Most modern applications are multithreaded Threads run within application Multiple tasks with the application can be implemented by separate threads Update display Fetch data Spell checking Answer a network request Process creation is heavy-weight while thread creation is light-weight Can simplify code, increase efficiency Kernels are generally multithreaded

1. Operating System 20
Threads
Prof Neeraj Bhargava
Vaibhav Khanna
Department of Computer Science
School of Engineering and Systems Sciences
Maharshi Dayanand Saraswati University Ajmer

2. Motivation
• Most modern applications are multithreaded
• Threads run within application
• Multiple tasks with the application can be implemented
by separate threads
– Update display
– Fetch data
– Spell checking
– Answer a network request
• Process creation is heavy-weight while thread creation is
light-weight
• Can simplify code, increase efficiency
• Kernels are generally multithreaded

3. Multithreaded Server Architecture

4. Benefits
• Responsiveness – may allow continued execution
if part of process is blocked, especially important
for user interfaces
• Resource Sharing – threads share resources of
process, easier than shared memory or message
passing
• Economy – cheaper than process creation, thread
switching lower overhead than context switching
• Scalability – process can take advantage of
multiprocessor architectures

5. Multicore Programming
• Multicore or multiprocessor systems putting pressure
on programmers, challenges include:
– Dividing activities
– Balance
– Data splitting
– Data dependency
– Testing and debugging
• Parallelism implies a system can perform more than
one task simultaneously
• Concurrency supports more than one task making
progress
– Single processor / core, scheduler providing concurrency

6. Multicore Programming (Cont.)
• Types of parallelism
– Data parallelism – distributes subsets of the
same data across multiple cores, same
operation on each
– Task parallelism – distributing threads across
cores, each thread performing unique
operation
• As # of threads grows, so does
architectural support for threading
– CPUs have cores as well as hardware threads
– Consider Oracle SPARC T4 with 8 cores, and 8
hardware threads per core

7. Concurrency vs. Parallelism
 Concurrent execution on single-core system:
 Parallelism on a multi-core system:

8. Single and Multithreaded Processes

9. Amdahl’s Law
• Identifies performance gains from adding additional cores to an
application that has both serial and parallel components
• S is serial portion
• N processing cores
• That is, if application is 75% parallel / 25% serial, moving from 1 to 2
cores results in speedup of 1.6 times
• As N approaches infinity, speedup approaches 1 / S
Serial portion of an application has disproportionate effect on
performance gained by adding additional cores
• But does the law take into account contemporary multicore systems?

10. User Threads and Kernel Threads
• User threads - management done by user-level threads library
• Three primary thread libraries:
– POSIX Pthreads
– Windows threads
– Java threads
• Kernel threads - Supported by the Kernel
• Examples – virtually all general purpose operating systems,
including:
– Windows
– Solaris
– Linux
– Tru64 UNIX
– Mac OS X

11. Thread Structure
• A thread, sometimes called a lightweight
process (LWP), is a basic unit of resource
utilization, and consists of a program counter,
a register set, and a stack.
• It shares with peer threads its code section,
data section, and operating-system resources
such as open files and signals, collectively
known as a task.

12. Thread Structure
• A traditional or heavyweight process is equal to a task with
one thread.
• A task does nothing if no threads are in it, and a thread
must be in exactly one task.
• The extensive sharing makes CPU switching among peer
threads and the creation of threads inexpensive, compared
with context switches among heavyweight processes.
• Although a thread context switch still requires a register set
switch, no memory-management-related work need be
done.
• Like any parallel processing environment, multithreading a
process may introduce concurrency control problems that
require the use of critical sections or locks.

13. User Level Thread Management
• Also, some systems implement user-level threads in user-level
libraries, rather than via system calls, so thread switching does not
need to call the operating system, and to cause an interrupt to the
kernel.
• Switching between user-level threads can be done independently
of the operating system and, therefore, very quickly.
• Thus, blocking a thread and switching to another thread is a
reasonable solution to the problem of how a server can handle
many requests efficiently.
• User-level threads do have disadvantages, however. For instance, if
the kernel is single-threaded, then any user-level thread executing a
system call will cause the entire task to wait until the system call
returns.

14. Advantages of threads over processes.
• With multiple processes, each process operates independently of
the others; each process has its own program counter, stack
register, and address space.
• This type of organization is useful when the jobs performed by the
processes are unrelated.
• Multiple processes can perform the same task as well. For instance,
multiple processes can provide data to remote machines in a
network file system implementation.
• However, it is more efficient to have one process containing
multiple threads serve the same purpose.
• In the multiple process implementation, each process executes the
same code but has its own memory and file resources.
• One multi-threaded process uses fewer resources than multiple
redundant processes, including memory, open files and CPU
scheduling,

15. Threads vs Process
• A thread within a process executes sequentially, and each thread
has its own stack and program counter.
• Threads can create child threads, and can block waiting for system
calls to complete; if one thread is blocked, another can run.
However, unlike processes, threads are not independent of one
another.
• Because all threads can access every address in the task, a thread
can read or write over any other thread's stacks.
• This structure does not provide protection between threads. Such
protection, however, should not be necessary.
• Whereas processes may originate from different users, and may be
hostile to one another, only a single user can own an individual task
with multiple threads.
• The threads, in this case, probably would be designed to assist one
another, and therefore would not require mutual protection.

16. Assignment
• Explain Multithreaded Server Architecture and
its benefits
• Explain the concept of multicore programming
• Explain the advantages of threads over
processes