Threads PDF

IT2106 Threads threads. Each process is represented by an object that includes Thread Functionality specific attributes and encapsulates...

IT2106 Threads threads. Each process is represented by an object that includes Thread Functionality specific attributes and encapsulates a number of actions or services A thread is a fundamental element of the central processing unit (CPU) that it may perform. A Windows process must also contain at least one utilization. It is generally composed of thread identification, a program counter, (1) thread to execute. This thread may then create other threads. a set of registers, and a stack. Threads that belong in the same process share Some attributes of a thread object resemble those of a process, and the same code section, data section, and other operating system (OS) in this case, the thread attribute value is actually derived from the resources. A traditional process has a single thread of control, and if a process process attribute value. has multiple threads of control, it can perform more than one task at a time (Silberschatz, Galvin, & Gagne, 2018). A thread should at least contain the Note that one of the attributes of a thread object is context, which following attributes: contains the values of the processor registers when the thread last o Thread execution state ran. This information enables threads to be suspended and resumed. o A saved thread context when not running Furthermore, in Windows OS, it is possible to alter the behavior of a o Some per-thread static storage for local variables thread by altering its context while it is suspended. o Access to the memory and resource of the process o An execution stack To differentiate the resource ownership and program execution (scheduling) characteristics of a process, particularly for recently developed systems, the As an example, the list of attributes below characterizes a Windows thread unit of resource ownership is usually referred to as process or task, while the object (Stallings, 2018): unit of dispatching processes is usually referred to as threads or lightweight processes. The following process and thread arrangement or relationship may Thread ID A unique value that identifies a thread when it calls a occur (Stallings, 2018): server One process: One thread (1:1) – Each thread of execution is a unique Thread context A set of register values and other volatile data that process with its own address space and resources. The MS-DOS and the defines the execution state of a thread Classic UNIX are examples of OS that support this kind of relationship. Dynamic priority The thread's execution priority at any given moment Multiple processes: One thread per process (M:1) – A thread may Base priority The lower limit of the thread's dynamic priority Thread processor affinity The set of processors on which the thread can run. migrate from one (1) process environment to another. This allows a thread Thread execution time The cumulative amount of time a thread has to be easily moved among distinct systems. Some variants of the UNIX executed in user mode and in kernel mode OS support this kind of arrangement. Alert status A flag that indicates whether a waiting thread may One process: Multiple threads (1:M) – A process defines a specific execute an asynchronous procedure call address space and a dynamic resource ownership. Thus, multiple threads Suspension count The number of times the thread's execution has been can be created and executed within the process. The Java runtime suspended without being resumed environment is an example of a system that encompasses this Impersonation token A temporary access token allowing a thread to arrangement. perform operations on behalf of another process Multiple processes: Multiple threads per processes (M:M) – This has Termination port An inter-process communication channel to which the combined attribute of the M:1 and 1:M arrangement. the process manager sends a message when the thread terminates Thread exit status The reason for a thread's termination The thread construct is also useful for systems with a single processor, since it simplifies the structure of a program that is logically performing several The object-oriented structure of Windows OS facilitates the different functions. Threads are applied in a single-user multiprocessing development of a general-purpose process facility that encompasses system for the enhancement of process execution, for implementing a modular 03 Handout 1 *Property of STI  [email protected] Page 1 of 4  IT2106 program structure, for asynchronous processing, and for other foreground and the environments of the other threads within the same process. It is therefore background work enhancement. necessary to synchronize the activities of various threads in the process to eliminate interference and maintain the data structure of the process. Thread State. The key states for a thread are Running, Ready, and Blocked. If a process is swapped out, noted that all of its threads are automatically Thread Library. Any application can be programmed to be multithreaded with swapped out because they all share the address space of the process. The the use of thread libraries. A thread library technically provides programmers following are the four (4) basic thread operations associated with a change in with an application programming interface (API) for creating and managing thread state (Stallings, 2018): threads. It may contain codes for creating and destroying threads; for passing 1. Spawn – This operation typically transpires whenever a new process messages and data between threads; for scheduling thread execution; and for is created, since a thread for that specific process is also formed. saving and restoring thread context/information. 