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Chapter 2: Memory and Process Management, THE MEANING, Preemptive…

- - - - Running
        
        A process in the running state has all of the resources that it needs for further execution, including a processor
      - Blocked
        
        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
      - Ready
        
        A process in the ready state has all of the resources that it needs for further execution except for a processor
        
        normally held in a ready queue until a processor becomes available
  - - - CPU time
        
        definition :
        the amount
        of time for which a central processing unit(CPU) was
        used for processing instructions of a computer program
        
        often measured in clock ticks or
        seconds
        
        also mentioned as percentage of the CPU's
        capacity at any given time on multi-tasking environment
        
        helps in figuring out how a CPU’s computational
        power is being shared among multiple computer
        programs
        
        elapsed real time (or simply real time, or wall
        clock time) is the time taken from the start of a computer
        program until the end as measured by an ordinary clock
      - Memory
        
        RAM
        
        Random access memory (RAM) is a form of computer data storage
        
        RAM allows stored data to be accessed in any
        order in very nearly the same amount of time for any storage location
        or size of memory device
        
        device such as a magnetic tape requires increasing time to access
        data stored on parts of the tape that are far from the ends
        
        Memory devices (such as floppy disc )can access
        the storage data only in a predetermined order, because of
        mechanical design limitations; the time to access a given part of the
        device varies significantly due to its physical location
        
        VIRTUAL MEMORY
        
        a memory
        management technique developed for
        multitasking kernels
        
        virtualizes a computer architecture's
        various forms of computer data storage allowing a program to
        be designed as though there is only one kind of memory,
        "virtual" memory, which behaves like directly addressable
        read/write memory (RAM)
      - Hard disk space
        
        'disk space' is an amount of computer storage
        space on random-access memory devices, such as on a hard
        drive, floppy or USB flash drive
        
        commonly measured in large numbers
        of bytes, such as Kilobytes, Megabytes, and Gigabytes, with
        each unit 1024x times larger than the previous unit
        
        term originated in the 1950s
        
        the total disk space can span multiple devices, such
        as areas on an array of disk drives
      - Network throughput
        
        the average
        rate of successful message delivery over a
        communication channel
        
        data may be delivered over a physical or logical link,
        or pass through a certain network node
        
        usually measured in bits per second
        (bit/s or bps), sometimes in data packets per second
        or data packets per time slot
      - Electrical power
        
        rate
        at which electric energy is transferred by an electric circuit
        
        SI unit of power is the watt, one joule per second.
      - External Devices
        
        component that can be connected to your computer
        
        called peripheral input or output
        
        two types of peripheral, input devices and output devices
        
        INPUT
        
        used to provide information (or data) to the computer system
        
        e.g : keyboard(typing text),mouse
        (pointing), scanner (scanning paper documents)
        
        OUTPUT
        
        used to bring out information
        from the computer system
      - Input/output operations
        
        IOPS ( Input/Output Operations Per Second, pronounced eye-ops) is
        a common performance measurement used to benchmark computer
        storage devices like hard disk drives (HDD), solid state drives (SSD),
        and storage area networks (SAN)
        
        as with any benchmark, IOPS numbers published by storage device
        manufacturers do not guarantee real-world application performance
        
        IOPS can be measured with applications, such as Iometer
- - - - Long-Term Scheduler
        
        a) The long-term, or admission, scheduler decides which
        jobs or processes are to be admitted to the ready queue.
        b) When an attempt is made to execute a program,its
        admission to the set of currently executing processes is
        either authorized or delayed by the long-term scheduler.
        c) Thus, this scheduler dictates what processes are to run on a system, and the degree of concurrency to be
        supported at any one time
        
         Long-term scheduler (or job scheduler) – selects which
        processes should be brought into the ready queue
         Long-term scheduler is invoked very infrequently
        (seconds, minutes)  (may be slow)
         The long-term scheduler controls the degree of
        multiprogramming
      - Medium-Term Scheduler
        
