Thrashing Swapping out a piece of a process just before that piece is needed The processor spends most of its time swapping pieces rather than executing user instructions
Principle of Locality Program and data references within a process tend to cluster Only a few pieces of a process will be needed over a short period of time Possible to make intelligent guesses about which pieces will be needed in the future This suggests that virtual memory may work efficiently
Thrashing If a process does not have “enough” pages, the page-fault rate is very high. This leads to: ▫low CPU utilization ▫operating system thinks that it needs to increase the degree of multiprogramming ▫another process added to the system Thrashing a process is busy swapping pages in and out
Demand Paging and Thrashing Why does demand paging work? Locality model ▫Process migrates from one locality to another ▫Localities may overlap Why does thrashing occur? size of locality > total memory size
Working-Set Model working-set window a fixed number of page references Example: 10,000 instruction WSS i (working set size of Process P i ) = 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 = WSS i total demand frames if D > m Thrashing Policy if D > m, then suspend one of the processes
Keeping Track of the Working Set Approximate with interval timer + a reference bit Example: = 10,000 ▫Timer interrupts after every 5000 time units ▫Keep in memory 2 bits for each page ▫Whenever a timer interrupts copy and sets the values of all reference bits to 0 ▫If one of the bits in memory = 1 page in working set Why is this not completely accurate? Improvement = 10 bits and interrupt every 1000 time units
Page Size Smaller page size, less amount of internal fragmentation Smaller page size, more pages required per process More pages per process means larger page tables Larger page tables means large portion of page tables in virtual memory Secondary memory is designed to efficiently transfer large blocks of data so a large page size is better
Page Size Small page size, large number of pages will be found in main memory As time goes on during execution, the pages in memory will all contain portions of the process near recent references. Page faults low. Increased page size causes pages to contain locations further from any recent reference. Page faults rise.
Frame Allocation: Issues Smaller no. of frames to a process – more processes can reside in MM – increases probability that OS will find at least one ready process Despite Principle of Locality, Page faults rate will be high Beyond a certain size, additional allocation will have no noticeable effect on page fault rate because of Principle of Locality
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.
Priority Allocation Use a proportional allocation scheme using priorities rather than size. If process P i generates a page fault, –select for replacement one of its frames. –select for replacement a frame from a process with lower priority number.
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.
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