Hi
Quoting this article which failed get to the list.
It sounds pretty technical, nevertheless, for those who can get 2 cents out of
it. Rest don't get worked up and think you seem to be stupid.
Harish Kotian
Quoting:
We all get error message like this "Windows Explorer is not responding" One of
the reason is because the system is in a deadlock situation. To discover
more what is computer deadlock click
here You go:
Deadlock
>From Wikipedia, the free encyclopedia
Process_deadlock
Enlarge
Both processes need both resources. P1 requires additional resource R1, P2
requires additional resource R2; neither process can continue.
A deadlock is a situation wherein two or more competing actions are each
waiting for the other to finish, and thus neither ever does.
In an
operating system,
a deadlock is a situation which occurs when a process enters a waiting
state
because a
resource
requested by it is being held by another waiting process, which in turn is
waiting for another resource. If a process is unable to change its state
indefinitely
because the resources requested by it are being used by other waiting process,
then the system is said to be in a deadlock.
[1]
Deadlock is a common problem in
multiprocessing
systems,
parallel computing
and
distributed systems,
where software and hardware
locks
are used to handle shared resources and implement
process synchronization.
[2]
In
telecommunication systems,
deadlocks occur mainly due to lost or corrupt signals instead of resource
contention.
[3]
Contents
[hide]
list of 9 items
1Examples
2Necessary conditions
3Deadlock handling
list of 4 items nesting level 1
3.1Ignoring deadlock
3.2Detection
3.3Prevention
3.4Avoidance
list end nesting level 1
4Livelock
5Distributed deadlock
list of 1 items nesting level 1
5.1Distributed deadlock prevention
list end nesting level 1
6See also
7References
8Further reading
9External links
list end
[
edit
]Examples
Deadlock situation can be compared to the classic "
chicken or egg"
problem.
[4]
It can be also considered a paradoxical "
Catch-22"
situation.
[5]
A real world analogical example would be an
illogical
statute
passed by the
Kansas
legislature in early 20th century which stated:
[1]
[6]
"
When two trains approach each other at a crossing, both shall come to a full
stop and neither shall start up again until the other has gone.
"
A simple computer based example is as follows. Suppose a computer has three CD
drives and three processes. Each of the three processes holds one of the
drives. If each process now requests another drive, the three processes will be
in a deadlock. Each process will be waiting for the CD drive released event,
which can be only caused by one of the other waiting processes. Thus, it
results in a
circular chain.
[
edit
]Necessary conditions
A deadlock situation can arise only if all of the following conditions hold
simultaneously in a system:
[1]
list of 4 items
Mutual Exclusion:
At least one resource must be non-shareable.
[1]
Only one process can use the resource at any given instant of time.
Hold and Wait or Resource Holding: A process is currently holding at least one
resource and requesting additional resources which are being held by other
processes.
No Preemption:
The operating system must not de-allocate resources once they have been
allocated; they must be released by the holding process voluntarily.
Circular Wait:
A process must be waiting for a resource which is being held by another
process, which in turn is waiting for the first process to release the resource.
In general, there is a
set
of waiting processes, P = {P1, P2, ..., PN}, such that P1 is waiting for a
resource held by P2, P2 is waiting for a resource held by P3 and so on till
PN is waiting for a resource held by P1.
[1]
[7]
list end
These four conditions are known as the Coffman conditions from their first
description in a 1971 article by
Edward G. Coffman, Jr.
[7]
Unfulfillment of any of these conditions is enough to preclude a deadlock from
occurring.
[
edit
]Deadlock handling
Most current operating systems cannot prevent a deadlock from occurring.
[1]
When a deadlock occurs, different operating system respond to them in
different non-standard manners. Most approaches work by preventing one of the
four
Coffman conditions from occurring, especially the fourth one.
[8]
Major approaches are as follows.
[
edit
]Ignoring deadlock
In this approach, it is assumed that a deadlock will never occur. This is also
called the
Ostrich algorithm.
[8]
[9]
This approach was initially used by
MINIX
and
UNIX.
[7]
This is used when the time intervals between occurrences of deadlocks is large
and the data loss incurred each time is tolerable. It is avoided in very
critical systems.
[
edit
]Detection
Under deadlock detection, deadlocks are allowed to occur. Then the state of the
system is examined to detect that a deadlock has occurred and subsequently
it is corrected. An algorithm is employed that tracks resource allocation and
process states, it rolls back and restarts one or more of the processes in
order to remove the a detected deadlock. Detecting a deadlock that has already
occurred is easily possible since the resources that each process has locked
and/or currently requested are known to the resource scheduler of the operating
system.
[9]
This approach is simpler than deadlock avoidance or deadlock prevention. It is
so because predicting a deadlock before it happens is difficult, as it is
generally an
undecidable problem,
which itself results in a
halting problem.
However, in specific environments, using specificmeans of locking resources,
deadlock detection may be decidable. In the general case, it is not possible
to distinguish between algorithms that are merely waiting for a very unlikely
set of circumstances to occur and algorithms that will never finish because
of deadlock.
Deadlock detection techniques include, but are not limited to
model checking.
This approach constructs a
finite state
-model on which it performs a progress analysis and finds all possible terminal
sets in the model. These then each represent a deadlock.
After a deadlock is determined, it can be corrected by using one of the
following methods:
list of 2 items
Process Termination: One or more process involved in the deadlock may be
aborted. We can choose to abort all processes involved in the deadlock. This
ensures
that deadlock is resolved with certainty and speed. But the expense is high as
partial computations will be lost. Or, we can choose to abort one process
at a time until the deadlock is resolved. This approach has high overheads
because after each abortion an algorithm must detect if the system is still
in deadlock. Several factors must be considered while choosing a candidate for
termination, such as priority and age of the process.
