thread to periodically check if it should be cancelled

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 1. Signal is generated by particular event 2. Signal is delivered to a process 3. Signal is handled (default/user-defined)  Signals received: Asynchronously Synchronously

Signal Handling  Signal handler is of two types 1. Default Signal Handler 2. User Defined Signal Handler  Options: 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  UNIX : pthread_kill(pid_t, int signal)  Windows : emulated through APCs

Thread Pools  Create a number of threads 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  The # of threads on a pool  # of CPUs, memory, concurrent # of request  Sophisticated architectures adjust dynamically.

Thread Specific Data  Allows each thread to have its own copy of data  Useful when you do not have control over the thread creation process (i.e., when using a thread pool)

Scheduler Activations  Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application  Many systems implement an intermediate data structure called Lightweight process(LWP)  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

Lightweight Processes (LWP)

Operating System Examples  Windows XP Threads  Linux Thread

Windows XP Threads  Implements the one-to-one mapping, kernel-level  Many-to-one model could be implemented throgh fiber-library  Each thread contains A thread id Register set Separate user and kernel stacks Private data storage area – for runtime and dynamic link libraries  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)

Windows XP Threads

Linux Threads  Linux refers to them as tasks rather than threads  Thread creation is done through clone() system call  clone() allows a child task to share the address space of the parent task, depending on if certain flags are set.  A unique kernel structure struct task_struct used for each task.

Linux Threads

End of Lecture 4