P3L2 - Memory Management

Question 1

How does the OS decouple physical memory from memory that a process interacts with?

Answer 1

By providing a address space

Question 2

T/F: The range of virtual addresses that are visible to a process can be much larger than the actual amount of physical memory?

Answer 2

True

Question 3

What is allocation?

Answer 3

Requires that the OS incorporate certain mechanisms and data structures so it can track how memory is used and what memory is free?

Question 4

What is arbitration?

Answer 4

Requires that the OS can quickly interpret and verity a process memory access and to translate a virtual address into a physical address and validate it to verify that it is a legal access

Question 5

____ are fixed-size segments that the virtual address space is divided into

Answer 5

Pages

Question 6

____ are the segments that the physical memory is divided into?

Answer 6

Page Frames

Question 7

How does the operating system map pages into page frames?

Answer 7

Via page tables

Question 8

T/F: Paging is not the only way to decouple the virtual and physical memories?

Answer 8

True - Another approach is segmentation, or a segment-based memory approach

Question 9

______ is the dominant memory management mechanism

Answer 9

Paging

Question 10

T/F: Memory management is not done by the OS alone?

Answer 10

True! Hardware mechanisms help

Question 11

What unit is responsible for converting virtual into physical addresses?

Answer 11

The MMU (Memory Management Unit)

Question 12

____ is a small cache of virtual-physical address translations

Answer 12

TLB (Translation Lookaside Buffer)

Question 13

What is the use of page tables?

Answer 13

To convert the virtual memory addresses into physical memory addresses

Question 14

T/F: For each virtual address, an entry in the page table is used to determine the actual physical address?

Answer 14

True!

Question 15

T/F: The sizes of the pages in virtual memory are identical to the sizes of the page frames in physical memory?

Answer 15

True

Question 16

When dealing with a single level page table, what is the first portion of the virtual address known as?

Answer 16

The Virtual Page Number (VPN)

Question 17

When dealing with a single level page table, what is the last portion of the virtual address known as?

Answer 17

The Offset

Question 18

What is produced by using the VPN in paging?

Answer 18

The Physical Frame Number - First physical address of the frame in DRAM

Question 19

Given the physical frame number how is translation completed when dealing with a single level page table?

Answer 19

By using the offset to index into the physical frame to find the appropriate address

Question 20

_____ is when physical memory is only allocated when the process is trying to access it?

Answer 20

Allocation on first touch

Question 21

What is the reason for allocation on first touch?

Answer 21

To make sure that physical memory is only allocated when it’s really needed

Question 22

What is the purpose of multi-level page tables?

Answer 22

To lower the size of a page table as the OS maintains one per process

Question 23

What are inverted page tables?

Answer 23

A page table managed on a system-wide basis as opposed to per process

Question 24

How does one determine the size of a page?

Answer 24

Number of bits in the offset

Question 25

How does the buddy allocator work?

Answer 25

Starts with some consecutive memory region that is free and is a power of two. Whenever a request comes in, the allocator subdivides the area into smaller chunks such that every one of them is also a power of two. IT will continue subdividing until it finds a small enough chunk that is power of two that can satisfy the request

Question 26

How does the slab allocator work?

Answer 26

Builds custom object caches on top of slabs where each slab represents contiguously allocated physical memory

Question 27

_____ is the process by which physical page frames can be repeatedly saved and stored to and from some secondary storage?

Answer 27

Paging/Demand Paging

Question 28

What happens when a page is not present in memory?

Answer 28

It has its present bit in the paging table entry set to 0

Question 29

When should pages be swapped out of main memory and on to disk?

Answer 29

Periodically when the amount of occupied memory reaches a particular threshold

Question 30

Which pages should be swapped out?

Answer 30

Pages that wont be used in the future

Question 31

____ is a page replacement policy in which we look at how recently a page has been used and use that to inform a prediction about the page’s future use?

Answer 31

Least Recently Used (LRU)

Question 32

In addition to LRU, what are other candidates for pages that can be freed from physical memory?

Answer 32

Pages that don’t need to be written out to disk

Question 33

What is the default replacement algorithm in Linux?

Answer 33

Second chance - Performs two scans before determining which pages are the ones that should be swapped out