Caching and reusable components

Question 1

Define caching.

Answer 1

The process of storing previously used data in a location (cache) so that it can be quickly accessed to speed up retrieval if it is needed in the future.

Question 2

Define pre-fetching.

Answer 2

Data is fetched and stored in a cache or buffer before it is needed.

Question 3

Define a cache miss.

Answer 3

Occurs when a system or application requests data from a cache, but the data is not found

Question 4

Define a library.

Answer 4

A collection of reusable subroutines that a programmer can “call” when writing code so that the programmer doesn’t have to write this code.

Question 5

Define reusable components.

Answer 5

Use of existing assets in some form within the software product development process including code e.g. subroutine and designs.

Question 6

Why do CPUs have onboard cache?

Answer 6

To store frequently used data/instructions.

Question 7

Why is it faster to access the cache than it is to access main memory (RAM)?

Answer 7

• Cache is physically located on the chip
• Cache uses SRAM which is faster than DRAM which is used for main memory

Question 8

What is the hierarchy of CPU cache?

Answer 8

L1 (smallest + quickest) to L3

Question 9

What happens if the processor needs to read or write a location in memory?

Answer 9

It first checks for a corresponding entry in the cache. If the processor finds that the memory location is in the cache, a cache hit has occurred and the processor immediately reads/writes to the cache. If not, a cache miss has occurred and the data is copied from main memory into cache and the request fulfilled from cache.

Question 10

What is a pro of CPU cache?

Answer 10

Faster response time.

Question 11

What are cons of CPU cache?

Answer 11

• Small size
• SRAM is expensive
• Fixed size

Question 12

What is hard drive cache (HDC) also known as?

Answer 12

A disk buffer.

Question 13

What does hard drive cache store?

Answer 13

The most recent and frequently used programs/data from the drive.

Question 14

What happens when the CPU requests data from the hard disk?

Answer 14

Modern hard disks come with fast memory, around 512 kilobytes. The hard-disk controller checks this memory before moving the mechanical parts of the hard disk (slow compared to memory). If it finds the data that the computer asked for in the cache, it will return the data stored in the cache without actually accessing data on the disk itself, saving a lot of time.

Question 15

What is a pro of hard drive cache?

Answer 15

Faster response time.

Question 16

What are cons of hard drive cache?

Answer 16

• Small size
• Fixed size

Question 17

What does web browser cache store?

Answer 17

Frequently used web pages to reduce the number of requests that go to web server.

Question 18

What are pros of web browser cache?

Answer 18

• Faster load time
• Reduced network traffic

Question 19

What is a con of web browser cache?

Answer 19

Cached resource may be but of date.

Question 20

What are proxies?

Answer 20

Proxies act as intermediaries between the client and the origin server.

Question 21

What does a cache server store? (proxy server)

Answer 21

Previously requested web pages or other content.

Question 22

What are pros of a cache server?

Answer 22

• Faster load time as HTTP request does not have to go to original web server
• Reduces network traffic

Question 23

What is a con of a cache server?

Answer 23

Cached resources might be out of date.

Question 24

What is a DNS? (Domain name system)

Answer 24

It is used to resolve a particular domain name to its IP equivalent

Question 25

What happens when a DNS server responds to a DNS request?

Answer 25

The operating system may store results in a local DNS cache

Question 26

What's a pro of DNS cache?

Answer 26

Reduced network traffic (fewer DNS requests)

Question 27

What are cons of DNS cache?

Answer 27

- Cached resource may be out of date - If a web page has crashed the location of its web server in the time since its IP address was cached, a web browser might return an HTML 404 error - although the site is still online. Cache could be maliciously changed to direct users to different sites.

Question 28

What are the general cons of cache?

Answer 28

- Algorithms for caching can be very complicated. When the cache is full, the algorithm must choose which items to discard to make room for the new ones. - Wrong data may be cached then it has to be removed and the correct data loaded. - Cache does not have the required content (cache miss) so time is wasted checking it.

Question 29

What are pros of reusable design components?

Answer 29

- Saves time and resources - Can sell code to a third party. The buyer knows how it works and how much it costs and can get it straight away. - Increased dependability, i.e the code has been tried and tested in working systems - A particular element can be written by a specialist. Good if the development team don't have experience for something specific. - Update one library and every program that uses it is updated - don't have to manually change code

Question 30

What are the cons of reusable design components?

Answer 30

- The reusable component must be available in the required language - Some programmers will prefer to write their own code - The reusable components have to be stored somewhere accessible - You have to find, understand and possibly adapt the reusable component