BY SALMAN MEMON
UNIVERSITY OF SINDH
A thread is contained inside a
process and different threads
in the same process share
some resources (most
commonly memory), while
different processes do not.
Process and threads
A thread is a basic unit of CPU utilization, consisting of a
program counter, a stack, and a set of registers, ( and a
thread ID. )
On a single processor, multithreading generally
occurs by as in multitasking, the processor switches
between different threads.
On a multiprocessor or multi-core system, the threads
or tasks actually do run at the same time, with each
processor or core running a particular thread or task
◦Has single thread of control
◦It allows the process to perform only 1 task at a time.
◦Has many threads
◦Simultaneous execution of different task
Take less time to create a new tread than a process.
Less time to terminate a tread than a presses.
less time to switch between two treads within the same process.
Hence treads within the same process share memory and files,
they can communicate with each other without invoking the
Resource sharing .
Utilization of MP Architectures.
Thread management done by user-level threads
◦Thread creation, scheduling, are done in user level
Fast to create and manage
◦If kernel is single thread, then user level thread performing a
blocking system call will cause entire process to block
Supported by OS
◦Thread creation, scheduling, are done in user level by kernel
Thread management is performed by os, thus kernel
thread are slow.
If thread perform blocking system call, kernel can
schedule another thread in application for execution
(In a specific implementation, the user threads must
be mapped to kernel threads, using one of the
following strategies. )
MANY-TO-ONEmany user-level threads are all
mapped onto a single kernel thread.
Thread management is handled by
the thread library in user space,
which is very efficient.
if a blocking system call is made, then
the entire process blocks, even if the
other user threads would otherwise
be able to continue.
Ex: Green threads for Solaris and GNU
Each user-level thread maps to kernel thread
Allow anther thread to run if block
Drawback: along with user thread kernel thread shld be
Ex: Linux and Windows from 95 to XP
The many-to-many model multiplexes
any number of user threads onto an
equal or smaller number of kernel
threads, combining the best features of
the one-to-one and many-to-one
Blocking kernel system calls do not
block the entire process.
Individual processes may be allocated
variable numbers of kernel threads,
depending on the number of CPUs
present and other factors.
Ex: RIX, HP-UX, and Tru64 UNIX
Thread libraries provide programmers with an API for
creating and managing threads.
Thread libraries may be implemented either in user space
or in kernel space.
There are three main thread libraries in use today:
POSIX Pthreads .
Java threads .
May be provided either as user-level or kernel-level.
A POSIX standard (IEEE 1003.1c) API for thread creation and
API specifies behavior of the thread library, implementation is up
to development of the library.
Common in UNIX operating systems (Solaris, Linux, Mac OS X).
Linux refers to them as tasks rather than threads.
Thread creation is done through clone() system
clone() allows a child task to share the address
space of the parent task (process).
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