An Introduction to threads

1. An Introduction to
threads
Presented by:
Zahra sadeghi

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history
 First: in late 60’s
 early 80’s : Unix community
 Windows 95 and OS/2
 Today: all operating systems have
multithreaded capability

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Terminology
 Thread:
 single sequence of execution
 independent path of execution
 Multiprogramming: Running multiple processes
concurrently
 Multithreading: Running multiple threads within one
process concurrently
 i.e: having several code executions within a
single process

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Process and thread
 A process is the "heaviest" unit of kernel
scheduling.
 process has its own memory area and data.
 A thread is the "lightest" unit of kernel
scheduling.
 the thread shares memory and data with the other
threads.
 A process/program,consists of many threads. (At
least one thread exists within each process.)

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Are threads actually needed!!!
 We are able to do the normal work with the
computer (like editing/saving a file) or drawing a
graphic and listening to music etc without getting
bothered with the print job.
 separate threads are executing all these tasks.
 the database or a web server interacts with a
number of users simultaneously.
 they maintain a separate thread for each user and
hence can maintain the state of all the users.
 If the program is run as one sequence then it is
possible that failure in some part of the program
will disrupt the functioning of the entire program.
 threads can execute independent of others

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drawbacks
 Is a Costly concept
 Many threads : less processor time
 memory : storing the context information of
each thread.
 Good designed program

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Multi_threading
 Each thread has its own run-time stack and
CPU state
 Two different threads can act on the same
object and same static fields concurrently

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example

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A thread control problem
multi-processor system:
 the operating system can allocate individual threads to the separate
processors, which thus fastens the execution of the program.
 the distribution of the threads on several processors is faster than
sharing time-slices on a single processor.

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Race Condition
 if shared data is accessed in an
unsynchronized manner
 Two threads are simultaneously reading or
modifying some shared data
 outcome :the order in which the
program's threads are allocated CPU time

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 On a single processor system,
threads can be run either in a preemptive
mode or in a cooperative mode.
threads
preemptive cooperative

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preemptive mode
 the operating system distributes the
processor time between the threads
 interrupts the threads at regular intervals
 The processor time given to each thread is
so small
 The switching between threads is so fast
that it appears as if all the threads are
running simultaneously.

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Cooperative mode
 each thread can control the CPU for as
long as it needs it.
 one thread can starve all the others
 Windows 3.x uses this kind of
implementation.
 A, B and C can be
considered as three
threads and are context
switched at the points
indicated by the small
circles.

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Implemntation of cooperative mode
 jmp_buf is defined in the ANSI C header file
setjmp.h.
 setjmp() : to save the execution environment
before switching out
 longjmp() : to switch to a place recorded in a
particular jump buffer.
 With this technique, we can implement a
non-preemptive threaded system with
minimal capability in about 100 lines!
 Note that a separate stack is required for each
executing function.

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Links & White Papers on Threads
 http://jamesthornton.com/linux/FAQ/Thre
ads-
 FAQ/ThreadLibs.html
 http://linuxdevices.com/articles/AT675369
9732.html
 http://www.gridbus.org/~raj/asc98.html