Operating Systems

Objectives • • • • • Describe the functions and layers of an operating system List the resources allocated by the operating system and describe the allocation process Explain how an operating system manages processes and threads Compare CPU scheduling methods Explain how an operating system manages memory Operating System Overview • Important part of all information systems • Manages all hardware resources and allocates them to users and applications as needed • Accesses files and directories, creates and moves windows, and accesses resources over a network Operating System Functions • Functions are divided into five main groups • Resource allocation: – Ensure that overall system objectives are achieved efficiently and effectively – Bridge between users, their processes, and the hardware resources used by these processes. Operating System Hardware Interface (device drivers) Interrupt Processing Resource Allocation Services (service layer) Users and Security CPU Timer interrupts Scheduling Process/thread management Account management Primary storage Page fault and protection interrupts Virtual memory management Interprocess communications Authentication Secondary storage Storage device interrupts Storage I/O control File/folder services Authorization I/O devices I/O device interrupts Interactive and network I/O Network services Console and GUI services Operating System Layers • Using layers makes the OS easier to maintain • Command layer (shell) • Command language or job control language (JCL) • Service layer • Service call • Kernel Resource Allocation • Single-tasking and multitasking • Resource allocation tasks • Real and virtual resources Single-Tasking Resource Allocation • Involves only two running programs – Application – OS (grants the application all unused hardware resources) • Single-tasking operating systems are small and efficient • There’s only one active program Multitasking Resource Allocation • Norm for modern general-purpose computers • Allows flexibility of application and system software • Resource allocation goals – Meet resource needs of each program – Prevent programs from interfering with one another – Efficiently use hardware and other resources Resource Allocation Tasks • Keep detailed records of available resources; know which resources can satisfy which requests • Schedule resources based on specific allocation policies • Update records to reflect resource commitment and release by programs and users Real and Virtual Resources • Real resources – Physical devices and associated system software • Virtual resources – Resources that are apparent to a process or user – Meet or exceed real resources by: • •Rapidly shifting resources unused by one program to other programs that need them • Substituting one type of resource for another Process Management • Process – Unit of executing software managed independently by OS – Can request and receive hardware resources and OS services – Can be stand-alone or part of a group that cooperates to achieve a common purpose – Can communicate with other processes executing on the same computer or on other computers Process Control Data Structures • Process control block (PCB) – Created, updated, and deleted by OS – Used by OS to perform many functions (e.g., resource allocation, secure resource access, protecting active processes from interference with other active processes) – Normally organized into a larger data structure (called a linked list, process queue, or process list) Process Control Data Structures (continued) • Processes can spawn other processes and communicate with them – Parent process – Child process – Sibling process – Process family Threads • Portion of a process that can be scheduled and executed independently • Can execute concurrently on a single processor or simultaneously on multiple processors • Share all resources allocated to parent process • Advantage: reduce OS overhead for resource allocation and process management • Thread control block (TCB) and run queues CPU Allocation • OS makes rapid decisions about which threads receive CPU control and for how long that control is retained • Threads usually share CPUs (concurrent or interleaved execution) Thread States • Ready – Waiting for access to the CPU • Running – Retains control of CPU until the thread or its parent process terminates normally or an interrupt occurs • Blocked – Waiting for some event to occur (completion of service request or correction of an error condition) Interrupt Processing • Thread can be blocked waiting for resources • Thread is put into a wait state and its state stored on the stack • Some interrupt handler will process the blocked thread’s request • When the resource has been allocated, thread is moved from block state to ready or running state • The thread remains in the blocked state until the request is satisfied or a time out occurs Scheduling • Decision-making process used by OS to determine which ready thread moves to the running state • Typical methods – Preemptive scheduling – Priority-based scheduling – Real-time scheduling Preemptive Scheduling • A thread can be removed involuntarily from the running state • Functions of the supervisor (portion of OS that receives control) – Calls appropriate interrupt handler – Transfers control to the scheduler • Functions of the scheduler – Updates status of any process or thread affected by last interrupt – Decides which thread to dispatch to the CPU – Updates thread control information and the stack to reflect the scheduling decision – Dispatches selected thread • Processing steps on left occur after Thread 1 makes an I/O service call • Processing steps on right occur after I/O device completes I/O operation Timer Interrupts • Generated at regular intervals by CPU to give scheduler an opportunity to suspend currently executing thread • Not a “real” interrupt • No interrupt handler to call • Supervisor passes control to the scheduler • Important CPU hardware feature for multitasking OSs Priority-Based Scheduling • Determines which ready thread should be dispatched to the CPU according to: – First come first served (FCFS) – Explicit priority – Shortest time remaining (STR) Real-Time Scheduling • Guarantees minimum amount of CPU time to a thread if the thread makes an explicit realtime scheduling request when it is created • Guarantees a thread enough resources to complete its function within a specified time • Often used in transaction processing, data acquisition, and automated process control Memory Allocation • The assignment of specific memory addresses to system software, application programs, and data • OS allocates memory • –When threads are created; responds to requests for additional memory during a thread’s lifetime – To itself and for other needs (buffers and caches) Physical Memory Organization • Main memory can be regarded as a sequence of contiguous, or adjacent, memory cells • Most significant byte • Least significant byte • Addressable memory • Physical memory Single-Tasking Memory Allocation • Bulk of OS normally occupies lower memory addresses – Application program is loaded immediately above it • Contiguous memory allocation • Address resolution – Process of determining physical memory address that corresponds to memory reference Multitasking Memory Allocation • The operating system: – Finds free memory regions in which to load new processes and threads – Reclaims memory when processes or threads terminate Multitasking Memory Allocation (continued) • Goals of multitasking memory allocation – Allow as many active processes as possible – Respond quickly to changing memory demands of processes – Prevent unauthorized changes to a process’s memory region(s) – Perform memory allocation and addressing as efficiently as possible Memory Fragmentation • Occurs when memory partitions allocated to a single process or purpose are scattered throughout physical memory • To address the problem – Compaction (large overhead) – Noncontiguous memory allocation Noncontiguous Memory Allocation • Portions of a process can be allocated to free partitions anywhere in memory • Uses small fixed-sized partitions • More flexible than contiguous memory allocation, but requires more complex partition tables and address calculations Virtual Memory Management • Allocates portions of processes (pages) to small memory partitions (page frames) • Swaps pages between memory and secondary storage as needed • Page hits, page faults, page tables • Page files and victims Memory Protection • Refers to protecting memory allocated to one program from unauthorized access by another program • Prevents errors in one program from generating errors in another • Adds overhead to each write operation Memory Management Hardware • Complex memory management procedures incur substantial overhead • Modern CPUs incorporate advanced memory allocation and address resolution functions in hardware (e.g., Intel Pentium)