Lecture 4&5-Concurrent vs sequential programming

Question 1

What is the Kernel vs User Mode?

Answer 1

Question 2

What is a Heavyweight processes?

Answer 2

A process is an operating system abstraction to represent what is needed to run a program.

It consists of:

• Sequential Program Execution Stream (includes state of CPU registers)
• Protected resources: memory state, I/O state

No concurrency inside heavyweight processes!

Question 3

What is PCB, address space and context switch?

Answer 3

The current state of the process is held in the PCB (Process Control Block)

To multiplex several processes we need to give CPU time using efficient scheduling policies.

• preemptive: SRT, RR
• non-preemptive: FCFS, SJF

Controlled access to non-CPU resources, e.g. memory, I/O.

Question 4

What are Threads?

Answer 4

A thread is an independent sequence of execution within a program

Question 5

What is the Concept of Thread?

Answer 5

A thread is similar to a process

• Both have a single sequential flow of control with a start and end
• At any time a thread has a single point of execution
• A thread has its execution stack & program counter
• Sometimes a thread is called a lightweight process

However, a thread differs from a process

• A thread cannot exist on its own. It exists within a process
• Usually created and/or controlled by a process
• Threads can share a process’s resources, including memory and open files

Question 6

What is Sequential Programming?

Answer 6

Traditional activity of constructing a program containing one process using a (sequential) computer language

The program is supposed to execute on a single processor architecture

Question 7

What is Single Processor Architecture?

Answer 7

A CPU is linked to RAM and I/O devices by buses
Both program instructions and data are stored in RAM
The CPU repeatedly executes the cycle of

• Fetching, decoding and executing the next instruction
• Referenced by the current value of program counter (PC)

Can at most be executing one instruction at any time

Question 8

What is Total Ordering?

Answer 8

What is meant by “P must precede Q”?
Q can only begin after P finishes
The execution sequence at the program level
P; Q; R;
implies the execution sequence at the system level
p, q1, q2, q3, r1, r2, r3
Single threaded computation, no overlap in the execution of the
statements — Total Ordering

Question 9

What is System Level Execution?

Answer 9

Each statement may be compiled into several machine instructions.

Question 10

Executing a Sequential Program?

Answer 10

The execution sequence is the sequence of values of PC

Deterministic: only one possible sequence of execution

A sequential program gives the system strict instructions on the order of executing the statements in the program.

Question 11

What is the Nature of Sequential Programming?

Answer 11

Total ordering
Deterministic: same input same output
Sequential programming ⇔ Finding a strict sequence of steps to achieve the desired end
This mode does not apply for many practical problems

Question 12

What is Parallel Computation and Partial Ordering?

Answer 12

The operations carried out by Bingxing’s 607279 soldiers were NOT in a total order.

They were carried out in parallel

Each individual soldier did his operations in sequence

The operations in the whole computation can be viewed as in a partial order

Question 13

What is Concurrent Programming?

Answer 13

The activity of constructing a program containing multiple
processes/threads that execute in parallel
These processes may run on
A multi-processor system
A single processor system
Needs language support, e.g., Java Thread and Socket

Question 14

Why Concurrent Programming?

Answer 14

Improve efficiency in program execution using multi-CPU hardware
(Chinese General Problem)
Improve CPU utilisation via multi-tasking on a uni-CPU system
(operating systems)
Some applications are inherently non-deterministic and concurrent,
e.g., embedded traffic lights controller
The order of program operations is determined by external events,
e.g., a sensor is triggered by a coming vehicle
Impossible to predict the order of these events, e.g., a car from the north comes first, and then one from the east, and so on

Question 15

How to Represent Concurrent Program?

Answer 15

Use COBEGIN/COEND to bracket the processes

The program ends only if all processes in COBEGIN/COEND
terminate

The statements in COBEGIN/COEND may overlap in the
execution, but we cannot say they must do so

Question 16

What is a Multi-Processor System?

Answer 16

A computer with multi-CPUs is called a Parallel Computer System

Parallel computation can be implemented on a parallel computer system

If each task is computed by its own CPU, the computation is called Maximum Parallel Computation

E.G., if a system has 607279 CPUs, each soldier’s task can be assigned to its own CPU

Maximum parallelism may not be always possible

Question 17

What is a Uni-Processor Multi-Tasking System?

Answer 17

A uni-CPU system can support multi-tasking/multi-thread
We can treat each soldier as a process or thread

Each process/thread has its own process counter

The program counter (PC) forks to produce many process/thread counters, which later join into the PC

In each CPU cycle, a process is non-deterministically chosen and its next command is loaded and executed

There may be many different possible paths

This CPU sharing technique is interleaving. The execution of processes in a concurrent program is interleaved

Question 18

Problems in Concurrent Programming?

Answer 18

The concurrent processes must interact with each other in order to share resources or exchange data

Synchronisation: when, how, and with what language
abstractions we can synchronise computation events to
eliminate unacceptable interleavings, and thus inacceptable outputs

Distribution: how we can distribute processes among a
number of processors, and how a process on one processor can interact with another process on a different processor.