detail about multi threading

1. Represnted By
Ezzat gul

2. The ability of an operating system to execute
different parts of a program, called threads,
simultaneously. The programmer must
carefully design the program in such a way
that all the threads can run at the same time
without interfering with each other.

3.  main ()
 {.....
 f1(.........) ;
 f2(..........) ;
 }
 f1(.............)
 f2(..............)
 }
Thread
Process
terminated
f1
f2

4.  main ()
 {
 ........
 Thread (t1,f1) ;
 Thread (t2,f2) ;
 .........
 }
 f1(.......)
 {...........}
 f2(........)
 {..........}
Process Address
Space
f1 f2
PC
PC

5. Threads compared with
processes :
Processes carry considerable state information,
where multiple threads within a process share state
as well as memory and other resources Processes
have separate address space, where threads share
their address space Context switching between
threads in the same process is typically faster than
context switching between processes. Processes are
typically independent, while threads exist as subsets
of a process.

6. Advantages of Multi-threading
:
If a thread can not use all the computing resources of
the CPU (because instructions depend on each other's
result), running another thread permits to not leave
these idle. If several threads work on the same set of
data, they can actually share its caching, leading to
better cache usage or synchronization on its values. If a
thread gets a lot of cache misses, the other thread(s)
can continue, taking advantage of the unused
computing resources, which thus can lead to faster
overall execution, as these resources would have been
idle if only a single thread was executed.

7. Two levels of thread :
Two levels of thread User level(for user
thread) Kernel level(for kernel thread)?

8. User Threads :
User threads are supported above the kernel and are
implemented by a thread library at the user level. The
library provides support for thread creation, scheduling,
and management with no support from the
kernel.Because the kernel is unaware of user-level
threads, all thread creation and scheduling are done in
user space without the need for kernel intervention.
User-level threads are generally fast to create and
manage User-thread libraries include POSIX
Pthreads,Mach C-threads,and Solaris 2 UI-threads.

9. Kernel Threads :
Kernel threads are supported directly by
the operating system:The kernel performs
thread creation, scheduling, and
management in kernel space. Because
thread management is done by the
operating system, kernel threads are
generally slower to create and manage
than are user threads. Most operating
systems-including Windows NT,
Windows 2000, Solaris 2, BeOS, and
Tru64 UNIX (formerly Digital UN1X)-

10. Multi-threading Models :
There are three models for thread libraries,
each with its own trade-offs many threads on
one LWP (many-to-one)? one thread per LWP
(one-to-one)? many threads on many LWPs
(many-to-many)?

11.  In this model, the library maps all
threads to a single lightweight process
 Advantages:
 totally portable
 easy to do with few systems
dependencies

12.  In this model, the library maps each
thread to a different lightweight
process
 Advantages:
 can exploit parallelism, blocking
system calls
 Disadvantages:
 thread creation involves LWP
creation
 each thread takes up kernel
resources
 limiting the number of total threads
 Used in LinuxThreads and other
systems where LWP creation is not too
expensive

13.  In this model, the library has
two kinds of threads: bound and
unbound
 bound threads are mapped
each to a single lightweight
process
 unbound threads may be
mapped to the same LWP
 Probably the best of both worlds
 Used in the Solaris
implementation of Pthreads
(and several other Unix
implementations)