Multithreaded Programming Part- II.pdf

Multithreaded Programming Part- II.pdf

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  Amrita School of Engineering, Bangalore Ms. Harika Pudugosula TeachingAssistant Department of Electronics & Communication Engineering
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  • Overview • MulticoreProgramming • Multithreading Models • Thread Libraries • Implicit Threading • Threading Issues • Operating System Examples 2
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  Multithreading Models • Typesof Threads 1. User Threads - Supported above the kernel and are managed without kernel support - These threads are managed by the user level thread library - Kernel knows nothing about the threads - The thread library contains code for creating and destroying threads, for passing message and data between threads, for scheduling thread execution and for saving and restoring thread contexts. - All thread creation and scheduling is done in user space - Advantage- fast to create and manage - Disadvantage- if the kernel is single threaded then any user level thread performing a blocking s/m call will cause entire process to block even if other threads available to run within application 3
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  Multithreading Models • Typesof Threads 2. Kernel Threads - Supported and managed directly by the operating system - These threads are managed by the kernel - All modern operating systems support kernel level threads - Kernel performs- thread creation, scheduling, management - Advantage - if thread performs blocking s/m call kernel can schedule another thread in application for execution - Disadvantage - slower to create and manage threads • Ultimately, a relationship must exist between user threads and kernel threads • Many-to-One Model • One-to-One Model • Many-to-Many Model 4
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  Many-to-One Model • Mapsmany user-level threads to one kernel thread • Thread management is done by the thread library in user space, so it is efficient • Limitation - the entire process will block if a thread makes a blocking system call • Limitation - because only one thread can access the kernel at a time, multiple threads are unable to run in parallel on multicore systems 5
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  One-to-One Model • Mapseach user thread to a kernel thread • Provides more concurrency than the many-to- one model by allowing another thread to run when a thread makes a blocking system call • Also allows multiple threads to run in parallel on multiprocessors • Limitation - creating a user thread requires creating the corresponding kernel thread • Because the overhead of creating kernel threads can burden the performance of an application, most implementations of this model restrict the number of threads supported by the systems 6
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  Many-to-Many Model • Multiplexesmany user-level threads to a smaller or equal number of kernel threads • The number of kernel threads may be specific to either a particular application or a particular machine • Developer to create as many user threads as necessary, and the corresponding kernel threads can run in parallel on a multiprocessor • When a thread performs a blocking system call, the kernel can schedule another thread for execution 7
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  Two-level model • Itenables the processor to execute two threads, or sets of instructions, at the same time • It allows threads to be executed in parallel, it is almost like having two seperate processors working together • Also known as Hyperthreading, Simultaneous Multithreading (SMT) 8
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  9 One thread blockingcauses all threads to block. Identify the type of multithreading model and explain with neat diagram.
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  Amdahl’s Law • Identifiesperformance gains from adding additional cores to an application that has both serial and parallel components • S is serial portion • N processing cores • That is, if application is 75% parallel / 25% serial, moving from 1 to 2 cores results in speedup of 1.6 times • As N approaches infinity, speedup approaches 1 / S • Principle behind Amdahl’s Law : Serial portion of an application has disproportionate effect on performance gained by adding additional cores • But does the law take into account contemporary multicore systems? • No, Amdahl’s Law may cease to be applicable as the number of processing cores continues to increase on modern computer systems
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  Amdahl’s Law • UsingAmdahl’s Law, calculate the speedup gain: 67 percent parallel with a) two processing cores b) four processing cores • If 60% of the computation can be parallelized, Using Amdahl’s law what is the maximum speed up achievable using 4 processor. • By adding cores will definitely have an impact on the improvement of the system • If the system has more serial operations than parallel operations, adding cores will not have much improvement
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  Threads Libraries • Athread library provides the programmer with an API for creating and managing threads • There are two primary ways of implementing a thread library • First approach - • To provide a library entirely in user space with no kernel support • All code and data structures for the library exist in user space • This means that invoking a function in the library results in a local function call in user space and not a system call • Second approach - • To implement a kernel-level library supported directly by the operating system • All code and data structures for the library exist in kernel space • This means invoking a function in the API for the library typically results in a system call to the kernel 12
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  Threads Libraries • Threemain thread libraries are in use today: POSIX Pthreads, Windows, and Java • Pthreads, the threads extension of the POSIX standard, may be provided as either a user-level or a kernel-level library • The Windows thread library is a kernel-level library available on Windows systems • The Java thread API allows threads to be created and managed directly in Java programs • Most instances the JVM is running on top of a host operating system, the Java thread API is generally implemented using a thread library available on the host system • So, on Windows systems, Java threads are typically implemented using the Windows API; on UNIX and Linux systems often use Pthreads • For POSIX and Windows threading, any data declared globally—that is, declared outside of any function—are shared among all threads belonging to the same process 13