        The mid-term scheduler may decide to swap out a process
        which :
         Has not been active for some time, or a process which has a low
        priority, or
         A process which is page faulting (kesalahan muka surat)
        frequently, or
         A process which is taking up a large amount (julat) of memory in order to free up main memory for other processes, swapping the process back in later when more memory is available, or
         When the process has been unblocked and is no longer waiting for
        a resource.
      - Short-Term Scheduler
        
        a) Also known as the dispatcher.
        b) Decides which of the ready, in-memory processes are to
        be executed (allocated a CPU) next following a clock interrupt, an IO interrupt, an operating
        system call or another form of signal.
        c) Makes scheduling decisions much more frequently
        than the long-term or mid-term schedulers
        d) Short-term scheduler
        
         Short-term scheduler (or CPU scheduler) – selects which
        process should be executed next and allocates CPU
         Short-term scheduler is invoked very frequently (milliseconds)
         (must be fast)
         Processes can be described as either:
        i) I/O-bound process – spends more time doing I/O than
        computations, many short CPU bursts
        ii) CPU-bound process – spends more time doing computations; few
        very long CPU bursts
- - - - 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
      - known as a context switch
      - normally carried out by a privileged task or part of the system known as a preemptive scheduler
      - which has the power to preempt, or interrupt, and later resume, other tasks in the system
    - - When scheduling takes place only under circumstances 1 and 2, we say that the scheduling scheme is non-preemptive
        
        Under non-preemptive scheduling, 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. (Windows 95 and earlier)
      - Otherwise, its called preemptive
        
        incurs a cost associated
        with access to share data.
        
        Consider the case of two processes that share a
        data
        
        It is preemptive so that while one process is updating the data, the second process then tries to read the data, which are in an inconsistent state
  - - - 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 to acquire a
        resource held by another process in the set.
    - - 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.
      - PRINTER SCENARIO
        
        A system may have two printers. These two printers may be defined to be in the same resource class if no one cares which printer prints which output.
        
        However, if one 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
  - - - Hold-and-wait condition: each thread holds one resource while
        waiting for another.
        
        No preemption condition: thread can only release resources
        voluntarily. No other thread (or OS) can force the thread to release.
        
        Circular wait condition: two or more processes form a circular chain where each process waits for a resource that the next process in the chain holds.
        
        All four conditions must occur before the deadlock
        happens.
        
        There will be no deadlock if at least one of the
        conditions unhold.
- - - - Mutual Exclusion: The mutual-exclusion condition must hold for non-sharable resources. (Weaknesses: resource utilization may be low, and starvation)
      - Hold and Wait: whenever a process requests a resource, it does not hold any other resources.
      - No Preemption: no preemption of resources that have already been allocated. (often applied to resources whose state can be easily saved and restored later, such as CPU registers and memory space)
      - Circular Wait: to ensure that this condition never holds is to impose a total ordering of all resource types and to require that each process requests resources in an increasing order of enumeration.
  - - - 1. Non-blocking synchronization algorithm
        
        a non-blocking algorithm ensures that threads competing for a shared resource do not have their execution indefinitely postponed by mutual exclusion. A non-blocking algorithm is lock-free if there is guaranteed system-wide progress; wait-free if there is also guaranteed per-thread progress
      - 2. Serializing tokens
        
        serializing tokens are a concept in concurrency control arising from the ongoing development of DragonFly BSD. According to Matthew Dillon, they are most akin to SPLs, except a token works across multiple CPUs while SPLs only work within a single CPU's domain.
      - 3.Lock-free and wait-free algorithm
        
        Lock-free data structures guarantee the progress of at least one thread when executing multithreaded procedures, thereby helping you avoid deadlock.
      - 4. Resource hierarchy solution
        
        The resource hierarchy solution avoids deadlocks, it is not always practical, especially when the list of required resources is not completely known in advance.
        
        For example, if a unit of work holds resources 3 and 5 and thendetermines it needs resource 2, it must release 5, then 3 before acquiring 2, and then it must re-acquire 3 and 5 in that order.
        
        Computer programs that access large numbers of database records would not run efficiently if they were required to release all higher-numbered records before accessing a new record, making the method impractical for that purpose.