2. What?
A thread is called a lightweight process:
●
it is comprised over a thread ID, program counter, a register set
and a stack
●
it shares with other threads belonging to the same process its code
section, data section and other OS resources (e.g., open files)
●
a process that has multiples threads can do more than one task at a
time
A traditional (or heavyweight) process has a single thread of
control.
Operating System 2011/2012
3. Why?
A single application may be required to perform several
similar tasks (such as Web server and Word processor).
●
One solution is to have the a single process for each Web
request or Word document. Each process perform on a single
request/document => a separate process is created to service
that request.
●
Process creation is time consuming and resource intensive.
●
Rather than creating another process, the server will create a
new thread to service the request and resume listening for
additional requests.
●
Operating System 2011/2012
5. Benefits
Responsiveness
●
One part of a program can continue running even if
another part is blocked
●
Resource Sharing
●
Threads of the same process share the same memory
space and resources
●
Economy
●
Much less time consuming to create and manage threads
than processes
●
Scalability
●
Each thread can run in parallel on a different processor
●
Operating System 2011/2012
6. User Threads
Thread management done by user-level threads library is
without the intervention of the kernel
●
Fast to create and manager
●
If the kernel is single threaded, any user-level thread
performing a blocking system call will cause the entire
process to block
User thread libraries
●
POSIX Pthreads
●
Mach C-threads
●
Solaris 2 UI-threads
Operating System 2011/2012
7. Kernel Threads
Supported by the Kernel
●
Slower to create and manage than user threads
●
If thread performs a blocking system call, the kernel can
schedule another thread in the application for execution
●
In multi-processor environments, the kernel can schedule
threads on multiple processors
Examples
●
Windows 95/98/NT/2000
●
Solaris
●
Linux
Operating System 2011/2012
9. One-to-One
Each user-level thread maps to kernel thread.
●
Another thread can run when one thread makes a blocking
call
●
Multiple threads an run in parallel on a MP machine
●
Overhead of creating a kernel thread for each user thread
●
Most implementations limit the number of threads supported
Examples
●
Windows 95-2000
●
OS/2
●
Linux
Operating System 2011/2012
10. Many-to-One
Many user-level threads mapped to single kernel thread.
●
Efficient - thread management done in user space
●
Entire process will block if a thread makes a blocking
system call
●
Only one thread can access the kernel, no parallel processing
in MP environment
Operating System 2011/2012
11. Many-to-Many
Allows many user level threads to be mapped to smaller or
equal number of kernel threads.
As many user threads as necessary can be created.
Corresponding kernel threads can run in parallel on a
multiprocessor.
When a thread performs a
blocking system call, the kernel
can schedule another thread for
execution.
Allows true concurrency in a MP
environment and does not
restrict number of threads that
can be created.
Operating System 2011/2012
12. Two Level Model
Has all the functionality of the many-to many model but also
allows a user-level thread to be bound to a kernel thread
Operating System 2011/2012
13. API
API for creating and managing threads
●
No kernel support, strictly in use space so no system calls
involved
●
Kernel level directly supported by the OS. All code and data
structures for the library exits in kernel spacer
●
An API call typically invokes a system call
●
Three main libraries in use
●
POSIX (Portable Operating System Interface) threads
●
Win32
●
Java
Operating System 2011/2012
14. Pthreads
Pthreads refers to the POSIX standard (IEEE 1003.1c)
deifining an API for thread creation and synchronization
●
Pthreads is an IEEE and Open Group certified product
●
The Open Group is a vendor-neutral and technology-neutral
consortium, whose vision of Boundaryless Information
Flow™ will enable access to integrated information, within
and among enterprises, based on open standards and global
interoperability.
●
This is a specification for thread behavior not an
implementation
●
Implemented by Solaris, Linux, MacOS X and Tru64
●
Operating System 2011/2012
15. Windows XP Threads
Implements the one-to-one mapping
●
Each thread contains: a thread id, register set, separate user
and kernel stacks private data storage area
●
A Windows XP application runs as a separate process and
each process may contain one or more threads
●
The register set, stacks, and private storage area are known
as the context of the threads
●
The primary data structures of a thread include: ETHREAD
(executive thread block), KTHREAD (kernel thread block),
TEB (thread environment block)
●
Also provides support for a fiber library which implements
the many-to-many model
●
Operating System 2011/2012
16. Java
Java threads may be created by extending the Thread class or
implementing the Runnable interface
●
Java threads are managed by the JVM
●
Not really a user- or kernel-level thread, support provided at
the language level
●
No global data in Java, data is shared by passing the
reference to the appropriate threads
●
The JVM specification does not indicate how threads should
be mapped to the underlying OS
●
Windows 95/98/NT/2000 use the one-to-one model (each
Java thread maps to a kernel thread)
●
Solaris 2 used the many-to-many model
●
Operating System 2011/2012
18. Thread pools
Though creating a new thread is more efficient than creating a
new process, there are still some issues
There is a delay in creating the thread and it will be discarded
when done
There is no limit put on how many threads are allowed to be
created and system resources could be exhausted
Create a number of threads in a pool where they await work.
When needed, thread is awoken, when finished returns to pool
to await more work
Operating System 2011/2012
19. Thread pools – cont.
Advantages:
●
●
●
Usually slightly faster to service a request with an existing
thread than create a new thread
Allows the number of threads in the application(s) to be
bound to the size of the pool
Size of the pool can be dynamically adjusted based on usage
patterns
Operating System 2011/2012
