Operating Systems Principles Process Management and Coordination Lecture 5: Process and Thread Sched

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Operating Systems PrinciplesProcess Management
and CoordinationLecture 5Process and Thread
Scheduling

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Content

• Organization of Schedulers
• Embedded and Autonomous Schedulers
• Priority Scheduling
• Scheduling Methods
• A Framework for Scheduling
• Common Scheduling Algorithms
• Comparison of Methods
• Priority Inversion

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Operating Systems PrinciplesProcess Management
and CoordinationLecture 5Process and Thread
Scheduling

• Organization of Schedulers

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Process Scheduling/Dispatching
We use scheduling to refer to both.

• Process scheduling
• Long term scheduling
• Move process to Ready List (RL) after creation
  (When and in which order?)
• Process Dispatching
• Short term scheduling
• Select process from Ready List to run

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Organization of Schedulers

• Embedded
• Called as function at end of kernel call
• Runs as part of calling process
• Autonomous
• Separate process
• May have dedicated CPU on a multiprocessor
• On single-processor,run at every
  quantumscheduler and other processes alternate

pi process S scheduler
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Organization of Schedulers

• Embedded
• Called as function at end of kernel call
• Runs as part of calling process
• Autonomous
• Separate process
• May have dedicated CPU on a multiprocessor
• On single-processor,run at every
  quantumscheduler and other processes alternate

S
S
S
S
Windows 2000
OS
Unix
S
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Priority Scheduling
Who runs next?
Based on priority

• Priority function returns numerical value P for
  process p
• P Priority(p)
• Static priority unchanged for lifetime of p
• Dynamic priority changes at runtime

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Priority-Leveled Processes

• Priority divides processes into levels
• Implemented as multilevel ready list, say, RL
• p at RLi run before q at RLj if i gt j
• p, q at same level are ordered by other criteria,
  e.g., the order of arrival.

RLn
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The Processes in Ready List
Status.Type ? running, ready_a, ready_s
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The Scheduler
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The Scheduler

• The scheduler may be invoked
• periodically
• when some events affect the priorities of
  existing processes, e.g.,
• a process is blocked/suspended
• a process is destroyed
• a new process created
• a process is activated

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The Scheduler
The scheduler/dispatcher should always keep the
np (processors) highest-priority active
processes running.
Some processes in RL can get CPU.

• The scheduler may be invoked
• periodically
• when some events affect the priorities of
  existing processes, e.g.,
• a process is blocked/suspended
• a process is destroyed
• a new process created
• a process is activated

Running processes may be preempted.
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Invoking the Scheduler

• The caller to the scheduler may be a process to
  be
• blocked/suspended or
• awakened/activated

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Invoking the Scheduler

• The caller to the scheduler may be a process to
  be
• blocked/suspended or
• awakened/activated

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An Embedded Scheduler

• The caller to the scheduler may be a process to
  be
• blocked/suspended or
• awakened/activated

Scheduler() do Find highest priority
ready_a process p Find a free cpu
if (cpu ! NIL) Allocate_CPU(p,cpu) while
(cpu ! NIL) do Find highest
priority ready_a process p Find lowest
priority running process q if (Priority(p)
gt Priority(q)) Preempt(p,q) while
(Priority(p) gt Priority(q)) if
(self-gtStatus.Type!running) Preempt(p,self)
If free CPUs are available, allocate these free
CPUs to high-priority processes in ready_a state
To preempt low-priority running processes by
high-priority ready_a ones.
If the caller is going to be idled, preempt
itself.
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An Embedded Scheduler

• The caller to the scheduler may be a process to
  be
• blocked/suspended or
• awakened/activated

Scheduler() do Find highest priority
ready_a process p Find a free cpu
if (cpu ! NIL) Allocate_CPU(p,cpu) while
(cpu ! NIL) do Find highest
priority ready_a process p Find lowest
priority running process q if (Priority(p)
gt Priority(q)) Preempt(p,q) while
(Priority(p) gt Priority(q)) if
(self-gtStatus.Type!running) Preempt(p,self)
To preempt low-priority running processes by
high-priority ready_a ones.
If the caller is going to be idled, preempt
itself.
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An Embedded Scheduler

• The caller to the scheduler may be a process to
  be
• blocked/suspended or
• awakened/activated

