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Cosc 4740

Cosc 4740. Chapter 4 & 5 Threads & Scheduling. Motivation. Threads run within application (process) Multiple tasks with the application can be implemented by separate threads Update display Fetch data Spell checking Answer a network request

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Cosc 4740

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  1. Cosc 4740 Chapter 4 & 5 Threads & Scheduling

  2. Motivation • Threads run within application (process) • Multiple tasks with the application can be implemented by separate threads • Update display • Fetch data • Spell checking • Answer a network request • Process creation is heavy-weight while thread creation is light-weight • Can simplify code, increase efficiency • Kernels are generally multithreaded

  3. Single and Multithreaded Processes

  4. Benefits • Responsiveness • Resource Sharing • Economy • Scalability • Utilization of MP Architectures

  5. Limitations • Schedulers view a process, not threads of a process • If 1 thread blocks for I/O or a signal, the schedulers switches to a different process • All threads in that process are blocked! • This is especially true in java with the jvm process • Scheduler allocates same amount of CPU time for a 100 thread process as 2 thread process

  6. Multithreaded Server Architecture More “light weight” then doing the same with processes.

  7. User Threads • Thread management done by user-level threads library • Three primary thread libraries: • POSIX Pthreads • Win32 threads • Java threads

  8. User Thread (2) • Call special library functions • develop multiple threads of controls in our programs that run concurrently • Fast switching: switching among peer threads does not incur an interrupt to the kernel • No short-term scheduler, no address change • Only PC & stack-address changed.

  9. Kernel Threads • Supported by the Kernel • Examples • Windows XP/2000 + • Solaris • Linux • Tru64 UNIX • Mac OS X

  10. Multi-thread Kernel • Kernel is a task of multiple threads, so • Threads supported directly by the O/S. • Fair scheduling • Solve the limitations of the user threads • Increased Kernel utilization • While 1 kernel thread is waiting on I/O, the kernel can accept another request

  11. Multithreading Models • Many-to-One • One-to-One • Many-to-Many

  12. Many-to-One • Many user-level threads mapped to single kernel thread • Limitations, all threads access a single kernel thread, so unable to run in parallel on multiprocessors • Examples: • Solaris Green Threads • GNU Portable Threads

  13. One-to-One • Each user-level thread maps to kernel thread • Examples • Windows NT/XP/2000 +, Linux, Solaris 9 and later

  14. One-to-One • Benefits • More concurrency then many-to-one • Allows parallel processing • Limitations • Most implementations restrict the number of threads • Creating kernel threads is burden to the application and O/S

  15. 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 • Examples • Solaris prior to version 9 • Windows NT/2000 with the ThreadFiber package

  16. Many-to-Many Model

  17. Many-to-Many Model (3) • Benfits • Allows threads to block on IO, while allowing other threads to be scheduled • Fewer kernel threads, so doesn’t burden application and OS • Limitations • While still being done parallel on multiprocessors, not every thread that can execute will.

  18. 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

  19. Two-level Model

  20. Threading Issues • Semantics of fork() and exec() system calls • Thread cancellation • Signal handling • Thread pools • Thread specific data • Scheduler activations

  21. Semantics of fork() and exec() • Does fork() duplicate only the calling thread or all threads?

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

  23. 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 • 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

  24. 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

  25. 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)

  26. 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

  27. Pthreads • A POSIX standard (IEEE 1003.1c) 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)

  28. Pthreads Example

  29. Pthreads Example (Cont.)

  30. 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)

  31. Win32 API Multithreaded C Program

  32. Win32 API Multithreaded C Program (Cont.)

  33. 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 (process)

  34. Java Threads • Java threads are managed by the JVM • Java threads may be created by: • Extending Thread class • Implementing the Runnable interface

  35. Java Thread States

  36. Java Multithreaded Program

  37. Java Multithreaded Program (Cont.)

  38. Java Thread Scheduling • JVM Uses a Preemptive, Priority-Based Scheduling Algorithm • FIFO Queue is Used if There Are Multiple Threads With the Same Priority

  39. Java Thread Scheduling (cont) JVM Schedules a Thread to Run When: • The Currently Running Thread Exits the Runnable State • A Higher Priority Thread Enters the Runnable State * Note – the JVM Does Not Specify Whether Threads are Time-Sliced or Not

  40. Time-Slicing Since the JVM Doesn’t Ensure Time-Slicing, the yield() Method May Be Used: while (true) { // perform CPU-intensive task . . . Thread.yield(); } This Yields Control to Another Thread of Equal Priority

  41. Thread Priorities PriorityComment Thread.MIN_PRIORITY Minimum Thread Priority Thread.MAX_PRIORITY Maximum Thread Priority Thread.NORM_PRIORITY Default Thread Priority Priorities May Be Set Using setPriority() method: setPriority(Thread.NORM_PRIORITY + 2);

  42. 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

  43. Pthread Scheduling • API allows specifying either PCS or SCS during thread creation • PTHREAD_SCOPE_PROCESS schedules threads using PCS scheduling • PTHREAD_SCOPE_SYSTEM schedules threads using SCS scheduling • Can be limited by OS – Linux and Mac OS X only allow PTHREAD_SCOPE_SYSTEM

  44. Pthread Scheduling API #include <pthread.h> #include <stdio.h> #define NUM THREADS 5 int main(int argc, char *argv[]) { int i; pthread t tid[NUM THREADS]; pthread attr t attr; /* get the default attributes */ pthread attr init(&attr); /* set the scheduling algorithm to PROCESS or SYSTEM */ pthread attr setscope(&attr, PTHREAD SCOPE SYSTEM); /* set the scheduling policy - FIFO, RT, or OTHER */ pthread attr setschedpolicy(&attr, SCHED OTHER); /* create the threads */ for (i = 0; i < NUM THREADS; i++) pthread create(&tid[i],&attr,runner,NULL);

  45. Pthread Scheduling API /* 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) { printf("I am a thread\n"); pthread exit(0); }

  46. Q A &

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