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Operating system

Operating system






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    Operating system Operating system Presentation Transcript

    • Operating System
    • Operating System Functions of OS • scheduler (time allocation to programs) , • synchronizer ( avoid conflicts for a resource at the same time) • memory manager (space allocation to programs)
    • Scheduling • Scheduling Algo • basically utilize IO time by scheduling another task • Switching from one thread to another (A --> B ) can occur when a) A is complete b) A is blocked on IO c) Timer interrupt forcefully preempts A
    • Synchronization • Synchronization is needed as process need to exchange information using a shared region in memory. • However, this shared region cannot be accessed in any manner. They needed to be accessed in correct sequence
    • Synchronization The code accessing Shared Data is called Critical Section No more than one process can access shared data at a given time Critical Sections must obey mutual exclusion
    • Synchronization • How to ensure Mutual exclusion (only one process is in critical section) • Disable interrupts • Test and set instructions • Semaphore
    • Synchronization • Semaphores • Used to solve Producer Consumer problems • Producer consumer problems occur for example in networks (sending and receiving packets), unix shell IO
    • Synchronization • Producer Code down(empty_slot) put item in buffer up(full_slot)
    • Synchronization • The above solution works for a single producer consumer • When multiple producers and consumers are present , then a race condition can happen resulting in writing to the same slot by two different producers • To overcome this problem, we need to make sure that only one producer is executing "put item in buffer". So we need to execute mutual exclusion within a critical section
    • Synchronization • down(empty_slot) • down(mutex) • put item in buffer • up(mutex) • up(full_slot)
    • Memory Management • How to run process when not enough physical memory? • How to protect one process from accessing another’s memory
    • Memory Management • How to run process when not enough physical memory? • How to protect one process from accessing another’s memory VIRTUAL MEMORY provides illusion of more memory Each process access only a window of memory – due to Locality of Reference Only a small portion of address space must be in physical memory
    • Memory Management • Each layer acts as “backing store” for layer above disk storage main memory cache registers size speed cost
    • vpn … vpn 3 vpn 2 vpn 1 vpn 0 P1 Virtual pages Frame # Present bit … … Blk 1031 0 Blk 1030 0 Fr # 15 1 Blk 1030 … Blk 1031 Fr #15 Fr #16 Fr # 16 1 … PAGE TABLE
    • Memory Management Page Replacement