Lecture 3 and 4 threads

Rushdi Shams, Dept of CSE, KUET 1

The Cake Baking Analogy
 The computer scientist baking the cake
has the ingredients (data) and recipe
(algorithm).
 Now, he needs to blend the egg
 Then that blended egg will be mixed up
with the dough made by flour
 The dough mixed with blended egg will
then be placed to syrup.

 Are they dependent or independent of each
other?
- Dependent
 Are they verbs? I mean are they executing
something?
- Of course, making something is always a verb
 Are they sharing some common place?
- mmm, that can be tricky! I can say they are
sharing common place if the mixing up of
blended egg with the dough takes place in the
same bowl! 

 So, blending egg is a process, mixing it
up with dough is a process and mixing
the dough with yummy syrup is a
process.
 But they are dependent on each other.
 We will thus call them Threads
 And threads share some common
resources that processes don’t!

Threads
 Because threads have some of the
properties of processes, they are
sometimes called lightweight
processes
 A thread comprises of
1. Thread ID
2. Program Counter (PC)
3. Register Set
4. Stack

Threads
 Threads share its code section and
data section and other operating
systems resources with other threads
belonging to the same process

Thread Property

Threads vs Processes
Similarities

Threads vs Processes
Dissimilarities

Why Threads?
 A web browser has a thread to display
contents and another thread that serves
you as you click Download button
 MS Word has a thread that displays the
content area, another thread that reads
keystrokes from the keyboard and the
third checks the grammar and spelling in
background

Why Threads?
 And of course process does the same
thing thread does! 
 A client poses 5 different requests (but
similar) to the web browser.
 The browser can create 5 different
processes to serve him
 But, what’s the wrong with this
architecture?

Benefits of Threads
 Responsiveness
A multithreaded web browser allows user to
interact on several buttons with less overhead
 Resource Sharing
Have you ever heard any time anyone saying I
have plenty of resources, use as much as you
like?? 

Benefits of Threads
 Economy
Allocating memory and resources for process
creation is costly. It is more economical to
create and switch between threads than
processes.
-Creating process is 30% slower than creating
threads
-Switching between processes is 5% slower
than switching between threads

User Thread
 User threads are supported above the
kernel and implemented by a thread
library at user level
 This library provides support for thread
creation, scheduling, and management
 Does not need support from the kernel
 Kernel is unaware of user level threads
 If kernel is single threaded, any user
level thread performing system block
call will cause the entire process
blocked

Kernel Thread
 Supported directly by the operating
system
 Slower to create and manage than user
threads

Multithread Model
 Many systems support for both user and
kernel threads.
1. Many to one Model
 Many user level threads to one kernel
thread
 Thread management is done in user
space. So, efficient but the entire process
will block if any of the user threads call a
system block.
 Multiple threads are unable to run in
parallel

Multithread Model
2. One to one Model
 More concurrency than many to many
model by allowing another thread to
run when a thread makes a system
call.
 Creating user thread requires creating
corresponding kernel thread. And
creating kernel thread is expensive!

Multithread Model
3. Many to Many Model
 Multiplexes many user level threads to
a smaller or equal number of kernel
threads

Thread Cancellation
 Thread cancellation is a task of
terminating threads before their
completion
 OS creates 5 threads for a database
operation. 1 executes the query, 1
fetches the record and 1 displays. The 2
more threads are cancelled by the OS

Thread Cancellation
 You are browsing through IE. Suppose,
3 threads are running. Suddenly you
press STOP button. All of them are
cancelled.

Asynchronous Cancellation
 The thread that is to be cancelled is
called Target Thread
 In asynchronous cancellation, one
thread immediately cancel the target
thread

Deferred Cancellation
 The target thread checks periodically
check if it should terminate, allowing the
target thread an opportunity to terminate
itself in an orderly fashion

Problem in Cancellation
 Well, cancellation of thread means
simply withdrawing OS resources from
the thread. Sometimes, resources are
GIVEN to a thread and sometimes
resources are ALLOCATED to a thread.
When GIVEN, it is really a problem to
retrieve that.
 If a thread is in a mid of update, and
cancelled, lost of data may occur.

Problem in Cancellation
 The problem is severe in case of
asynchronous cancellation. OS often
cannot retrieve all of its resources from
the cancelled thread.
 Most OS, however, do the
asynchronous thread cancellation! 

Signal Handling
 A signal is used to notify a process that
a particular event has occurred. All
signals follow the pattern-
1. A signal is generated by the
occurrence of a particular event
2. A generated signal is delivered to a
process
3. Once delivered, the signal MUST be
handled.

Types of Signals
 Synchronous Signal
A signal is generated and delivered to a
particular process if the occurrence is
caused by that process only
 Asynchronous Signal
A signal is generated and delivered to all
of the process created by the process
responsible for the event.

Signal Handler
 Every signal has a default signal handler
run by kernel.
 User defined signal handler can
overtrump it if required.
 Some signals can be ignored
(maximizing the window), others can be
handled by terminating the program
(illegal memory access)

Signal Delivery
 Delivery of a signal is simple in single
threaded programs (hand to hand delivery)
 In multithreaded environment, any one of the
following can be accomplished-
1. Deliver the signal to the thread to which the
signal applies
2. Deliver the signal to every thread of the
process
3. Deliver the signal to certain threads of the
process
4. Assign a specific thread to receive all signals
for processes.

Thread Pool
 We said earlier that in web browser we can
serve the client faster by creating 5 threads
for 1 process (browsing) than by creating 5
processes (saving, downloading, clicking,
maximizing, minimizing)
 Okay, but still there is a problem- if you
create too many threads, that would again
embarrass our POOR OS!! 
 Too many threads can be just a little bit
reduction in servicing time than creating
independent processes

Thread Pool
 That’s why OS creates a pool of threads
at start up
 While starting up, the OS puts, say, 50
threads in its pool (10 system threads,
10 browsing threads, 10 nitty bitty, 20
bla bla)
 Then it will invite those threads from the
pool when required.

Lecture 3 and 4 threads

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