Resource Preemption: Resource allocated to various processes may be
successively preempted and allocated to other processes until deadlock is
broken.
list end
[
edit
]Prevention
Deadlock prevention works by preventing one of the four Coffman conditions from
occurring.
list of 4 items
Removing the mutual exclusion condition means that no process will have
exclusive access to a resource. This proves impossible for resources that cannot
be
spooled.
But even with spooled resources, deadlock could still occur. Algorithms that
avoid mutual exclusion are called
non-blocking synchronization
algorithms.
The hold and wait or resource holding conditions may be removed by requiring
processes to request all the resources they will need before starting up (or
before embarking upon a particular set of operations). This advance knowledge
is frequently difficult to satisfy and, in any case, is an inefficient use
of resources. Another way is to require processes to request resources only
when it has none. Thus, first they must release all their currently resources
before requesting all the resources they will need from scratch. This too is
often impractical. It is so because resource may be allocated and remain unused
for long periods. Also, a process requiring a popular resource may have to wait
indefinitely as such a process may be always allocated to some process,
resulting in
resource starvation.
[1]
(These algorithms, such as
serializing tokens,
are known as the all-or-none algorithms.)
The no
preemption
condition may also be difficult or impossible to avoid as a process has to be
able to have a resource for a certain amount of time, or the processing outcome
may be inconsistent or
thrashing
may occur. However, inability to enforce preemption may interfere with a
priority algorithm. Preemption of a "locked out" resource generally implies a
rollback,
and is to be avoided, since it is very costly in overhead. Algorithms that
allow preemption include
lock-free and wait-free algorithms
and
optimistic concurrency control.
The final condition is the circular wait condition. Approaches that avoid
circular waits include disabling interrupts during critical sections and using
a hierarchy to determine a
partial ordering
of resources". If no obvious hierarchy exists, even the memory address of
resources has been used to determine ordering and resources are requested in
the increasing order of the enumeration.
[1]
The
Dijkstra's solution
can also be used.
list end
[
edit
]Avoidance
Deadlock can be avoided if certain information about processes are available to
the operating system before allocation of resources, such as which resources
a process will consume in its lifetime. For every resource request, the system
sees if granting the request will mean that the system will enter an unsafe
state, meaning a state that could result in deadlock. The system then only
grants requests that will lead to safestates.
[1]
In order for the system to be able to determine whether the next state will be
safe or unsafe, it must know in advance at any time:
list of 3 items
resources currently available
resources currently allocated to each process
resources that will be required and released by these processes in the future
list end
It is possible for a process to be in an unsafe state but for this to not
result in a deadlock. The notion of safe/unsafe states only refers to theability
of the system to enter a deadlock state or not. For example, if a process
requests A which would result in an unsafe state, but releases B which would
prevent circular wait, then the state is unsafe but the system is not in
deadlock.
One known algorithm that is used for deadlock avoidance is the
Banker's algorithm,
which requires resource usage limit to be known in advance.
[1]
However, for many systems it is impossible to know in advance what every
process will request. This means that deadlock avoidance is often impossible.
Two other algorithms are Wait/Die and Wound/Wait, each of which uses a
symmetry-breaking technique. In both these algorithms there exists an older
process
(O) and a younger process (Y). Process age can be determined by a timestamp at
process creation time. Smaller timestamps are older processes, while larger
timestamps represent younger processes.
table with 3 columns and 3 rows
Wait/Die
Wound/Wait
O needs a resource held by Y
O waits
Y dies
Y needs a resource held by O
Y dies
Y waits
table end
[
edit
]Livelock
A livelock is similar to a deadlock, except that the states of the processes
involved in the livelock constantly change with regard to one another, none
progressing.
[10]
Livelock is a special case of
resource starvation
; the general definition only states that a specific process is not progressing.
[11]
A real-world example of livelock occurs when two people meet in a narrow
corridor, and each tries to be polite by moving aside to let the other pass, but
they end up swaying from side to side without making any progress because they
both repeatedly move the same way at the same time.
Livelock is a risk with some algorithms that detect and recover from deadlock.
If more than one process takes action, the deadlock detection algorithm can
be repeatedly triggered. This can be avoided by ensuring that only one process
(chosen randomly or by priority) takes action.
[12]
So what to do when Windows shown this kind error message? Well there are many
reasons this kind error occur, no one perfect explanation for all different
situation. Sometimes, when you insert a disk you will be unable to play what's
inside the disk and you get an error like this. Sometimes, when you try
to
install programs
and it never ends error like this again. This is the most common error, there
are two ways you can solve this, manually stop this, if you click "close
the program" doesn't work then
list of 2 items
Tip #1 press "control + Shift + Esc" this will bring you the
task manager,
yes it's not the old "control + Alt + delete". When the task manage is up you
can find which program uses the most CPU/memory, and manually stop them.
Hopefully your computer won't get frozen again.
Tip #2 this applies when you constantly getting this kind error, say whenever
you do something your PC froze. So if you ever have this kind situation than
there is a real problem. Inside
the Windows
7 you can try to use a very useful tool that has already been build-in, it's
called the
Problem Steps Recorder.
( find it just type "problem Steps recorder" in the start manual)
list end
Good luck ! and Enjoy your
new Windows 7
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