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Chapter 2.5 : Threads
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Process concept 
Process scheduling 
Interprocess communication 
Deadlocks
Threads
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These lecture notes have been adapted from
• How to program with threads
An introduction to multithreaded programming
By Bil Lewis and Daniel J. Berg
and
• Tanenbaum slides
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Processes & Threads
• Processes and threads are related concepts
• A process is a kernel-level entity
– Process structure can only be accessed through
system calls
• A thread (or a lightweight process) is a
user-level entity
– The thread structure is in user space
– It is accessed directly with the thread library
calls, which are just normal user-level functions
(threads do not use system calls)
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Threads
The Thread Model (1)
(a) Three processes each with one thread
(b) One process with three threads
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The Thread Model (2)
• Items shared by all threads in a process
• Items private to each thread
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The Thread Model (3)
Each thread has its own stack
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Process and Thread Data
Structures
TCB1
Code
TCB2
TCB3
Data
Stack
User Space
Kernel Space
PCB
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Characteristics of Threads
• The TCB (thread control block) consist of
– program counter
– register set
– stack space
Thus the TCB is a reduced PCB
• A traditional process is equal to a task with one
thread
• All threads in a process share the state of that
process
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Characteristics of Threads (Cont.)
• They reside in the exact same memory space (user
memory), see the same code and data
• When one thread alters a process variable (say, the
working directory), all the others will see the
change when they next access it
• If one thread opens a file to read it, all the other
threads can also read from it.
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Characteristics of Threads (Cont.)
• Because no system calls are involved, threads are
fast
• There are no kernel structures affected by the
existence of threads in a program, so no kernel
resources are consumed -- threads are cheap
• The kernel doesn't even know that threads exist
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Thread Scheduling (1)
Possible scheduling of user-level threads
• 50-msec process quantum
• threads run 5 msec/CPU burst
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Thread Scheduling (2)
Possible scheduling of kernel-level threads
• 50-msec process quantum
• threads run 5 msec/CPU burst
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Threads of a Task
Threads
Threads
Program
Counter
Code segment
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Task
Data
segment
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Implementing Threads in User Space
A user-level threads package
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Implementing Threads in the Kernel
A threads package managed by the kernel
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Hybrid Implementations
Multiplexing user-level threads onto kernel- level
threads
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Single vs. Multiple Threads of
Execution
Single Thread
Start
Multiple Threads
Start
Edit Document
Edit Document
Print Document
Print Document
End
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End
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Thread Usage (1)
A word processor with three threads
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Thread Usage (2)
A multithreaded Web server
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Thread Usage (3)
• Rough outline of code for previous slide
(a) Dispatcher thread
(b) Worker thread
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Some Benefits of Writing
Multithreaded Programs:
 Performance gains from multiprocessing
hardware (parallelism)
 Increased application throughput
 Increased application responsiveness
 Enhanced process-to-process
communications
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Parallellism
• Different threads can run on different
processors simultaneously with no special
input from the user and no effort on the part
of the programmer
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Throughput
• When a traditional, single-threaded program
requests a service from the operating
system, it must wait for that service to
complete, often leaving the CPU idle
• Multithreading provides progress even
though one or more threads wait for an
event as long as other threads are active
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Responsiveness
• Blocking one part of a process need not block the
whole process. Single-threaded applications that
do something lengthy when a button is pressed
typically display a "please wait" cursor and freeze
while the operation is in progress
• If such applications were multithreaded, long
operations could be done by independent threads,
allowing the application to remain active and
making the application more responsive to the
user
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Communications
• An application that uses multiple processes to
accomplish its tasks can be replaced by an
application that uses multiple threads to
accomplish those same tasks
• Processes-to-process communication through
traditional IPC (interprocess communications)
facilities (e.g., pipes or sockets)
• The threaded application can use the inherently
shared memory of the process
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