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  Threads Libraries • Javahas no notion of global data, access to shared data must be explicitly arranged between threads • Data declared local to a function are typically stored on the stack • Two general strategies for creating multiple threads: - Asynchronous threading - Synchronous threading • Asynchronous threading • Once the parent creates a child thread, the parent resumes its execution, so that the parent and child execute concurrently • Each thread runs independently of every other thread, and the parent thread need not know when its child terminates • Because the threads are independent, there is typically little data sharing between threads • Example: Multithreaded server (refer topic: Motivation) 14
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  Threads Libraries • Twogeneral strategies for creating multiple threads: - Asynchronous threading - Synchronous threading • Synchronous threading • When the parent thread creates one or more children and then parent must wait for all of its children to terminate before it resumes - fork-join strategy • Here, the threads(children) created by the parent perform work concurrently, but the parent cannot continue until threads(children) work has been completed • Once each thread(child) has finished its work, it terminates and joins with its parent • Only after all of the children have joined can the parent resume execution • Involves significant data sharing among threads • Example: The parent thread may combine the results calculated by its various children 15
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  Pthreads • Pthreads refersto the POSIX standard (IEEE 1003.1c) defining an API for thread creation and synchronization • This is a specification for thread behavior, not an implementation • Operating-system designers may implement the specification in any way they wish • In a Pthreads program, separate threads begin execution in a specified function • Example: the basic Pthreads API for constructing a multithreaded program that calculates the summation of a non-negative integer in a separate thread 16
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  Pthreads Example 17 #include <pthread.h> #include<stdio.h> int sum; /* this data is shared by the thread(s) */ void *runner(void *param); /* threads call this function */ int main(int argc, char *argv[]) { pthread t tid; /* the thread identifier for the thread that we create */ pthread attr t attr; /* represents the attributes for the thread, we set the attributes in the function call pthread attr init(&attr)*/ if (argc != 2) { fprintf(stderr,"usage: a.out <integer value>n"); return -1; } if (atoi(argv[1]) < 0) { fprintf(stderr,"%d must be >= 0n",atoi(argv[1])); return -1; }
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  Pthreads Example 18 pthread attrinit(&attr); /* get the default attributes */ pthread create(&tid,&attr,runner,argv[1]); /* get the default attributes */ pthread join(tid,NULL); /* wait for the thread to exit */ printf("sum = %dn",sum); } void *runner(void *param) /* The thread will begin control in this function */ { int i, upper = atoi(param); sum = 0; for (i = 1; i <= upper; i++) sum += i; pthread exit(0); }
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  Pthreads Code forJoining 10 Threads 19 #define NUM THREADS 10 /* an array of threads to be joined upon */ pthread t workers[NUM THREADS]; for (int i = 0; i < NUM THREADS; i++) pthread join(workers[i], NULL);
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  Windows Threads • Thetechnique for creating threads using the Windows thread library is similar to the Pthreads technique • Must include the windows.h header file when using the Windows API • Data shared by the separate threads (similar to Pthreads) 20
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  Windows Threads 21
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  Windows Threads 22
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  Windows Threads • Insituations that require waiting for multiple threads to complete, the WaitForMultipleObjects() function is used. This function is passed four parameters: 1. The number of objects to wait for 2. A pointer to the array of objects 3. A flag indicating whether all objects have been signaled 4. A timeout duration (or INFINITE) • For example, if THandles is an array of thread HANDLE objects of size N, the parent thread can wait for all its child threads to complete with this statement: WaitForMultipleObjects(N, THandles, TRUE, INFINITE); 23
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  Java Threads • Threadsare the fundamental model of program execution in a Java program, and the Java language and its API provide a rich set of features for the creation and management of threads • All Java programs comprise at least a single thread of control—even a simple Java program consisting of only a main() method runs as a single thread in the JVM • There are two techniques for creating threads in a Java program • One approach is to create a new class that is derived from the Thread class and to override its run() method • An alternative—and more commonly used— technique is to define a class that implements the Runnable interface public interface Runnable{ public abstract void run(); } 24
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  Java Threads • Analternative—and more commonly used— technique is to define a class that implements the Runnable interface public interface Runnable { public abstract void run(); } • When a class implements Runnable, it must define a run() method • The code implementing the run() method is what runs as a separate thread • Example: • Java version of a multithreaded program that determines the summation of a non- negative integer • The Summation class implements the Runnable interface • Thread creation is performed by creating an object instance of the Thread class and passing the constructor a Runnable object 25
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  Java Threads • Creatinga Thread object does not specifically create the new thread; rather, the start() method creates the new thread. Calling the start() method for the new object does two things: 1. It allocates memory and initializes a new thread in the JVM 2. It calls the run() method, making the thread eligible to be run by the JVM • If two or more threads are to share data in a Java program, the sharing occurs by passing references to the shared object to the appropriate threads • The parent threads in the Pthreads and Windows libraries use pthread join() and WaitForSingleObject() to wait for the summation threads to finish before proceeding • The join() method in Java provides similar functionality 26
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  Java Multithreaded Program 27
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  Java Multithreaded Program(Cont.) 28
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  29 References 1. Silberscatnz andGalvin, “Operating System Concepts,” Ninth Edition, John Wiley and Sons, 2012.
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  30 Thank you