Scheduler() do Find highest priority
ready_a process p Find a free cpu
if (cpu ! NIL) Allocate_CPU(p,cpu) while
(cpu ! NIL) do Find highest
priority ready_a process p Find lowest
priority running process q if (Priority(p)
gt Priority(q)) Preempt(p,q) while
(Priority(p) gt Priority(q)) if
(self-gtStatus.Type!running) Preempt(p,self)
If the caller is going to be idled, preempt
itself.
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An Embedded Scheduler

• The caller to the scheduler may be a process to
  be
• blocked/suspended or
• awakened/activated

Scheduler() do Find highest priority
ready_a process p Find a free cpu
if (cpu ! NIL) Allocate_CPU(p,cpu) while
(cpu ! NIL) do Find highest
priority ready_a process p Find lowest
priority running process q if (Priority(p)
gt Priority(q)) Preempt(p,q) while
(Priority(p) gt Priority(q)) if
(self-gtStatus.Type!running) Preempt(p,self)
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An Embedded Scheduler
How to simplify the scheduler for a uniprocessor
system?
Scheduler() do Find highest priority
ready_a process p Find a free cpu
if (cpu ! NIL) Allocate_CPU(p,cpu) while
(cpu ! NIL) do Find highest
priority ready_a process p Find lowest
priority running process q if (Priority(p)
gt Priority(q)) Preempt(p,q) while
(Priority(p) gt Priority(q)) if
(self-gtStatus.Type!running) Preempt(p,self)
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Operating Systems PrinciplesProcess Management
and CoordinationLecture 5Process and Thread
Scheduling

• Scheduling Methods

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Scheduling Policy

• Scheduling based on certain criteria.
• For examples
• batch jobs vs. interactive users
• System processes vs. user processes
• I/O bound processes vs. CPU bound processes

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The General Framework

• Decision mode
• Priority function
• Arbitration Rule

A scheduling policy is determined by

• When to schedule?
• Preemptive
• Nonpreemptive
• Who to schedule?
• Priority evaluation
• Arbitration to break ties

Decision Mode
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The Decision Modes

• Decision mode
• Priority function
• Arbitration Rule

A scheduling policy is determined by

• Preemptive scheduler called
• periodically (quantum-oriented) or
• when system state changes
• More costly
• Nonpreemptive scheduler called
• when process terminates or blocks
• Not adequate in real-time or time-shared systems

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The Priority Function

• Decision mode
• Priority function
• Arbitration Rule

A scheduling policy is determined by

• Possible parameters
• Attained service time (a)
• Real time in system (r)
• Total service time (t)
• Period (d)
• Deadline (explicit or implied by period)
• External priority (e)
• Memory requirements (mostly for batch)
• System load (not process-specific)

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The Arbitration Rules

• Decision mode
• Priority function
• Arbitration Rule

A scheduling policy is determined by

• Break ties among processes of the same priority
• Random
• Chronological (First In First Out FIFO)
• Cyclic (Round Robin RR)

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Common Scheduling Algorithms

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

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Example Processesfor Scheduling

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

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FIFO

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

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FIFO

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

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SJF

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

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SJF

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

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SRT

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

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SRT

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

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RR

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

Assume a time-quantum of 0.1 time units.
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RR

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

Assume a time-quantum of 0.1 time units.
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ML

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

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ML

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

Each process has a fixed priority.
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ML (Nonpreemptive)

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

Each process has a fixed priority.
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ML (Preemptive)

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

Each process has a fixed priority.
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ML (Nonpreemptive)

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

ep1 15
ep2 14
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MLF

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

• Like ML, but priority changes dynamically
• Every process enters at highest level n
• Each level P prescribes maximum time tP
• tP increases as P decreases
• Typically
• tn T (constant)
• tP 2 ? tP1

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MLF

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

a attained service time

• The priority of a process is a function of a.
• Find P for given a
• priority attained time
• n altT
• n 1 altT2T
• n 2 altT2T4T
• . . . . . .
• n i alt(2i11)T

P n i n ?lg2(a/T1)?
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MLF

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

a attained service time

• The priority of a process is a function of a.
• Find P for given a
• priority attained time
• n altT
• n 1 altT2T
• n 2 altT2T4T
• . . . . . .
• n i alt(2i11)T

P n i n ?lg2(a/T1)?
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RM EDF

• First-In/First-Out (FIFO)
• Shortest-Job-First (SJF)
• Shortest-Remaining-Time (SRT)
• Round-Robin (RR)
• Multilevel Priority (ML)
• Multilevel Feedback (MLF)
• Rate Monotonic (RM)
• Earliest Deadline (EDF)

