Concept of thread

Concept of Thread

Munmun Das Bhowmik

Objectives

● Resource sharing
● Performance

● Parallel computing

Few terms to be used :

● Processor : runs a code stream
● Function : a particular code stream
● Schedule : time slice given by processor
● Process : instance of a program being executed
● Registers : physical storage of CPU
● Kernel : interface between hardware & software
● Thread : an execution context for processor
● Thread context : The register set, stacks & private
storage area are known as context of thread.

What is a Thread ?
light-weight process
is the smallest sequence of programmed instructions that
can be managed independently by an operating system
scheduler

Benefits :

• Resourse sharing : On a single-processor machine, the
operating system is switching rapidly between the threads,
giving the appearance of simultaneous execution.
• Responsiveness : Maintain a responsive user interface
while background tasks are executing
• Distinguish tasks of varying priority
• Economy : Perform operations that consume a large
amount of time without stopping the rest of the application.

• Another example is a web server - Multiple threads allow
for multiple requests to be satisfied simultaneously,
without having to service requests sequentially or to fork
off separate processes for every incoming request. ( The
latter is how this sort of thing was done before the
concept of threads was developed. A daemon would
listen at a port, fork off a child for every incoming request
to be processed, and then go back to listening to the
port. )

User & Kernel threads :

● User threads :
● User threads are scheduled in user space by the
process itself. Usually these are provided as a
feature of a computer language.
● Kernel threads :
● These can be scheduled across different CPU's in
a multiprocessor system
● Kernel threads interact intelligently with the I/O
subsystems of the Kernel. This means that while
one of your threads waits on I/O, the others will
continue to run

Models :

User thread(N:1) : Kernel thread (1:1): Hybrid (M:N) :
user library
●
N number of application
● ●
simplest possible
no kernel threads onto some M number
●
threading implementation
modifications of kernel entities, or "virtual
●
scheduling/ processors."
flexible & low cost synchronization
●
thread may block
●
are more complex to
coordination implement than either kernel
entire process, no
parallelism
●
less overhead than or user threads, because
●
Eg : Win 32 process, suitable for changes to both kernel and
parallel application user-space code are required
●
Eg : Windows 7

Questions ?

● What happens when too many user threads are created ?
● Does all sequential process converted to mutiple threads
has benefits ?
● Which type of thread is used to create new tab in web
browser ?
● What type of threads are handled by device drivers ?
● What is hardware virtualization ?

Threads in C# library
Namespace : System.Threading
States :

The ThreadState property is useful for diagnostic purposes.

C# supports parallel execution of code through multithreading

● A C# client program (Console, WPF, or Windows Forms) starts
in a single thread created automatically by the CLR and
operating system (the “main” thread), and is made multithreaded
by creating additional threads.

● WCF, ASP.NET, and Web Services applications are
implicitly multithreaded

If threads are so good, why not use them everywhere?

●
When mutiple threads tries to access shared data
●
Concurrent access to variables
●
To avoid race conditions : .This is a scenario where
two or more threads are competing(racing) to
read/write from a shared memory location leading to
the possibilities of leaving the program in an
inconsistent state.

Synchronization
Block methods : Sleep , Wait, Task.Wait
Lock Constructs :
Construct Purpose Cross process

Lock(Monitor.Enter/ only one thread can access a resource -
Monitor.Exit) or enter the critical zone

Mutex yes

SemaphoreSlim (.Net 4.0) Not more than a specified number of -
threads can access a resource or enter
Semaphore the critical zone yes

ReaderWriterLockSlim (.Net Allows multiple readers to coexist -
3.5) with a single writer

ReaderWriterLock -

Protecting shared data in safer way

● Thread – safety concept :

wait()
T1 Shared data
T2 Exclusive
T3
Sleep() lock

While waiting on a Sleep or Join, a thread is blocked
and so does not consume CPU resources.

Points to consider while designing
Threading has resource requirements & potential
conflicts.
● Few threads should be used as it will minimize the
operating system resources & improving performance.
● Memory consumed for context those required by
AppDomain objects , processes & threads.
● Consumes processor time. If most of current threads are
in one process, then other threads processes are
scheduled less frequently.
● Thread pooling / thread spawning
● Shared access to resources creates conflicts.
● Failure to synchronize causes deadlocks or race
conditions.

Questions :

● Benefits of thread pooling ?
● How to handle long running threads ?
● What about the performance ?
● What is the mechanism for Worker thread ?
● Different ways to create muti-threaded applications
in .Net ?
● Explicit thread class , task parallel library , lambda
functions , BeginInvoke , BackgroundWorker

Concept of thread

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