Operating Systems Lecture 38 Frame Allocation Read Ch. 10.5 - 10.6

1. Operating SystemsLecture 38 Frame Allocation Read Ch. 10.5 - 10.6 Operating System Concepts

2. Page Replacement Algorithms • Page replacement algorithms select the page to be replaced. • Want lowest page-fault rate. • Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string. • In all our examples, the reference string is 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5. • Last time: • FIFO algorithm • Optimal algorithm Operating System Concepts

3. Least Recently Used (LRU) Algorithm • Replace the page that has not been used for the longest period of time. • Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 • Considered a good replacement algorithm. • Question: How do we implement it? Operating System Concepts

4. LRU Counter implementation • Add a logical clock or counter that is incremented with each memory reference. • When a reference is made to a page, the clock register is copied into the time-of-use field in the page table entry. • Replace the page with the smallest time value. • This requires a search of the page table to find the page with the lowest clock value. • It also requires an extra write to memory (of the clock value) for each memory access. Operating System Concepts

5. LRU Algorithm, Stack Implementation • Keep a stack of page numbers. • If a page is referenced, move it to the top of the stack. • The LRU page ends up on the bottom of the stack. • No search is required to find the LRU page. • Because of the need to remove a page from the middle of the stack, implement the stack with a doubly linked list. • Example: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 Operating System Concepts

6. Use Of A Stack to Record The Most Recent Page References Operating System Concepts

7. Allocation of Frames • Each process has a maximum number of frames allocated--we cannot allocate more frames than there are in memory. • Each process needs minimum number of pages. • As the number of frames per process decreases, the fault rate increases. • When a page fault occurs before an instruction is complete, we must restart the instruction. • There must be enough frames to hold all the pages that a single instruction can reference. • Example: IBM 370 – 6 pages to handle Storage to Storage MOVE instruction (SS MOVE): • instruction is 6 bytes, might span 2 pages. • 2 pages to handle from. • 2 pages to handle to. • Two major allocation schemes. • fixed allocation • priority allocation Operating System Concepts

8. Fixed Allocation • Equal allocation – e.g., if 100 frames and 5 processes, give each 20 pages. • Proportional allocation – Allocate according to the size of process. Operating System Concepts

9. Priority Allocation • Use a proportional allocation scheme using priorities rather than size. • If process Pi generates a page fault, either • select for replacement one of its frames. • OR select for replacement a frame from a process with lower priority number. Operating System Concepts

10. Global vs. Local Allocation • Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another. • Local replacement – each process selects from only its own set of allocated frames. • In global replacement a process cannot control its own page fault rate. It's turnaround time may vary depending on the behavior of other processes. • With local replacement, a process may be hindered because it does not have access to other, less used pages in memory. • Global replacement generally has greater system throughput, so it is more commonly used. Operating System Concepts

11. Thrashing • If a process does not have “enough” pages, the page-fault rate is very high. • Thrashing a process is busy swapping pages in and out. • Cause of thrashing: • If using a global replacement scheme, if one process starts needing more frames it will take them from other processes. • These other processes may start faulting more. • There could end up being many processes in queue waiting for pager to swap in needed pages. • This leads to low CPU utilization. • operating system thinks that it needs to increase the degree of multiprogramming to increase CPU utilization. • another process added to the system, making the problem worse. Operating System Concepts

12. Thrashing • Why does paging work?Locality model • Process migrates from one locality to another. • Localities may overlap. • Why does thrashing occur? size of locality > total memory size Operating System Concepts

13. Locality In A Memory-Reference Pattern Operating System Concepts

14. Working-set model • The working-set model is a way of estimating the size of the current locality for a process. •   working-set window  a fixed number of page references • The working set is the set of pages in the most recent D page references. Operating System Concepts

15. Working-Set Model • WSSi (working set of Process Pi) =total number of pages referenced in the most recent  (varies in time) • if  too small will not encompass entire locality. • if  too large will encompass several localities. • if  =   will encompass entire program. • D =  WSSi  total demand frames • if D > m  Thrashing • Policy if D > m, then suspend one of the processes. Operating System Concepts

16. Page-Fault Frequency Scheme • The Page-Fault Frequency scheme is an alternative to the working-set model. • Establish “acceptable” page-fault rate. • If actual rate too low, process loses frame. • If actual rate too high, process gains frame. Operating System Concepts