P3L5 - I/O Management

Question 1

T/F: The execution of applications doesn’t rely ONLY On CPU and Memory

Answer 1

True! It relies on other hardware components as well

Question 2

List some examples of I/O devices

Answer 2

Keyboards
Microphones
Displays
Speakers
Mice
NIC Cards (Network Interface)

Question 3

T/F: The device space is extremely diverse?

Answer 3

True

Question 4

List the types of registers traditionally found within a device

Answer 4

1. Command Registers
2. Data Registers
3. Status Registers

Question 5

What is the purpose of command registers within a device?

Answer 5

CPU uses these to control the device

Question 6

What is the purpose of the data registers in a device?

Answer 6

These are used by the CPU to transfer data in and out of the device

Question 7

What is the purpose of the status registers in a device?

Answer 7

These are used by the CPU to understand what is happening on the device

Question 8

_____ is basically the device’s CPU

Answer 8

microcontroller

Question 9

How do devices interface with the rest of a system?

Answer 9

Via a controller that is typically integrated as part of the device packaging that is used to connect the device with the rest of the CPU complex via some CPU/device interconnect

Question 10

What does PCI stand for

Answer 10

Peripheral Component Interconnect

Question 11

What is the purpose of PCI?

Answer 11

To connect all controllers of devices to the rest of the system

Question 12

T/F: The PCI bus is the only possible interconnect that can be present in a system

Answer 12

False!

Question 13

Operating systems support devices via ___ ___

Answer 13

device drivers

Question 14

Device drivers are ___ ___ software components

Answer 14

device specific

Question 15

Who is responsible for making drivers available for all the operating systems where a device will be used?

Answer 15

The device manufacturer!

Question 16

List the high level groups of devices

Answer 16

1. Block Devices
2. Character Devices
3. Network Devices

Question 17

What types of devices are encompassed within block devices?

Answer 17

Disks – which operate at the granularity of blocks of data that are delivered to and from the device via the CPU

Question 18

What types of devices are encompassed in character devices?

Answer 18

Keyboards – devices which work with a serial sequence of characters and support a get/put character interface

Question 19

What types of devices are encompassed in network devices?

Answer 19

Devices which work with more of a stream of data chunks as opposed to blocks over network

Question 20

On Unix-like systems, where do all devices appear as files?

Answer 20

/dev directory

Question 21

Device registers appear to the CPU as ____ ____

Answer 21

Memory Locations

Question 22

What is responsible for routing accesses to devices

Answer 22

The PCI controller

Question 23

What is the portion of physical memory on the system dedicated for device interactions controlled by ?

Answer 23

Base Address Registers (BAR)

Question 24

What is it called when a portion of physical memory on the system is dedicated for device interactions?

Answer 24

memory-mapped I/O

Question 25

What is it called when CPU can access device via special instructions

Answer 25

I/O Port Model

Question 26

What are the two routes that can be taken on the path from the device to the CPU

Answer 26

1. Devices can generate interrupts to the CPU | 2. CPUs can poll devices by reading their status registers to determine if they have some response/data fro the CPU

Question 27

What is the downside of using interrupts when communicating between device/CPU

Answer 27

Interrupt handlers and the associated overhead

Question 28

What is the pro of using interrupts when communicating between device/CPU

Answer 28

Interrupts can be triggered by the device as soon as the device has information for the CPU

Question 29

What is the downside of using polling when communicating between device/CPU

Answer 29

Delay can be introduced based on when/how frequently the CPU polls

Question 30

With just basic support from an interconnect like PCI, CPU can request operation from an I/O device using ______

Answer 30

programmed I/O (PIO)

Question 31

When using PIO how does the CPU issue instructions to a device?

Answer 31

By writing into the command registers of the device

Question 32

Name an alternative to PIO based communication

Answer 32

Direct Memory Access (DMA)

Question 33

Describe how Direct Memory Access (DMA) works

Answer 33

CPU still writes commands to the command register but the data movement is controlled by configuring the DMA controller to know which data needs to be moved from memory to the device, and vice-versa

Question 34

T/F: Memory access takes more cycles than DMA configuration

Answer 34

False! DMA configuration takes many more cycles

Question 35

List the steps of typical device access from a user process

Answer 35

1. Use process makes a system call specifying appropriate operation 2. kernel may do some preprocessing of the data/form data 3. OS will invoke the appropriate device driver for the device 4. Device will then perform action Result is returned in the reverse order of events

Question 36

T/F: It is not necessary to go through the the kernel to get to a device?

Answer 36

True! Known as Operating System Bypass

Question 37

What part is the OS still involved in with operating system bypass?

Answer 37

Making device registers available to user process on create

Question 38

What do you need to leverage OS bypass?

Answer 38

User-level drivers (a library)

Question 39

Describe the difference between sync and async access

Answer 39

For synchronous operations the calling thread will block | With asynchronous operations the thread is allowed to continue as soon as it issues the request

Question 40

What is the purpose fo the VFS layer?

Answer 40

Allows the OS to hide all details regarding the underlying filesystem(s) from the higher level consumers

Question 41

The _____ abstraction represents the elements on which the VFS operates

Answer 41

File

Question 42

For each file the VFS maintains a persistent data structure called an _____

Answer 42

inode

Question 43

What is the purpose of an inode

Answer 43

To maintain a list of all data blocks corresponding to the file as well as other important information for that file

Question 44

To help with certain operations on directories, linux maintains a data structure called a ____

Answer 44

dentry (directory entry)

Question 45

The _____ abstraction provides information about how a particular filesystem is laid out on some storage device

Answer 45

superblock

Question 46

T/F: The virtual file system data structures are hardware entities

Answer 46

False, they are software

Question 47

What was the default filesystem in Linux until it was replaced?

Answer 47

the ext2 filesystem

Question 48

What is the benefit of using inodes?

Answer 48

Easy to perform both sequential and random accesses to the file

Question 49

What is the downside of using inodes?

Answer 49

There is a limit on the file size for files that can be indexed using inodes

Question 50

What was one way to solve the issue of file size limits in inodes?

Answer 50

Use of indirect pointers!

Question 51

What is the benefit of using indirect pointers

Answer 51

Allows us to use relatively small inodes while being able to address larger and larger files

Question 52

What is the downside of using indirect pointers?

Answer 52

File access is slowed down

Question 53

List some techniques that filesystems employ to minimize the accesses to disk and improve the file access overheads?

Answer 53

1. Buffer Caches 2. I/O scheduling 3. Prefetching 4. Journaling