In computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

, a multilevel feedback queue is a scheduling

Scheduling (computing)

In computer science, a scheduling is the method by which threads, processes or data flows are given access to system resources . This is usually done to load balance a system effectively or achieve a target quality of service...

 algorithm. It is intended to meet the following design requirements for multimode systems:

1. Give preference to short jobs.
2. Give preference to I/O bound processes.
3. Separate processes into categories based on their need for the processor

Multiple FIFO

FIFO

FIFO is an acronym for First In, First Out, an abstraction related to ways of organizing and manipulation of data relative to time and prioritization...

 queues are used and the operation is as follows:

1. A new process is positioned at the end of the top-level FIFO
   FIFO
   FIFO is an acronym for First In, First Out, an abstraction related to ways of organizing and manipulation of data relative to time and prioritization...
   
    queue.
2. At some stage the process reaches the head of the queue and is assigned the CPU
   Central processing unit
   The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
   
   .
3. If the process is completed it leaves the system.
4. If the process voluntarily relinquishes control it leaves the queuing network, and when the process becomes ready again it enters the system on the same queue level.
5. If the process uses all the quantum time, it is pre-empted
   Preemption (computing)
   In computing, preemption is the act of temporarily interrupting a task being carried out by a computer system, without requiring its cooperation, and with the intention of resuming the task at a later time. Such a change is known as a context switch...
   
    and positioned at the end of the next lower level queue.
6. This will continue until the process completes or it reaches the base level queue.

• At the base level queue the processes circulate in round robin
  Round-robin scheduling
  Round-robin is one of the simplest scheduling algorithms for processes in an operating system. As the term is generally used, time slices are assigned to each process in equal portions and in circular order, handling all processes without priority . Round-robin scheduling is simple, easy to...
  
   fashion until they complete and leave the system.
• Optionally, if a process blocks for I/O, it is 'promoted' one level, and placed at the end of the next-higher queue. This allows I/O bound processes to be favored by the scheduler and allows processes to 'escape' the base level queue.

In the multilevel feedback queue, a process is given just one chance to complete at a given queue level before it is forced down to a lower level queue.

See also

• Lottery scheduling
  Lottery Scheduling
  Lottery Scheduling is a probabilistic scheduling algorithm for processes in an operating system. Processes are each assigned some number of lottery tickets, and the scheduler draws a random ticket to select the next process. The distribution of tickets need not be uniform; granting a process more...
  
  
• Scheduling (computing)
  Scheduling (computing)
  In computer science, a scheduling is the method by which threads, processes or data flows are given access to system resources . This is usually done to load balance a system effectively or achieve a target quality of service...
  
  
• Fair-share scheduling
  Fair-share scheduling
  Fair-share scheduling is a scheduling strategy for computer operating systems in which the CPU usage is equally distributed among system users or groups, as opposed to equal distribution among processes....
  
  
• Round-robin scheduling
  Round-robin scheduling
  Round-robin is one of the simplest scheduling algorithms for processes in an operating system. As the term is generally used, time slices are assigned to each process in equal portions and in circular order, handling all processes without priority . Round-robin scheduling is simple, easy to...