Introduction to Operating systems and Advancing later on, Exams of Operating Systems

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OPERATING SYSTEM

Threads & Concurrency

Dr Rahul Nagpal Computer Science

OPERATING SYSTEM

Threads & Concurrency

Dr. Rahul Nagpal Computer Science

Motivation

• Process creation is heavy-weight while thread creation is light-weight
• Most modern applications are multithreaded
• Threads run within application
• Multiple tasks in application can be implemented by threads
  • Update display
  • Fetch data
  • Spell checking
• Can simplify code, increase efficiency
• Kernels are generally multithreaded

Single and Multithreaded Processes

Benefits Responsiveness – may allow continued execution if part of process is blocked, especially important for user interfaces Resource Sharing – threads share resources of process, easier than shared memory or message passing Economy – cheaper than process creation, thread switching lower overhead than context switching Scalability – process can take advantage of multiprocessor architectures

Multicore Programming Multicore or multiprocessor systems putting pressure on programmers, challenges include: Dividing activities Balance Data splitting Data dependency Testing and debugging Parallelism implies a system can perform more than one task simultaneously Concurrency supports more than one task making progress Single processor / core, scheduler providing concurrency

Concurrency vs. Parallelism

• Concurrent execution on single-core system:
• Parallelism on a multi-core system:

User Threads and Kernel Threads

• User threads - management done by user-level threads library
• Three primary thread libraries:
  • POSIX Pthreads
  • Windows threads
  • Java threads
• Kernel threads - Supported by the Kernel
• Examples – virtually all general purpose operating systems, including: - Windows - Solaris - Linux - Tru64 UNIX - Mac OS X

Many-to-One

• Many user-level threads mapped to single kernel thread
• One thread blocking causes all to block
• Multiple threads may not run in parallel on muticore system because only one may be in kernel at a time
• Few systems currently use this model
• Examples:
  • Solaris Green Threads
  • GNU Portable Threads

One-to-One

• Each user-level thread maps to kernel thread
• Creating a user-level thread creates a kernel thread
• More concurrency than many-to-one
• Number of threads per process sometimes restricted due to overhead
• Examples
  • Windows
  • Linux
  • Solaris 9 and later

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

Thread Scheduling

• Distinction between user-level and kernel-level threads
• When threads supported, threads scheduled, not processes
• Many-to-one and many-to-many models, thread library schedules user-level threads to run on LWP - Known as process-contention scope (PCS) since scheduling competition is within the process - Typically done via priority set by programmer
• Kernel thread scheduled onto available CPU is system- contention scope (SCS) – competition among all threads in system

Pthreads Example

Slides Adapted from Operating System Concepts 9/e © Authors Pthread Scheduling API #include <pthread.h> #include <stdio.h> #define NUM_THREADS 5 int main(int argc, char argv[]) { int i, scope; pthread_t tid[NUM THREADS]; pthread_attr_t attr; / get the default attributes / pthread_attr_init(&attr); / first inquire on the current scope / if (pthread_attr_getscope(&attr, &scope) != 0) fprintf(stderr, "Unable to get scheduling scope\n"); else { if (scope == PTHREAD_SCOPE_PROCESS) printf("PTHREAD_SCOPE_PROCESS"); else if (scope == PTHREAD_SCOPE_SYSTEM) printf("PTHREAD_SCOPE_SYSTEM"); else fprintf(stderr, "Illegal scope value.\n"); } / set the scheduling algorithm to PCS or SCS / pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM); / create the threads / for (i = 0; i < NUM_THREADS; i++) pthread_create(&tid[i],&attr,run ner,NULL); / now join on each thread / for (i = 0; i < NUM_THREADS; i++) pthread_join(tid[i], NULL); } / Each thread will begin control in this function */ void *runner(void param) { / do some work ... */ pthread_exit(0); }