2. Block – This happens when a thread needs to wait for a particular event. In this operation, all the necessary information are Types of Threads (Stallings, 2018) automatically saved for the thread's execution resumption. User-Level Threads 3. Unblocked – This operation moves a blocked thread into the ready In implementing user-level threads, all the thread management is done by the queue for the continuation of its execution. application. Tthe system kernel is not aware of the existence of the threads 4. Finish – This happens whenever a thread completes its entire and continues to schedule the process as a unit. Below are some advantages execution. Its register settings and stacks are deallocated. of implementing user-level threads: o Thread switching does not require kernel-mode privileges since the As an example, the thread states used by Windows are illustrated below: overall thread data management structure is within the user address space of a single process. Thus, saves the overhead of two (2) mode switches. o Scheduling can be application-specific. The scheduling algorithm can be tailored for the application without disturbing the operating system scheduler. o User-level threads can run on any operating system. There are no changes required to the underlying kernel in order to support user- level threads. Figure 1. The Windows thread states. Source: Operating Systems: Internal and Design Principles (9th ed.), 2018 p. 200 Thread Synchronization. All threads of a process share the same address Figure 2. A pure user-level thread. space and resources. Hence, any alteration of a resource by one thread affects Source: Operating Systems: Internal and Design Principles (9th ed.), 2018 p. 184 03 Handout 1 *Property of STI  [email protected] Page 2 of 4  IT2106 Kernel-Level Threads level threads. The Solaris is a good example of an operating system that In implementing kernel-level threads, all thread management is performed by implements the combined approach. the operating system's kernel. There are no thread management code at the application level, only an application programming interface (API) to the kernel thread section. The kernel maintains the context information for the process as a whole and for the individual threads within the process. In addition, scheduling by the kernel is done on a thread basis. Below are some of the advantages of implementing kernel-level threads: o The kernel can simultaneously schedule multiple threads from the same process on multiple processors. o If one thread in a process is blocked, the kernel can schedule another thread from the same process. o The kernel routines themselves can be multithreaded. Figure 4. A combined user-level thread and kernel-level thread. Source: Operating Systems: Internal and Design Principles (9th ed.), 2018 p. 184 Multithreading Multithreading pertains to the ability of an operating system (OS) to support multiple, concurrent paths of execution within a single process. The use of multicore systems to provision applications with multiple threads affects the application design and performance. The potential performance benefits of a multicore system may also be subjected to the ability of a multithreaded application to effectively exploit the parallel resources available to the application. Moreover, an object-oriented multithreaded process is an efficient Figure 3. A pure kernel-level thread. means of implementing a server application (Stallings, 2018). Source: Operating Systems: Internal and Design Principles (9th ed.), 2018 p. 184 Below are some of the general characteristics of multithreading (Gregg, 2021): Combined User-Level and Kernel-Level Approach The memory overhead in multithreading is small. It only requires an extra Some operating systems provide a combined user-level thread and kernel- stack, register space, and space for thread-local data. level thread facility. Thread creation is performed completely in the user space, The central processing unit (CPU) overhead is small since it uses so is the scheduling and synchronization of threads within an application. application programming interface (API) calls. Multiple user-level threads from a single application are then mapped onto The communication within the process and with other processes are specific kernel-level threads. faster. The crash resilience in implementing multithreading is low. Any bug can In this combined approach, multiple threads within the same application can crash the entire application run in parallel on multiple processors, and a blocking system call does not The memory usage is monitored via a system allocator, which may incur block the entire process. If properly designed, the combined approach should some CPU contention from multiple threads, and fragmentation before the also combine the advantages of the pure user-level threads and the kernel- memory is reused. 03 Handout 1 *Property of STI  [email protected] Page 3 of 4  IT2106 The following are some examples of multithreaded applications: An application that creates thumbnails of photos from a collection of images A web browser that displays images or texts while retrieving data from the network A word processor that displays texts and responds to keystrokes or mouse clicks while performing spelling and grammar check The benefits of multithreaded programming can be categorized as follows (Silberschatz, Galvin, & Gagne, 2018): Responsiveness: Incorporating the multithreading concept in an interactive application allows a program to continue running even if some part of it is blocked or performing a lengthy operation. Thus, increasing the responsiveness of the application to the user. Resource Sharing: Generally, threads share the memory and resources of the process to which they belong. This allows applications to have several different threads of activities within the same address space. Economical: Allocating memory and resources for process creation is inefficient, but thread creation consumes less time and memory, making it economical. Scalability: The benefit of multithreading is greater when dealing with multiprocessor architecture, where threads can run in parallel on different processing cores. Windows is a good example of an OS that supports multithreading. Threads in different processes may execute concurrently or appear to run at the same time. Additionally, multiple threads within the same process may be allocated to different processors and execute simultaneously. In a Windows OS, threads within the same process can exchange information through their common address space and access shared resources of the process. Threads in different processes can exchange information through some shared memory that has been set up between two (2) processes References: Gregg, B. (2021). System performance: Enterprise and cloud (2nd ed.). Pearson Education, Inc. Silberschatz, A., Galvin, P. & Gagne, G. (2018). Operating systems concepts (10th ed.). John Wiley & Sons, Inc. Stallings, W. (2018). Operating systems: Internal and design principles (9th ed.). Pearson Education Limited 03 Handout 1 *Property of STI  [email protected] Page 4 of 4