CPU%20Scheduling

1
CPU Scheduling

• Chapter 7

2
CPU Scheduling

• Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
• FCFS (FIFO)
• SJN SRTN
• Priority Scheduling
• Round Robin Scheduling
• Multilevel Queue
• Multilevel Feedback Queue

3
CPU Scheduling

• A multiprogramming OS allows more than one
  process to be loaded in main memory at a time.
• Processes share the CPU using time-multiplexing
• A process execution consists of a cycle of CPU
  computation--I/O operations.
• I/O operations require orders of magnitude more
  time to complete.
• Basic Idea When the running process requests an
  I/O operation, allocate CPU to another process.

4
CPU Scheduler

• CPU Scheduler that part of the Process Manager
  than is responsible for
• handling removal of running process from CPU
• Selection of another process
• Two major issues
• Scheduling mechanism how is it all done?
• Scheduling policy
• when is it time for a process to be removed from
  CPU?
• Which ready process should be allocated the CPU
  next?

5
Scheduling Mechanism
6
Scheduling Mechanism CNTD

• Three parts
• enqueuer
• dispatcher
• context-switcher
• Data Structures
• Process Descriptor
• Ready List

7
Scheduling Mechanism CNTD

• When a process is moved to the Ready-List
• Process Descriptor (PD) is updated
• the enqueuer places a pointer to PD in the
  Ready-List
• When the Scheduler switches CPU from one process
  to another process
• the Context-Switcher saves the state of the
  current process in its PD.
• How context-switching occurs depends on how CPU
  multiplexing technique used
• voluntary multiplexing
• involuntary multiplexing

8
Scheduling Mechanism CNTD

• Voluntary multiplexing Running process gives up
  CPU voluntarily
• context-switcher is invoked by running process.
• Involuntary multiplexing an interrupt causes
  running process to be removed from CPU.
• Interrupt generated by an I/O operation requested
  by another process.
• Most commonly a timer generated interrupt.
• In either case, interrupt handler invokes
  context-switcher.

9
Scheduling Mechanism--Dispatcher

• After state of "old" process is saved by
  context-switcher, the CPU is allocated to the
  Dispatcher
• Dispatcher state is loaded on CPU
• Dispatcher selects one of the ready processes
  enqueued in the Ready-List.
• Dispatcher performs another context-switch from
  itself to selected process (saves its state and
  loads state of selected process).
• The Process Descriptor of selected process is
  changed from Ready to Running.

10
Process Scheduling
11
Scheduling Policy Criteria
12
Optimization Criteria
13
First-Come-First-Served (FCFS) Scheduling
14
FCFS Scheduling (cont.)
15
Shortest-Job-Next (SJN) Scheduling
16
Example of Non-Preemptive SJN
17
Example of Preemptive SJN
18
Priority Scheduling
19
Round Robin (RR) Scheduling
20
Example RR with time quantum20
21
Deadline Scheduling

• Real Time Systems
• Processes must complete their task by specific
  deadlines
• Main performance criteria
• Scheduler must have complete knowledge of service
  time of each process
• All function must be predictable no virtual
  memory
• A process is admitted to ready list only if OS
  can guarantee deadline can be met.

22
Example

• Process Service Time Deadline
• 0 350 575
• 1 125 550
• 2 475 1050
• 3 250 none
• 4 75 200

23
Multi-Level Queue

• Extension of priority scheduling, which also
  combines other strategies
• Ready list is partitioned into multiple sub-lists
• Each process is assigned to a queue based on some
  criteria (type, priority, etc.)
• E.g. one Q for foreground processes and one for
  background processes
• Scheduler in-queue strategy
• cross-queue strategy
• E.g. In-queue strategy RR for foreground Q and
  FCFS for background A
• Cross-queue strategy Serve higher-level
  processes first

24
Multi-Level Queue

• Example
• Cross-Queue Strategy Each queue get a percentage
  of CPU time (in a round robin fashion) which it
  can schedule among its processes.
• E.g. Level 1 80 of CPU time
• Level 2 20 of CPU time

25
Multi-Level Feedback Queue
26
ExampleMulti-Level Feedback Queue

• Gives shorter jobs higher priority without
  needing to predict a jobs service time
  requirement.

27
BSD UNIX Scheduling

• Multiple-level feedback queue approach
• 32 queues
• System processes are placed in Q0 Q7
• User processes are placed in Q8 Q31
• Dispatcher always selects a process from highest
  priority queue to run
• RR is used in each queue (time slice varies but
  always lt 100µs)
• A processs changes over time based on nice()
  system calls and processs utilization of CPU.

28
Windows NT/2K

• Multiple-level feedback queues for thread
  scheduling
• Priority to those threads that need very rapid
  response
• 32 levels
• 16 highest priority Qs are called real-time level
  queues
• Next 15 Qs are variable-level queue
• Lowest priority Q is called system-level Q.
• System-level Q contains a single thread called
  zero-page thread. It is run only when there are
  no other runnable threads.
• Scheduling goes from highest level down
• Scheduling is preemptive if a high-priority
  thread becomes runnable while a lower priority
  thread is running, the latter is preempted and
  the higher level thread will begin to use the
  processor