Multi-Level Queues

Classify jobs and maintain separate queues for each class.
Each queue can have its own scheduling algorithm.

Two examples of ways of choosing processes from the queues:

• Take jobs from the highest priority non-empty queue.
• Time slice among queues: each queue get a fixed share of the CPU time.

Examples of methods of assigning priorities:

• foreground vs. background
• batch vs. real time
• student vs. administrative

Multi-Level Feedback Queues

Allow jobs to change priority (and therefore their queue) dynamically.
To specify such a system you must specify:

• The number of queues
• The scheduling algorithm for each queue
• An initial queue assignment algorithm
• An upgrade priority algorithm
• A downgrade priority algorithm

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VAX VMS Scheduling

The VAX uses a multi-level feedback queue system with 32 priority levels:

• 16-31 are static real-time queues.
  They are scheduled according to priorities on a pre-emptive basis.
  A process is assigned a priority when it starts and that determines its queue.
• 0-15 are dynamic queues.
  All processes from a given queue are executed before processes from any lower number queue is considered.
  This is non -preemptive except for the quantum.
• Each system call has a priority increment.
  When an event occurs which causes a process to become executable, its priority is computed as the sum of the base and the increment.
• Each time a process is scheduled its priority is decremented, but not below the base.
• Processes also age if they spend a long time in a queue.
  This allows them to increase their priority.

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Windows NT Scheduling (also W2000, XP, maybe Vista)

Similar to the VAX (since it was designed by the VMS designers)

• Processes waiting for keyboard or mouse get largest priority increment.
• The foreground process in the current window get a larger quantum.
• Scheduling is preemptive even on the dynamic queues.

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Traditional UNIX Scheduling Algorithm

UNIX 4.3 uses multilevel feedback queues.

• Each runnable process is assigned a number that determines which queue they are placed in.
• Lower numbers mean higher priority.
  Negative numbers are for system processes which cannot be killed by signals.
• First process in lowest nonempty queue is run.
  You can nice to reduce your priority.
• Time quantum of 0.1 second is used, and priorities are recalculated once a second.
• No preemption except for quantum expiration.
• Under some flavors of Unix, an interactive process running in a window that has the focus is given a higher priority.
• The process's user-mode priority is:
  p_usrpri = P_USER + .25 p_cpu + 2 p_nice
  here p_cpu is incremented each time the system clock ticks and the process is found to be executing.
  It is also adjusted once per second for ready processes using a digital decay filter.
  
  where load is the sampled average length of the run queue over the previous 1-minute interval of system operation.
• When a process is blocked for an event, it cannot accumulate CPU time. When a process has been sleeping for longer than 1 second
  
  where p_slptime is an estimate of how long its been blocked and load is the current system load
• System does not preempt processes executing in kernel mode. This means it is not good for real-time system.

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Linux Scheduling Algorithm
This uses two separate process scheduling algorithms:
one for time-sharing which concentrates on fairness, and
one for real-time tasks where absolute priorities are more important.

Time sharing processes based on a credit system.
Each process has a fixed priority and a variable number of credits.

• When a new task must be chosen, the one with the most credits is run.
• Every time the timer interrupt occurs, the running process loses one credit and is removed from the CPU when its credits run out.
• If no runnable process has any credits, the credits of all processes in the system are adjusted using the formula:
  credits = credits/2 + priority
  
• What does this do if left to run?
• Can this starve out processes?

Real-time scheduling, each process has a priority and a scheduling class. The scheduling classes are FIFO=FCFS and RR.

• the highest priority real-time task is run first
• only if there are no runnable real-time tasks will a time-sharing task run.
• for tasks of the same priority it is FCFS
• if scheduling class is FCFS, will run until it requests I/O
• real-time processes do not preempt other processes

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