• RM
• Intended for periodic (real-time) processes
• Preemptive
• Highest priority shortest period P d
• EDF
• Intended for periodic (real-time) processes
• Preemptive
• Highest priority shortest time to next deadline
• r ? d number of completed periods
• r d time in current period
• d r d time remaining in current period
• P (d r d)

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Common Scheduling Algorithms
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Comparison of Methods

• FIFO, SJF, SRT Primarily for batch systems
• FIFO is the simplest
• SJF SRT have better average turnaround times

ri the real time that the ith process spends
in the system.
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Example
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Comparison of Methods

• Time-sharing systems
• Response time is critical
• RR or MLF with RR within each queue are suitable

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Comparison of Methods

• Time-sharing systems
• Response time is critical
• RR or MLF with RR within each queue are suitable
• Choice of quantum determines overhead
• When q ? ?, RR approaches FIFO
• When q ? 0, context switch overhead ? 100
• When q gtgt context switch overhead,n processes
  run concurrently at 1/n CPU speed

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Interactive Systems

• Most dynamic schemes tend to move interactive and
  I/O-bound processes to the top of the priority
  queues and to let CPU-bound processes drift to
  lower levels.
• MLF puts I/O-completed processes into highest
  priority queue.

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Comparison of Methods

• Real-time systems
• Feasible All deadlines are met
• CPU utilization is defined as U? ti/di
• Schedule is feasible if U ? 1
• EDF always yields feasible schedule (if U ? 1)
• RM yields feasible schedule if U ? 0.7

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Example RM and EDF
CPU Utilization
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Example RM and EDF
CPU Utilization
?
fail
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Operating Systems PrinciplesProcess Management
and CoordinationLecture 5Process and Thread
Scheduling

• Priority Inversion

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Priority Inversion Problem

• Assume priority order p1 gt p2 gt p3.
• p1 and p3 share common resource.
• p2 is independent of p1 and p3 .

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Priority Inversion Problem

• Assume priority order p1 gt p2 gt p3.
• p1 and p3 share common resource.
• p2 is independent of p1 and p3 .

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Priority Inversion Problem

• Assume priority order p1 gt p2 gt p3.
• p1 and p3 share common resource.
• p2 is independent of p1 and p3 .

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Priority Inversion Problem

• Assume priority order p1 gt p2 gt p3.
• p1 and p3 share common resource.
• p2 is independent of p1 and p3 .

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(Unrelated) p2 may delay p1 indefinitely.
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Solutions

• Make CSs nonpreemptable
• Practical is CSs are short and few
• Unsatisfactory if high-priority process often
  found themselves waiting for lower-priority
  process, especially unrelated ones.
• Naïve solution Always run CS at priority of
  highest process that shares the CS
• Problem p1 cannot preempt lower-priority process
  inside CS, even when it does not try to enter CS
  -- a different form of priority inversion.
• Dynamic Priority Inheritance
• See Next

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Dynamic Priority Inheritance
Sha, Rajkumar, and Lehocsky 1990

• p3 is in its CS
• p1 attempts to enter its CS
• p3 inherits p1s (higher) priority for the
  duration of CS

• Assume priority order p1 gt p2 gt p3.
• p1 and p3 share common resource.
• p2 is independent of p1 and p3 .

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Dynamic Priority Inheritance

• p3 is in its CS
• p1 attempts to enter its CS
• p3 inherits p1s (higher) priority for the
  duration of CS

• Assume priority order p1 gt p2 gt p3.
• p1 and p3 share common resource.
• p2 is independent of p1 and p3 .

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Dynamic Priority Inheritance

• p3 is in its CS
• p1 attempts to enter its CS
• p3 inherits p1s (higher) priority for the
  duration of CS

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• Assume priority order p1 gt p2 gt p3.
• p1 and p3 share common resource.
• p2 is independent of p1 and p3 .

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References

• Rensselaer Polytechnic Institute
  http//www.cs.rpi.edu/academics/courses/fall04/os/
  c8/
• University of east London
• Inside the Windows NT Scheduler, Part 1
• Inside the Windows NT Scheduler, Part 2

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Reading Assignment

• Chapter 6 CPU Scheduling in
• Operating System Concepts, Seventh Edition, by
  Abraham Silberschatz, Peter Baer Galvin, Greg
  Gagne, John Wiley Sons, Inc.