SSC - Concurrency and Multi-threading Basic conceptsszh/teaching/ssc/... · Concurrency: background and concepts Concurrent programming: Two basic units I Process: a program I has

SSC - Communication and Networking

SSC - Concurrency and Multi-threadingBasic concepts

Shan He

School for Computational ScienceUniversity of Birmingham

Module 06-19321: SSC


Outline

Outline of Topics

Concurrency: background and concepts



Concurrency: history

I In the beginning, a computer was only capable of executing asingle program at a time.

I Multitasking: Execute multiple programs “at the same time”.I Modern operating systems are all multitaskingI Essentially shares the computing resources, such as CPU(s),

main memory, and I/O channels.I Single-CPU machine: only one task can be executed at one

time through time-slicing of the CPU.I Multi-CPU machine: tasks can be executed simultaneously,

distributed among or time-slicing the CPUs.



Multitasking: Cooperative vs PreemptiveI Cooperative multitasking: processes control the CPU

I Used in early multitasking operating systems, e.g., Win 3.1I Transferring control explicitly from one process to another

(co-routines) according to a cooperative modelI Runtime support is simpler to implementI Programmers have to handle cooperation: bugs in processes

may lock the systems

I Pre-emptive multi-tasking: Kernel schedules CPU to eachtask

I Used in modern multitasking operating systemsI Operating system’s kernel manages (schedules) process’ access

to CPUI Pre-emption: an action performed by kernel: it forces a process

to abandon its running state even if it could safely executeI Used time slicing mechanism: a process is suspend when a

specified amount of time has expired



Multitasking: Cooperative vs Preemptive

P1 P2

Switch()

Switch()

Switch()

Switch()

1

2

35

46

P1 P21

35

24

6

Interrupt Interrupt

InterruptInterrupt

Cooperative multitasking Pre-emptive multitasking



Preemptive Multitasking

ready

waiting

running

Selection

Preemption

Synchronisation statement executed

by the process

Synchronisation statement executed by other processes

Process creation

Process termination



Concurrent programming: Two basic units

I Process: a programI has a self-contained execution environmentI has a complete, private set of basic run-time resourcesI has its own memory spaceI totally controlled by the operating system

I Thread: also called lightweight process, which is adispatchable unit of work in a process.

I has a definite beginning and an endI run inside a single processI share the same address space, the resources allocated and the

environment of that process



More about threads

I Both processes and threads provide an execution environment,but creating a new thread requires fewer resources thancreating a new process.

I A process can be divided into multiple independent threads

I A process can be a single thread, which is called main thread.

I A standalone Java application starts with main thread (main() )

I This main thread can start new independent threads.

I Multithreading: executing multiple threads inside the sameprocess (program).



Concurrency: Two basic units

Single threaded process Multithreaded process

Thread execution



Main thread and threads

Time

Begin End Suspended/Resumed

Main thread

Thread 2

Thread 3



Difference between threads and processes

I Processes are typically independent and might consist ofmultiple threads

I Processes have separate address spaces for code, data andother resources, whereas threads share the address space ofthe process that created it

I Threads are easier to create than processesI Multithreading requires careful programming

I A thread has its own stacks and registersI Stack: a reserved region of memory data that is operated in a

last-in-first-out mannerI Register: a small amount of storage available as part of CPU

I Processes use inter-process communication mechanismsprovided by the OS to communicate with other processes,while threads can directly communicate with other threads inthe same process



Difference between threads and processes

Code DataOther

resources

Register Stack

Single thread process

Code DataOther

resources

Register

Stack

Multithreaded process

Stack Stack

Register Register

Memory segments



Context switching

I Context switching: a procedure of multi-threading for thesystem to switch between threads running on the availableCPUs

I A context is the minimal set of data used by this task thatmust be saved to allow a task interruption at any point inttime

I Data to be saved include:I Registers: one of a small set of data holding places in CPU,

which may hold a computer instruction, a storage address, orany kind of data

I Program counter: known as an instruction address register,which is a small amount of fast memory that holds the addressof the instruction to be executed immediately after the currentone

I other necessary operating system specific data



Context switching: how it works

I A CPU timer: determines the end of the timeslice for eachthread, and signals at the end of the timeslice

I Steps for context switching:I Step 1: interrupt current thread by the CPU timerI Step 2: The CPU saves all information required , e.g., register

and programme counter for the current thread onto a stackI Step 3: Move this information from the stack into a data

structure called context structure.I Step 4: A scheduler algorithm decides which thread to run nextI Step 5: Run the new thread until a interrupt-return is called to

switch back to a previously executing threadI Step 6: Transfers all the information from the context structure

associated with the previously executing thread to the stackI Step 7: resume previous executing thread



Advantages of multi-threading

Advantages compared with multi-processingI Improves the performance of the program by better usage of

system resources:I Share the same address space, less overhead for operating

systemI Context-switching between threads is normally inexpensiveI Better usage of CPU time, e.g., while one thread is blocked

(e.g., waiting for completion of an I/O operation), anotherthread can use the CPU time to perform computations

I Simpler program design: Control and communication betweenthreads is easy and inexpensive.

I More responsive programs



Disadvantages of multi-threading

Disadvantages/costs compared with single-threading

I Context switching overhead: even lighter thanmulti-processing, CPU still needs to save the register, programcounter etc. of the current thread, and load the same of thenext thread to execute.

I More complex design: data shared and accessed by multiplethreads needs special attention

I Increased resource consumption: CPU time, memory to keepits local stack, and operating system resources to manage thethread



Concurrent programming - three important concepts

I Atomicity : An operation is said atomic when it cannot beinterrupted.

I Visibility: If an action in one thread is visible to anotherthread, then the result of that action can be observed by thesecond thread.

I Ordering: Since in multi-threading program, “the order ofexecution is not guaranteed”, we need ordering constraints todefine what order should the actions in threads executed.

We shall see these concepts in Java examples in the next fewlectures.

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SSC - Concurrency and Multi-threading Basic conceptsszh/teaching/ssc/... · Concurrency: background and concepts Concurrent programming: Two basic units I Process: a program I has