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Memory and Process Management, Jazrin, Jazrin, Jazrin, Haziq, Haziq -…

- - - - MUTUAL EXCLUSION
        
        A resources that cannot be used more than one process at time
      - HOLD-AND-WAIT
        
        Each thread holds one resource while waiting for another
      - NO PREEMPTION
        
        Thread can only release resources voluntarily.
      - CIRCULAR WAIT
        -Two or more processes form circular chain , each process wait for a resources that the next process in the chain holds.
    - - PREVENTION
        
        MUTUAL EXCLUSION
        
        The mutual-exclusion condition must hold for non-sharable resources.
        
        WEAKNESS: resource utilization may be low and starvation
        
        HOLD-AND-WAIT
        -Whenever a process requests a resource, it does not hold any other resources
        
        CIRCULAR WAIT -never holds is to impose a total ordering of all resource types
        
        NO PREEMPTION
        
        Applied resources that easily saved and restored later such as CPU registers and memory space
      - AVOIDANCE
        
        Restraining how requests can be made.
        
        The restraints ensure that one necessary conditions for deadlock cannot occour or hold
      - SOLUTION
        
        NON-BLOCKING SYNCHRONIZATION ALGORITM
        
        Competing for a shared resource do not have their execution indefinitely postponed by mutual exclusion.
        
        Lock free if there is guaranted system-wide progress
        
        Wait-free if there is also guaranteed per-thread progress.
        
        SERIALIZING TOKENS
        
        A concept in concurrency control arising.
        
        LOCK-FREE AND WAIT-FREE ALGORITHM
        
        The progress of at least one thread when executing multithreaded procedures
        
        RESOURCE HIERARCHY SOLUTION
        
        It is not always practical
        
        When the list of required resources is not completely known in advance.
  - - - A process requests resources; and if the resources are not
        available at that time, the process enters a waiting state.
        
        Sometimes, a waiting process is never again able to change state, because the resources it has requested are held by other waiting processes.
        
        This situation is called a Deadlock
    - - Deadlock is a common problem in multiprocessing where many processes share a specific type of mutually exclusive resource known as a software lock or soft lock.
        
        Deadlocks are particularly troubling because there is no general solution to avoid (soft) deadlocks.
        
        A set of blocked processes each holding a resource and waiting acquire a resource held by another process in the set.
      - Example
        
        This situation may be likened to two people who are drawing
        diagrams, with only one pencil and one ruler between them
        
        If one person takes the pencil and the other takes the ruler, a deadlock occurs when the person with the pencil needs the ruler and the person with the ruler needs the pencil to finish his work with the ruler.
        
        Both requests can't be satisfied , so a deadlock occurs.
        
        Bridge Crossing Example
        
        Traffic only in one direction.
        
        Each section of a bridge can be viewed as a
        resource.
        
        If a deadlock occurs, it can be resolved if one
        car backs up (preempt resources and rollback).
        
        Several cars may have to be backed up if a
        deadlock occurs.
        
        Starvation is possible.
        
        Printers Scenario
        
        A system may have two printers. These 2 printers may be defined to be in the same resource class if no one cares which printer prints which output.
        
        However, if 1 printer is on the ninth floor and the other is in the basement, then people on the ninth floor may not see both printers as equivalent, and separate resource classes may need to be defined for each printer.
- - - - those from error conditions or problems
        with memory) that are so important ( can't be ignore)
      - non-maskable interrupts (NMIs) must be dealt with immediately, regardless of the
        other tasks at hand.
    - - a processor
        execute instructions
        
        Prevents a process from monopolizing the system
      - OS
        
        sets interrupting clock/interval timer to allow user to
        run in a specific time interval.
  - - - carried out by the
        short-term scheduler (or CPU scheduler).
    - - Medium-Term Scheduler
        
        may decide to swap out a process
        
        Has not been active for some time / has a low
        priority
        
        page faulting frequently
        
        taking up a large amount of memory in
        order to free up main memory for other processes
      - Short-Term Scheduler
        
        A.K.A the dispatcher.
        
        Decides which of the ready in:
        
        memory processes are to be executed (allocated a CPU)
        
        a clock interrupt, an IO interrupt, an operating
        system call or another form of signal.
        
        scheduling decisions much more frequently
        than the long-term or mid-term schedulers
      - Long-Term Scheduler
        
        long-term A.K.A admission scheduler
        
        decides which jobs or processes are to be admitted to the ready queue.
        
        this scheduler dictates what processes are to run on a system, and the degree of concurrency to be supported at any one time
  - - - the act of temporarily interrupting a task being carried out by a computer system, without requiring its cooperation, and with the intention of resuming the task at a later time.
    - - 2.Terminates
      - 3.Switches from waiting to ready
      - 1.Switches from running to waiting state
      - Switches from running to ready state
    - - When scheduling takes place only under
        circumstances 1 and 2
      - once the CPU has been allocated to a process, the process keep the CPU until it release the CPU either by terminating or by switching to waiting state
    - - incurs a cost associated
        with access to share data.
      - -Consider the case of two processes that share a data.
  - - - A process is executed until it must wait, typically for the completion of some I/O request. In a simple computer system, the CPU then just sits idle. All this waiting time is wasted; no useful work is accomplished. With multiprogramming, we try to use this time productively. Several processes are kept in memory at one time. When one process has to wait, the operating system takes the CPU away from that process and gives the CPU to another process. This pattern continues.
        Every time one process has to wait, another process can take over use of the CPU.
      - Scheduling of this kind is a fundamental operating- system function. Almost all computer resources are scheduled before use. The CPU is, of course, one of the primary computer resources. Thus, its scheduling is central to operating-system design.
      - The success of CPU scheduling depends on an observed property of processes:
        
        Process execution consists of a cycle of CPU execution and I/O wait.
      - Processes alternate between these two states. Process execution begins with a CPU burst. That is followed by an I/O burst, which is followed by another CPU burst, then another I/O burst, and so on. Eventually, the final CPU burst ends with a system request to terminate execution
      - The durations of CPU bursts may vary greatly from process to process and from computer to computer, they tend to have a frequency curve similar to that shown in next figure. (exponential or hyper-exponential) with a large number of short CPU bursts and a small number of long CPU bursts.
      - An I/O-bound program typically has many short CPU bursts. A CPU-bound program might have a few long CPU bursts. This distribution can be important in the selection of an appropriate CPU-scheduling algorithm.
    - - Note that the ready queue is not necessarily a first-in, first-out (FIFO) queue.
        
        As we shall see when we consider the various scheduling algorithms, a ready queue can be implemented as a FIFO queue, a priority queue, a tree, or simply an unordered linked list. Conceptually, however, all the processes in the ready queue are lined up waiting for a chance to run on the CPU. The records in the queues are generally process control blocks (PCBs) of the processes.
- - - - Textbook uses the terms “job” and “process” almost interchangeably
      - Process –a program in execution; process execution must progress in sequential fashion
      - A process includes:
        
        program counter
        
        stack
        
        data section
      - An operating system executes a variety of programs: Batch system –jobs Time-shared systems –user programs or tasks
  - - - A process in the ready state has all of the resources that it needs for further execution except for a processor. It is normally held in a ready queue until a processor becomes available.
    - - A process in the running state has all of the resources that it needs for further execution, including a processor.
    - - A process that needs some resource other than a processor for further execution is in a blocked state. It is usually placed in a queue waiting for the needed resource.