Week 7 and 8

Question 1

What is Belady’s Anomaly?

Answer 1

In Page Referencing the phenomenon in which increasing the number of page frames results in an increase in the number of page faults for certain memory access patterns

Question 2

local vs global frame allocation

Answer 2

local: choose from a frame already owned by process (number of frames is fixed, cannot surrender extra pages)
global: processes can steal frames from other processes (allows pages to be reshuffled, more commonly used)

Question 3

What is thrashing? how do you detect it? and how do you fix it

Answer 3

Thrashing occurs when there is not enough page frames so you constantly keep page faulting (can detect because the page fault frequency is high and CPU utilization low)
- to fix it put processes to sleep and reallocate their pages

Question 4

What do you do if the page fault rate is too small?

Answer 4

take frames away

Question 5

What do you do if the page fault rate is too large

Answer 5

give the process more frames (may have to suspend another process to give more frames)

Question 6

What is a segment?

Answer 6

A segment is a separate logical address space used for various elements of the program process

Question 7

What is the file system?

Answer 7

Abstract layer above secondary storage (system most people interact with) - smallest allotment of secondary storage, all user data stored in files

Question 8

Single level directory vs two level directory

Answer 8

single: organization difficult (small disks)

Two: top level is directory, second level is user file directory, files are in user directories, path concept

Question 9

what is mounting?

Answer 9

Before a directory is accessible to the file system it must be mounted

• operating system checks the disk to make sure it has a valid file system on it
• loads info about the file system into internal structures for future access

Question 10

What is a mount point?

Answer 10

Where a file is accessed through

Question 11

The first mounted system is mounted at location ‘/’ - know as the what? of the file system?

Answer 11

root

Question 12

What are the levels of the file system

Answer 12

• logical file system
• file organization level
• basic file system
• I/O control
• devices

Question 13

What is the boot control block?

Answer 13

block containing code to start the operating system

Question 14

What is the partition control block?

Answer 14

aka the super block. Controls information inside of the partition

Question 15

What is the file control block?

Answer 15

one for each file. Contains information about each file

Question 16

opening a file

Answer 16

• search directory structure
• see if FCB is already in memory
• allocate entry in process FCB table

Question 17

What is the virtual file system?

Answer 17

Layer above the file system that maps file system specific view to operating system view

Question 18

What is a ‘Bit Vector’?

Answer 18

Free space management

one or more blocks, each bit represents data block in the system, easy to get contiguous space

Question 19

What is 80 - 20 reffering to?

Answer 19

Efficiency
it means 80% of processes are short vs 20% being long processes. It was that graph he showed like week 5 for the quantum

Question 20

What are journaled file systems?

Answer 20

• transaction based
• reliable log is used
  (actions are written to log and put on disk before actions is taken, then removed from log -> don’t have to check entire disk, just look at files who have entries in the log)

Question 21

What is DMA?

Answer 21

Direct memory access - DMA controller for the bust mediates the transfer of data in and out of the controllers

• DMA holds memory bus for controller
• DMA steals cycle from CPU access