Job control (computing)
 |
Job control in computing refers to the control of multiple tasks or Jobs on a computer system, ensuring that they each have access to adequate resources to perform correctly, that competition for limited resources does not cause a deadlock where two or more jobs are unable to complete, resolving such situations where they do occur, and terminating jobs that, for any reason, are not performing as expected.
Job control has developed from the early days of computers where human operators were responsible for setting up, monitoring and controlling every job, to modern operating systems which take on the bulk of the work of job control.
Even with a highly sophisticated scheduling system, some human intervention is desirable. Modern systems permit their users to stop and resume jobs, to execute them in the foreground (with the ability to interact with the user) or in the background. Unix-like systems follow this pattern.
History
bem seus filhos da puta agora esstudam pela merda do livro seus retardados. sempre a ir na wikipedia seus punheteiros
Real-time computing
Pre-emptive multitasking with job control assures that a system operates in a timely manner most of the time. In some environments (for instance, operating expensive or dangerous machinery), a strong design constraint of the system is the delivery of timely results in all circumstances. In such circumstances, job control is more complex and the role of scheduling is more important.
Job control languages
Early computer resident monitors and operating systems were relatively primitive and were not capable of sophisticated resource allocation. Typically such allocation decisions were made by the computer operator or the user who submitted a job. Batch processing was common, and interactive computer systems rare and expensive. Job control languages (JCLs) developed as primitive instructions, typically punched on cards at the head of a deck containing input data, requesting resources such as memory allocation, serial numbers or names of magnetic tape spools to be made available during execution, or assignment of filenames or devices to device numbers referenced by the job. A typical example of this kind of language, still in use on mainframes, is IBM's Job Control Language (also known as JCL). Though the format of early JCLs was intended for punched card use, the format survived the transmission to storage in computer files on disk.
As time sharing systems developed, interactive job control emerged. An end-user in a time sharing system could submit a job interactively from his remote terminal, communicate with the operators to warn them of special requirements, and query the system as to its progress. He could assign a priority to the job, and terminate (kill) it if desired. He could also, naturally, run a job in the foreground, where he would be able to communicate directly with the executing program. During interactive execution he could interrupt the job and let it continue in the background or kill it. This development of interactive computing in a multitasking environment led to the development of the modern shell.