Operating Systems

Operating Systems Prof. Navneet Goyal Department of Computer Science & Information Systems BITS, Pilani

Topics for Today • Thrashing • Working Set

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

Locality In A Memory-Reference Pattern

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

Thrashing (Cont.)

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 • WSSi (working set size 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

Working-set model

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

Frame Allocation Schemes • Fixed Allocation • Equal Allocation • Proportional Allocation • Priority Allocation

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 Pi 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.

Operating Systems