Operating Systems Distributed Systems (PPT 8)

Question 1

What do networks do?

Answer 1

Make it easy for work to be distributed between multiple machines

Question 2

What are some different types of network?

Answer 2

• Large distance networks
• Client-server model
• Middleware
• Distributed message passing
• Remote Procedure Calls
• Clusters of processing nodes

Question 3

What do programs on a network need to do?

Answer 3

• send data over the network
• ensure that all data arrives in the correct order
• set up high level dialogue

Question 4

What is client-server computing?

Answer 4

A program is split into two parts, each running on a different computer

Question 5

What does the Client part do in client-server computing?

Answer 5

• runs on a PC and provides a GUI or user-friendly interface to the user
• processes their commands and sends commands/requests to the server part
• displays answers back to the user once the server has sent them

Question 6

What does the Server part do in client-server computing?

Answer 6

-carries out the command and sends the answer back to the client

Question 7

What is Middleware?

Answer 7

It is an extra layer which is inserted to try and standardise interfaces and hide low level mechanics from the client application. This is because Operating Systems and proprietary systems can make it difficult to writer general applications

Question 8

What is Distributed Message Passing?

Answer 8

-Processes on different machines can only talk to each other by messages. No semaphores or shared memory available so messages are the synchronisation tools

Question 9

What are the two minimum commands?

Answer 9

send(machine,process, message)

• where machine and process specific where the message is to go (destination address)
• the message can be a simple string or a complex data structure

receive(machine,process,message)

• where machine and process may be used to specify which machines/processes the caller is prepared to accept message from
• wildcards,for example “all”, may be allowed. They are updated to show where the message actually came from

Question 10

What are the two types of transport mechanism?

Answer 10

Reliable
- makes the application program simpler

Unreliable
-systems make the message passing part easier

Question 11

What does Blocking mean?

Answer 11

Means that the process is held up until the message is known to have reached its destination. May cause considerable delays but is “well understood” when it comes to debugging

Question 12

What does Non-Blocking mean?

Answer 12

This is the most efficient in terms of parallel processing. Sends the message and then gets on with something else. Care has to be taken not to overwrite sent data before it is actually sent. OS usually makes a copy but not always

Question 13

What is Remote Procedure Calls?

Answer 13

This is a mechanism for executing a procedure on another machine. When a program calls a procedure it waits until the procedure has procedure has produced the required effects and then carries on. It does not care how this is accomplished and it is no concern of the calling program if the procedure goes to another machine. RPC is a simple method of hiding the inter machine mechanism from the calling program

Question 14

What are four parts to using an RPC?

Answer 14

• The actual procedure P(x,y) that will be run on the remote machine
• A dummy procedure or stub P(x,y) which simply takes the parameters, packages them up into a message and sends them to the remote machine (It waits for a reply then acts if it had done it locally)
• A suitable transport system
• A driver program, running on the remote machine

Question 15

What is Synchronous RPC?

Answer 15

This is when the calling program waits until the results of the call are delivered

Question 16

What is Asynchronous RPC?

Answer 16

This is when the calling program is immediately released once the data has been passed to the remote machine

Question 17

What are the principles of a cluster approach?

Answer 17

Absolute scalability
-scale up using any number of computers
Incremental scalability
-add more computers in small numbers
High availability
-failure of one node does not take it all down
Superior price/performance
-use commodity PCs to build a monster

Question 18

What are some attributes of Cluster Middleware?

Answer 18

• single entry point
• single user interface
• single file hierarchy
• single I/O space
• single control PC
• single process space
• single virtual network
• check pointing
• single memory space
• single job management system
• process migration

Question 19

What is Beowulf?

Answer 19

It is a NASA funded project to perform big computations using clustering. It uses mass market commodity components and a dedicated private network. It has a freely available software base and most implementations use Linux

Question 20

What is BPROC?

Answer 20

Beowulf distributed process space

• it allows a process ID space to span multiple nodes in a cluster environment
• needs to provide a single system image to the user

Question 21

What is Beowulf Ethernet Channel Bonding?

Answer 21

Joins multiple networks into one logical network with higher bandwidth. Uses multiple Ethernets