Computer Science 330
Operating Systems

Fall 2022, Siena College

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Agenda

• Announcements
  • Sign up for the CS Holiday Party! We need to send catering numbers today.
  • Programming Project 4: Roger Bacon Shell with Pipes and Redirection [HTML] [PDF] hard deadline on Wednesday
  • If you haven't completed the readings/activities from OS zyBook: Chapter 8, get them done before we talk about that chapter on Monday
  • The next (and last) readings/activities from OS zyBook: Chapter 9, due Friday, December 9
• Frame allocation
  • each process needs some number of frames
    • pages for instructions, local data, global data
  • how do we decide how many frames are allocated to each process?
    • equal allocation - everyone gets the same
    • proportional allocation - processes get frames proportional to the amount of logical memory
    • priority allocation - prioritize memory like we prioritize CPU scheduling
  • local replacement vs. global replacement - does a victim have to be selected from the page faulting process' frames?
  • thrashing
    • if we have too many processes, not enough frames to keep a reasonably low page fault rate
    • the medium-term scheduler may notice a low CPU utilization (everyone's waiting on page faults) and add more processes to the mix
    • this will only make the situation worse
    • need to be able to detect this and have the medium-term or long-term scheduler reduce the degree of multiprogramming
  • Working set model
    • each process will have a set of pages on which it is working frequently at a given time
    • this is the working set of a process
    • a process' working set changes over time
    • approximate the working set as the set of pages that a process has accessed within some window of time into the recent past
    • if this working set window is too small, the working set we detect will not encompass the true locality
    • if it is too large, pages will remain in the working set too long after the process has moved on
    • total demand for frames in the system is the sum of the working sets of all processes
    • thrashing is likely when this total demand outstrips availability
    • this is used in reality in many modern systems (Solaris, Windows, to name two)
    • we will look more closely at this in a Friday lab exercise
• Combinations of Segmentation and Paging
  • Intel Pentium memory hardware
    • Lots of details in Intel 64 and IA-32 Architecture Software Developer's Manual, Don't print it - it's over 1900 pages. Memory management is described in great detail in Chapter 3.
• Looking at the exercises for OS zyBook Chapter 7
  • Section 7.2, all
  • Section 7.3, all
  • Section 7.4, all
  • Section 7.5, all
• Lab 8: Working Set Simulation [HTML] [PDF]