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OPERATING SYSTEM
CONCEPTS
Lecture Contents
Motivation for threads
Thread Concept
Similarity and Difference between Process and Thread
Advantages of threads
Implementation of Multithreading
Multithreading Models
Thread Library Concept
Multithreading Issues
Case Study
*Please follow Silberchatz, Galvin and Gagne (Chapter 4), 7th edition
Issues with Processes
• So far we have discussed Process, Creation of new
process concept and its execution; However there are
some issues with processes.
Processes are EXPENSIVE ! (E.g. resources that are to
be allocated with every process)
Hardware resources (memory), kernel resources (PCB)
Link
establishment
for
processes/related processes)
IPC
(unrelated
Concurrency
Motivation
? .. A mechanism that provides an
alternative of a process (so that a new process is not
needed)
Analogy
• Active Entity
• Works simultaneously with others (many threads
executing)
• Requires coordination (if many parts of process are
executing in parallel, then coordination among resources
is required)
• Resources: I/O devices,CPU etc
Process VS Thread
• Process are represented in two ways:
• In RAM ? Physical representation..
• In Execution context ? CPU registers..
• OS represents all this information by using PCB
• Separate PCB for every process..
• PCB in case of multi-threading..
Threads within a same process share…
• Multiple and concurrent threads within a process share:
Code
Threads are in the same
process address space
Data
PCB (Process Control Block)
For a thread creation, only need to allocate CPU
context and Stack !!
Single and Multithreaded Processes
How processes are similar to threads or
different ?
• A thread can be in a state, similar to process (new, ready,
running, waiting, terminate)
• Threads can also create other threads.
• Difference
• Processes execute only within their own address space,
whereas threads of same process execute within address
space of one major process
• Synchronization issues ?
Advantages of thread
Responsiveness - allow a program to continue running even if
part of it is blocked (e.g. In a browser, multiple threads are
working simultaneously to increase responsiveness)
Resource sharing - threads share the memory and resources of
the process (lower main memory requirement)
Economy - it is more economical to create and context-switch
threads (because they share resources of the process to which
they belong)
Parallelization (computation speedup)
Specialization (repeatedly executing same code)
IPC not required!
 Scalability (utilizing multiprocessor architectures)
Some other things..
Some of these may block from time to time. By
decomposing such an application into multiple sequential
threads that run in parallel, the programming model
becomes simpler.
Any benefit to OS ? OS can assist multiple threads if it’s
multithreaded
Making OS multithreaded is helpful because they can
support multiple apps at same time and execute them on
different CPUs.
Benefits of multithreading (Single
CPU)..
Are threads useful in Single CPU architecture or
when num of threads > num of CPU ?
If t_idle (disk reading e.g.) > t_cxt_switch time, then
it makes sense that switching among threads is useful
t_cxt_switch < p_cxt_switch
Benefits of multithreading (Multi CPU
system)..
THREAD
Thread- A piece of code in a process which runs
concurrently with execution of other piece of codes
within the same process (same address space)
No need to create a new process
• Resources that thread need to update/allocate privately:
Thread ID
CPU Context (Program Counter (PC), register set)
Stack
Priority
Basic thread mechanism
• How to support threads?
• Must have data structure that allows to distinguish
between process and thread
Goals
• To identify threads and keep record of them like PCB
• Mechanisms to create and manage threads
• Mechanisms to allow threads to communicate with
eachother
Thread Creation
• Thread type data structure that contains all information
specific to that thread e.g. Program counter, registers,
stack etc
• Fork (proc,args) {new type thread data structure is
created}
• Join(thread) {parent thread blocked until all executed in
case of bunch of threads processing on some input array}
How multithreading is implemented ?
(with respect to benefits to applications and OS)
• OS level threads are visible to OS scheduler
• OS scheduler decides the execution/mapping of these OS
level threads
• Some of the OS (kernel) level threads are linked to user level
threads (in a process) and some are made to run their own
code.
• For a user level thread to be executed, it must be
RECOGNIZED (mapped to a kernel level thread and then OS
scheduler will schedule it on underlying hardware).
Multithreading Models
• Depending on this concept, we can have three types of
multithreading models:
A. One to One model: (for each user level thread, one
kernel level thread)
One-to-One Model
 Each user-level thread maps to kernel thread
 OS sees/understands that app is multithreaded
 Benefits
Since OS already provide mechanisms to support its threads, user threads
can directly benefit from this.
True concurrency - Another thread will be able to run when a thread makes
a blocking system call.
 Drawback:

Creating a user thread requires creating a corresponding kernel thread.
(Overhead)

