In computer science, a deadlock is a condition whereby two processes are waiting for a particular resource which the other has and will release only after receiving the resource for which it is waiting. Thus, the only way the resource will become available for one of the processes is if the other releases it — which neither will do, until it receives the resource that the other has.

For example, suppose Process A has a block of memory, and needs to access a disk drive. Meanwhile, Process B has control of the disk drive, and needs to put its information in memory — specifically, it needs the block of memory Process A controls. Process A will hold onto its block of memory and wait for the disk drive to become available, while Process B will hold onto the disk drive and wait for the memory to become available. Both processes are dependent on the other, thus they will wait for each other forever.

Thus, this situation is a vicious circle (as mentioned earlier) and is a good example of a Deadlock. This type of situation has also been referred to as a deadly embrace. A deadlock is, of course, an undesirable state in a computing system.

The deadlock is not only an informatical problem, the situation occurs in human society and my be refered to as Catch 22, or vicious cycle (or circle).
A familiar example of this circumstance occurs in the context of job searching. In moving from school to a career, one may encounter a Catch-22 where one cannot get a job without work experience, but one cannot gain experience without a job. Catch-22 situations are also sometimes called the chicken and egg problems.

Italiano:

***work in progress***

Da "Operating System Concepts" di "A. Silberschatz", "P. B. Galvin" e "G. Gagne" edizione "John Wiley & Sons, Inc."

(Sistema Operativo): È un programma che controlla ...

English:
(Underlined terms have/will have a voice in this glossary)

For example, a time sharing system must illude the user that more processes are running at the same time. The processes must alternate each other at a fairly high speed. We must also consider the waits that occur while switching between processes that should not exceed the CPU work time, in the worst case scenario the machine could take more time to save the proccesses state than the time it uses to actually run them, and this is obviously not acceptable. This situation would create an excessive Overhead.

Esxamples:
-FIFO
-SRTN
-RR
-IRQ
-Multilevel

Italiano:
(I termini sottolineati hanno/avranno una voce all'interno del glossario)

Con questo termine si intende la strategia usata dal nucleo per assegnare la risorsa CPU ai vari processi.

Per esempio un sistema di tipo time sharing deve gestire le risorse in modo tale da dare l'impressione che i processi di tutti gli utenti procedano simultaneamente. I processi si devono alternare in modo rapido, ma deve essere considerato anche che la sospensione di un processo e l'avvio di un altro richiede, da parte del sistema, un certo lavoro che non deve influire in maniera eccessiva, in calcolo percentuale, sul tempo macchina complessivo; altrimenti il sistema impiegherebbe più tempo per la sua manutenzione che per i processi utente e questo, per ovvi motivi, non può essere accettabile: ci sarebbe un eccessivo Overhead.

Esempi:
-Politica FIFO
-Politica SRTN
-Poltica RR
-Poltica IRQ
-Poltica Multilevel