P4L3 - Distributed Shared Memory

Question 1

What is distributed shared memory?

Answer 1

A service that manages memory access across multiple nodes so that applications will have the illusion that they are running on a single shared-memory machine

Question 2

In DSM, each node in the system owns some portion of ____ ____

Answer 2

physical memory

Question 3

In DSM, each node needs to be involved in some _____ ______

Answer 3

consistency protocols

Question 4

T/F: Overall memory access will be slower in a DSM environment

Answer 4

True, due to the network costs

Question 5

What are the two ways in which DSM can be supported?

Answer 5

1. Hardware

2. Software

Question 6

What does hardware-supported DSM rely on?

Answer 6

physical interconnect

Question 7

What is a downside of hardware-supported DSM?

Answer 7

Typically very expensive

Question 8

What are some options for granularity of sharing with DSM?

Answer 8

1. Variable Granularity
2. Page Granularity
3. Object Granularity

Question 9

What is one side effect of increasing the granularity of sharing?

Answer 9

False Sharing

Question 10

In the scope of DSM, describe the concept of false sharing

Answer 10

When different entities that may exist on the same memory page can trigger coherence mechanisms falsely. An example would be one process modifying a variable x and another modifying a variable y that happen to exist in the same page

Question 11

In the scope of DSM, what is the simplest kind of application?

Answer 11

Single Reader/Single Writer since there are no consistency or sharing related challenges

Question 12

For a DSM solution to be useful, it must provide what

Answer 12

Good performance to applications

Question 13

What is the core performance metric for analyzing DSM performance?

Answer 13

Access latency

Question 14

Describe the difference between migration and replication

Answer 14

Migration - whenever a process on another node needs to access remote state, copy the state over to that node

Replication - Copy state on multiple nodes

Question 15

The node where a page is located is typically referred to as the ___ ____ for that page?

Answer 15

Home Node

Question 16

In the scope of DSM, what is the purpose of the home node?

Answer 16

Responsible for driving all of the coherence operations related to that page ( keeping track of who the current owner is)

Question 17

In the scope of DSM, what is the owner node?

Answer 17

The node that currently OWNS the page, and can control all of the state modifications and drive the coherence mechanisms

Question 18

In DSM you need ____ ___ for performance (latency)

Answer 18

Local Caches

Question 19

In DSM, all nodes are responsible for part of ___ ____

Answer 19

distributed memory

Question 20

In DSM, _____ manages accesses and tracks page ownership

Answer 20

home node

Question 21

In DSM, ____ ____ is possible for load balancing, performance or reliability

Answer 21

explicit replication

Question 22

In DSM, what are the parts of an address for a page

Answer 22

Node ID and page frame number on that node

Question 23

T/F: In DSM, metadata for local pages is partitioned across the system, while manager information for every page is replicated

Answer 23

True!

Question 24

T/F: The DSM layer does not need to intercept every single access to the shared state

Answer 24

False!

Question 25

What are the reasons the DSM must intercept all shared state accesses?

Answer 25

1. To detect whether the access is local or remote
2. To detect if update is being performed on locally-controlled portion of the memory, in order to trigger coherence mechanisms

Question 26

How can we avoid overheads when accessing local,non-shared state in DSM?

Answer 26

Dynamically engage/disengage the DSM by leveraging the memory management unit (MMU)

Question 27

What is a consistency Model?

Answer 27

Is a guarantee that the state changes will behave in a certain way as long as the upper software layers follow a certain set of rules

Question 28

Describe strict consistency model in DSM

Answer 28

Every node in the system will see all writes in the system in the exact same order immediately as they are applied

Question 29

Is the strict consistency model in DSM achievable?

Answer 29

Not really, its a nice theoretical model but its not sustainable in practice

Question 30

Describe sequential consistency in DSM

Answer 30

With sequential consistency, not important that we see updates immediately but ordering of updates must occur the same across processes

Question 31

T/F: In sequential consistency, operations from the same process always appear in the order they were issued

Answer 31

True!

Question 32

Describe causal consistency in DSM

Answer 32

Guarantees that if possible causal relationships exist between updates, then the same order will be honored in other processes.

Question 33

Describe weak consistency in DSM

Answer 33

Introduces more than just read/write operations in the form of synchronization points

Question 34

T/F: In weak consistency, the synchronization point must be called by both processes to see an update

Answer 34

True!

Question 35

What are the three key issues in accessing data items in DSM address space while keeping data consistent?

Answer 35

DSM Algorithm (how the access actually executes)
Implementation Level (where the access is implemented)
Memory Consistency Model (How to define consistency)

Question 36

Does sequential consistency imply causal consistency?

Answer 36

Yes. Causal consistency is strictly weaker than sequential consistency. Ie - sequential accomplished everything causal does.