Portability issues: User level threads is relying support from kernel
level thread management so an app running in one environment
may not be able to run on other
Many to one Model
• Many user level threads mapped to one OS level thread
• There’s a thread management library on user level that
•
•
•
•
decides which user level thread should be mapped to
THAT ONE OS level thread.
That user level thread will run only when its scheduled by
OS scheduler
Totally portable (everything done at user level, no need of
support from OS level)
OS doesn’t know whether the app is multithreaded or not,
it just sees kernel level thread
Whole process can get blocked !
Many-to-Many Model (Hybrid Model)
 Multiple user level threads multiplexed over a smaller or equal number of
kernel threads
 Allows the operating system to create a sufficient number of kernel threads
• Allow hybrid multithreading !
Difference between the two techniques…
Advantages of User level threads (process scope)
Management and scheduling of thread is done by threading library in user
space, so user level threads don’t invoke kernel for doing these tasks
User level threads can run on any OS, no changes are required to do on
underlying kernel (Portability)
Process wide thread management (only limited to one process)
Disadvantages
Using User Level threading strategy, a multithreaded application cannot take
advantage of multiprocessor system. (different threads of a same process
cannot run on different CPU’s because OS considers a process a single
threaded program)
If a thread is blocked, all the threads within a process gets blocked.
Advantages of Kernel level threads (system scope)
 Since kernel threads use the kernel scheduler, different kernel threads
can run on different CPUs (taking advantage of multiprocessor system)
 If a thread within a process gets blocked (e.g. waiting for an I/O), the
scheduler can schedule another thread
If a process has large number of threads, it will receive large fraction of
underlying hardware CPU.
Disadvantages
Frequent invoking of kernel for management of threads
Examples
 User Threads
Thread management done by user-level threads library
Three primary thread libraries:

POSIX Pthreads
 Win32 threads
 Java threads
 Kernel Threads
 Supported by the Kernel
 Examples

Windows XP/2000
 Solaris
 Linux
Multithreading Models
Based on the number of kernel threads allocated
correspondingly to the user level threads, we have three
multithreading models
Many-to-one Model
One-to-one Model
Many-to-Many Model
Thread Libraries
Thread library provides programmer with API for creating and managing threads
Two primary ways of implementing
Library entirely in user space
Kernel-level library supported by the OS -> invoking a function in the library
results with a system call
Examples of threading Libraries
Pthreads
Win32
Java
Pthreads
A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization
API specifies behavior of the thread library, implementation is up to development
of the library (i.e. the OS designers)
This standard has been defined to make the writing of PORTABLE threaded
program possible
May be provided either as user-level or kernel-level.
Common in UNIX operating systems (Solaris, Linux, Mac OS X)
Threading Issues
Thread cancellation
Asynchronous or deferred
Signal handling
Thread pools
Thread-specific data
Scheduler activations
Thread Cancellation
Terminating a thread before it has finished
Example: multiple threads concurrently searching through a database
and one thread returns the result
Stopping a webpage from loading any further
Two general approaches:
 Asynchronous cancellation terminates the target thread immediately
– troublesome if it’s in middle of updating some data
 Deferred cancellation allows the target thread to periodically check if it
should be cancelled – checking a flag.
Can only be cancelled at defined points called cancellation points
Signal Handling
•
•
•
•
Signals are used in UNIX systems to notify a process that a particular event
has occurred
Example: illegal memory access, division by zero
A signal handler is used to process signals
1. Signal is generated by particular event
2. Signal is delivered to a process
3. Signal is handled
 In a multithreaded process, where should a signal be delivered?
Options:
• Deliver the signal to the thread to which the signal applies
• Deliver the signal to every thread in the process
• Deliver the signal to certain threads in the process
• Assign a specific thread to receive all signals for the process
Thread Pools
Reason for the need of thread pool:
Time consumed to create a new thread and destroy the previous ones – leads to
slow performance
Unlimited threads could exhaust system resources, such as CPU time or memory
Solution:
Thread Pools
• Create a number of threads at process startup and place them in a pool where they
await work
Advantages:
Usually slightly faster to service a request with an existing thread than create a
new thread
Also allows an application to specify the number of threads it requires
Thread Specific Data
Allows each thread to have its own copy of specific data
Example, in a transaction-processing system, service each transaction in a separate
thread. Furthermore, each transaction might be assigned a unique identifier. To
associate each thread with its unique identifier, use thread-specific data.
Scheduler activation
 User-Level Threads: If a single user-level thread blocks, the operating system blocks
the entire multithreaded process. Another limitation to a many-to-one thread
mapping is that a process's threads cannot simultaneously execute on multiple
processors. Scheduler activations attempt to address these limitations to userlevel threads. A scheduler activation is a kernel thread that can notify a userlevel threading library of events (e.g., a thread has blocked or a processor is
available). This type of kernel thread is called a "scheduler activation,“ because the
user-level threading library can perform thread-scheduling operations when
"activated" by an event notification, sometimes called an upcall.
Case Study
• Windows XP
• Linux
Windows XP
Windows XP Threads
Linux Threads
• Linux refers to them as tasks rather than threads
• Thread creation is done through clone() system call
• clone() accepts arguments that specify which resources to share with
the child process