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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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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
