I/O Management

Question 1

What are an I/O device’s control registers?

Answer 1

Use to communicate with OS. Three types: command (send commands to device), data (send data), status (status of device).

Question 2

Describe the I/O device stack

Answer 2

Hardware 
Controller (incl. with device)  
Driver 
kernel I/O Subsystem  
Kernel

Question 3

What s a device driver?

Answer 3

A piece of software that is responsible for accessing, managing, and controlling a device. Each OS has an interface that each device must implement.

Question 4

What are the types of I/O devices?

Answer 4

1. Block (e.g. disk) - read blocks of data
2. Character (keyboard) - get/put a char
3. Network - stream data chunks of different sizes

Question 5

Name some virtual devices

Answer 5

/dev/null, /dev/random

Question 6

How does an OS represent a device?

Answer 6

A file, can use file abstractions to write to device. Can use a special file system (tempfs, devfs)

Question 7

Name the two types of device–>CPU interactions

Answer 7

Memory-mapped and I/O port

Question 8

What is memory-mapped I/O?

Answer 8

Part of the OS’s physical memory is dedicated to I/O interactions and devices’ registers appear to OS as physical memory addresses. The PCI bus translates physical memory to appropriate device

Question 9

What is BAR?

Answer 9

Base Address Registers - device registers that control how much memory the device needs mapped into the OS (in memory-mapped I/O)

Question 10

What is I/O Port?

Answer 10

Device-CPU interaction where the CPU has special instructions to interact with the device

Question 11

How does a device communicate back to the OS?

Answer 11

Interrupts or Polling

Question 12

What is PIO?

Answer 12

Programmed I/O - device access method where CPU can ‘program’ the device using the command registers and passing data through data registers. Requires no additional hardware support, but is slow

Question 13

What is DMA?

Answer 13

Device Memory Access - device access method that relies on additional hardware support, but is faster than PIO for large data transfers. CPU writes commands to command registers, but configures DMA controller to point to MM where data is located. DMA then copies data over.

Question 14

What is an OS Bypass device access?

Answer 14

Can have user process directly access device by mapping device into user address space. Requires a user-level driver that is included while linking and the device needs to be able to support multiple processes accessing it - sufficient resources to split them + know which process to send a result back to

Question 15

What is an inode?

Answer 15

Persistent representation of a file. A list of blocks that contain the file’s data

Question 16

What is a dentry?

Answer 16

Directory entry, represents a directory and is not stored on disk.

Question 17

What is the dentry cache?

Answer 17

List of all the directories we traversed to get where we are now

Question 18

What is a superblock?

Answer 18

File system specific information on how the FS is laid out

Question 19

What is the limitation on inodes and how do you fix it?

Answer 19

There is a limit to the size of the file. You can fix is by using indirect pointers

Question 20

What is an inode with an indirect pointer?

Answer 20

inode contains: metadata, list of blocks, indirect pointer, double indirect pointer, etc
Indirect pointer is just another list of of blocks
Double indirect is a list to a list of blocks

Question 21

What are some ways to optimize disk I/O access?

Answer 21

1. Caching file in main memory
2. I/O scheduling - schedule how / when we access blocks (faster to modify blocks in order)
3. Prefetching - if a block is accessed, it’s likely that blocks around it will be accessed soon
4. Journaling / logging - write updates to a log on device, then periodically write changes to device