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Review: Chapters 1 – 7 Chapter 1: OS is a layer between user and hardware to make life easier for user and use hardware efficiently Control program or resource allocator Computer organization CPU(s), memory, and I/O devices connect to a common bus Devices request CPU attention through interrupts Storage hierarchy: speed, cost, volatility Caching: copy frequently used data to faster storage Multiprogramming: multiple jobs in memory efficiency Timesharing: frequently switch between jobs interactive, short response time users get the impression that each has his/her own computer Dual mode operation: user and kernel modes Protect OS and users from each other Privileged instructions executed only in kernel mode Timer to prevent processes from holding resources forever 1.2 Operating-System Operations OS is interrupt driven: it sits idle till something happens Interrupts are generated by devices (hardware) Traps (or exceptions) are software-generated interrupts due to software errors, e.g., divide by zero Request for operating system services (system calls) Dual-mode operation allows OS to protect itself and other system components User mode and kernel mode Mode bit provided by hardware Provides ability to distinguish when system is running user code or kernel code Some instructions designated as privileged, only executable in kernel mode System call changes mode to kernel, return from call resets it to user 1.3 Transition from User to Kernel Mode 1.4 Chapter 2: OS Services and Structures OS provides two sets of services for user convenience and efficient use of resources System calls: programming interface to OS services Typically used through APIs for portability and ease OS structures monolithic layered microkernel modular 1.5 Chapter 3: Processes Process is a program in execution OS maintains process info in PCB Process State diagram Creating and terminating processes (fork) Process scheduling Long-, short-, and medium-term schedulers Scheduling queues 1.6 Scheduling: The Big Picture Midterm sched. Jobs Disk Job sched. CPU sched. In most small and interactive systems (UNIX, WinXP, …), only the CPU scheduler exists 1.7 Process Lifetime 1.8 CPU Switch From Process to Process (Context Switch) When switching occurs, kernel Saves state of P0 in PCB0 (in memory) Loads state of P1 from PCB1 into registers State = values of the CPU registers, including the program counter, stack pointer 1.9 Interprocess Communications Models Message Passing Shared Memory 1.10 Chapter 4: Threads A thread is a basic unit of CPU utilization, a process is composed of one or more threads Each thread has: Program counter, stack, registers Threads share: code, data, OS resources (e.g., open files and signals) 1.11 Single and Multithreaded Processes Shared among threads 1.12 User level threads vs. kernel threads 1.13 Chapter 5: CPU Scheduling Process execution: cycle of CPU bursts and I/O bursts CPU bursts lengths: many short bursts, and few long ones Scheduler selects one process from ready queue Dispatcher performs the switching Scheduling criteria (usually conflicting) CPU utilization, waiting time, response time, throughput, … Scheduling Algorithms FCFS, SJF, Priority, RR, Multilevel Queues, … 1.14 First-Come, First-Served (FCFS) Scheduling Process Burst Time P1 24 P2 3 P3 3 Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: P1 P2 0 24 Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 1.15 P3 27 30 Multilevel Feedback Queues 1.16 CPU Scheduling Multiprocessor Scheduling Processor affinity vs. load balancing Evaluation of Algorithms Modeling, simulation, implementation 1.17 Chapter 6: Synchronization Processor Synchronization Race condition Techniques to coordinate access to shared data Multiple processes manipulating shared data and result depends on execution order Critical section problem Three requirements: mutual exclusion, progress, bounded waiting Software solution: Peterson’s Algorithm Hardware support: TestAndSet(), Swap() Busy waiting (or spinlocks) Semaphores: Not busy waiting wait(), signal() must be atomic moves the CS problem to kernel Monitors High-level (programming language) constructs 1.18 Synchronization Some classical synchronization problems Consumer-producer Dining philosopher Readers-writers 1.19 Chapter 7: Deadlock A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set Four necessary (but not sufficient) conditions Mutual exclusion: only one process at a time can use a resource Hold and wait: a process holding at least one resource and is waiting to acquire additional resources held by other processes No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task Circular wait: there exists a set {P0, P1, …, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0 1.20 Deadlock Handling Prevention: ensure that at least one of the necessary conditions does not hold Avoidance: decide for each request whether or not the issuing process should wait to avoid leaving the system in unsafe state Resource-allocation graph: single instance of a resource type Banker’s algorithm: multiple instances of a resource type Detection and Recovery Detection algorithm Recovery: process termination or resource preemption 1.21 Good Luck on the Exam! 1.22