P2L5. Thread Performance Considerations

Question 1

What are the pros/cons of a multi-process (MP) implementation?

Answer 1

Pros:

• Simple programming

Cons:

• High memory usage

Costly to context switch

• Hard/costly to maintain shared state

Question 2

What are the pros/cons of a multi-threaded (MT) approach?

Answer 2

Pros:

• Shared address space
• Shared space
• Cheap context switch

Cons:

• Not simple implementation
• Requires sychnonization
• Need underlying support for threads on OS (not an issue today)

Question 3

How does the Event Driven Model work? (TODO: Ask a better question?)

Answer 3

Question 4

How does the Event-Driven Model achieve concurrency?

Answer 4

MP and MT: 1 request per execution context (process/thread)

Event-driven: Many requests interleaved in an execution context.

Question 5

What are the pros/cons of Event-Driven Model?

Answer 5

Pros:

• Single address space
• Single flow of control
• Smaller memory requirement
• No context switching
• No sychronization

Cons:

• Loss of cache locality
• A blocking request/handler will block the entire process

Question 6

What does the Event-Driven Model use helpers for?

Answer 6

When async calls are not available, helpers can be used instead.

Helpers are:

• designated for blocking I/O operations only
• pipe/socket based communication w/ event dispatcher => select()/poll() still ok!
• helper blocks, but main event loop (and process) will not

At the time the Flash paper was written not all OSes were multi-threaded, so they used processes for helpers.

Question 7

What are the pros/cons of helper processes in the Event-Drive Model (AMPED/AMTED)?

Answer 7

Question 8

Compare/contrast performance of Flash, SPED, MT, MP when the data is cached and when it’s not cached.

Answer 8