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 Process creation is heavy-weight while thread creation is light-weight Can simplify code, increase efficiency Kernels are generally multithreaded

Single and Multithreaded Processes

Benefits Responsiveness Resource Sharing Economy Scalability Utilization of MP Architectures

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

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

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

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.

Kernel Threads Supported by the Kernel Examples –Windows XP/ –Solaris –Linux –Tru64 UNIX –Mac OS X

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

Multithreading Models Many-to-One One-to-One Many-to-Many

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

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

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

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

Many-to-Many Model

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.

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

Two-level Model

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

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

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

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 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

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

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 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

Pthreads 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)

Pthreads Example

Pthreads Example (Cont.)

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)

Win32 API Multithreaded C Program

Win32 API Multithreaded C Program (Cont.)

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)

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

Java Thread States

Java Multithreaded Program

Java Multithreaded Program (Cont.)

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

Java Thread Scheduling (cont) JVM Schedules a Thread to Run When: 1.The Currently Running Thread Exits the Runnable State 2.A Higher Priority Thread Enters the Runnable State * Note – the JVM Does Not Specify Whether Threads are Time-Sliced or Not

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

Thread Priorities PriorityComment Thread.MIN_PRIORITYMinimum 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);

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

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

Pthread Scheduling API #include #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);

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); }

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