Processes and Threads

1
Processes and Threads

• Chapter 2

2.1 Processes 2.2 Threads 2.3 Interprocess
communication 2.4 Classical IPC problems 2.5
Scheduling
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ProcessesThe Process Model

• Multiprogramming of four programs
• Conceptual model of 4 independent, sequential
  processes
• Only one program active at any instant

3
Process Creation

• Principal events that cause process creation
• System initialization
• Execution of a process creation system
• User request to create a new process
• Initiation of a batch job

4
Process Termination

• Conditions which terminate processes
• Normal exit (voluntary)
• Error exit (voluntary)
• Fatal error (involuntary)
• Killed by another process (involuntary)

5
Process Hierarchies

• Parent creates a child process, child processes
  can create its own process
• Forms a hierarchy
• UNIX calls this a "process group"
• Windows has no concept of process hierarchy
• all processes are created equal

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Process States (1)

• Possible process states
• running
• blocked
• ready
• Transitions between states shown

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Process States (2)

• Lowest layer of process-structured OS
• handles interrupts, scheduling
• Above that layer are sequential processes

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Implementation of Processes (1)

• Fields of a process table entry

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Implementation of Processes (2)

• Skeleton of what lowest level of OS does when an
  interrupt occurs

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ThreadsThe Thread Model (1)

• (a) Three processes each with one thread
• (b) One process with three threads

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The Thread Model (2)

• Items shared by all threads in a process
• Items private to each thread

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The Thread Model (3)

• Each thread has its own stack

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Thread Usage (1)

• A word processor with three threads

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Thread Usage (2)

• A multithreaded Web server

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Thread Usage (3)

• Rough outline of code for previous slide
• (a) Dispatcher thread
• (b) Worker thread

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Thread Usage (4)

• Three ways to construct a server

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Implementing Threads in User Space

• A user-level threads package

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Implementing Threads in the Kernel

• A threads package managed by the kernel

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Hybrid Implementations

• Multiplexing user-level threads onto kernel-
  level threads

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Scheduler Activations

• Goal mimic functionality of kernel threads
• gain performance of user space threads
• Avoids unnecessary user/kernel transitions
• Kernel assigns virtual processors to each process
• lets runtime system allocate threads to
  processors
• Problem Fundamental reliance on kernel
  (lower layer)
• calling procedures in user space (higher
  layer)

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Pop-Up Threads

• Creation of a new thread when message arrives
• (a) before message arrives
• (b) after message arrives

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Making Single-Threaded Code Multithreaded (1)

• Conflicts between threads over the use of a
  global variable

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Making Single-Threaded Code Multithreaded (2)

• Threads can have private global variables

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Interprocess CommunicationRace Conditions

• Two processes want to access shared memory at
  same time

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Critical Regions (1)

• Four conditions to provide mutual exclusion
• No two processes simultaneously in critical
  region
• No assumptions made about speeds or numbers of
  CPUs
• No process running outside its critical region
  may block another process
• No process must wait forever to enter its
  critical region

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Critical Regions (2)

• Mutual exclusion using critical regions

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Algorithm 1

• Shared variables
• int turninitially turn 0
• turn - i ? Pi can enter its critical section
• Process Pi
• do
• while (turn ! i) /infinite loop /
• critical section
• turn j
• reminder section
• while (1)

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Mutual Exclusion with Busy Waiting (1)

• Proposed solution to critical region problem
• (a) Process 0. (b) Process 1.

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Mutual Exclusion with Busy Waiting (2)

• Peterson's solution for achieving mutual exclusion

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Synchronization Hardware

• Test and modify the content of a word
  atomically.
• boolean TestAndSet(boolean target)
• boolean rv target
• tqrget true
• return rv

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Mutual Exclusion with Test-and-Set

• Shared data boolean lock false
• Process Pi
• do
• while (TestAndSet(lock))
• critical section
• lock false
• remainder section

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Mutual Exclusion with Busy Waiting (3)

• Entering and leaving a critical region using the
• TSL instruction

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Synchronization Hardware

• Atomically swap two variables.
• void Swap(boolean a, boolean b)
• boolean temp a
• a b
• b temp

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Mutual Exclusion with Swap

• Shared data (initialized to false) boolean
  lock
• Process Pi
• do
• key true
• while (key true)
• Swap(lock,key)
• critical section
• lock false
• remainder section

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Semaphores

• Synchronization tool that does not require busy
  waiting.
• Semaphore S integer variable
• can only be accessed via two indivisible (atomic)
  operations
• wait/down (S)
• while S? 0 do no-op S--
• signal/up (S)
• S

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Critical Section of n Processes

• Shared data
• semaphore mutex //initially mutex 1
• Process Pi do wait(mutex)
  critical section
• signal(mutex) remainder section
  while (1)

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Semaphore Implementation

• Define a semaphore as a record
• typedef struct
• int value struct process L
  semaphore
• Assume two simple operations
• block suspends the process that invokes it.
• wakeup(P) resumes the execution of a blocked
  process P.

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Implementation

• Semaphore operations now defined as
• wait/down(S) S.value--
• if (S.value lt 0)
• add this process to S.L block
• signal/up(S) S.value
• if (S.value lt 0)
• remove a process P from S.L wakeup(P)
  

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Mutexes

• Implementation of mutex_lock and mutex_unlock

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Semaphore as a General Synchronization Tool

• Execute B in Pj only after A executed in Pi
• Use semaphore flag initialized to 0
• Code
• Pi Pj
• ? ?
• A wait/down(flag)
• signal/up(flag) B

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Deadlock and Starvation

• Deadlock two or more processes are waiting
  indefinitely for an event that can be caused by
  only one of the waiting processes.
• Let S and Q be two semaphores initialized to 1
• P0 P1
• wait(S) wait(Q)
• wait(Q) wait(S)
• ? ?
• signal(S) signal(Q)
• signal(Q) signal(S)
• Starvation indefinite blocking. A process may
  never be removed from the semaphore queue in
  which it is suspended.

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Two Types of Semaphores

• Counting semaphore integer value can range over
  an unrestricted domain.
• Binary semaphore integer value can range only
  between 0 and 1 can be simpler to implement.
• Can implement a counting semaphore S as a binary
  semaphore.

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Implementing S as a Binary Semaphore

• Data structures
• binary-semaphore S1, S2
• int C
• Initialization
• S1 1
• S2 0
• C initial value of semaphore S

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Implementing S

• wait operation
• wait(S1)
• C--
• if (C lt 0)
• signal(S1)
• wait(S2)
• signal(S1)
• signal operation
• wait(S1)
• C
• if (C lt 0)
• signal(S2)
• else
• signal(S1)

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Sleep and Wakeup

• Producer-consumer problem with fatal race
  condition

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Semaphores

• The producer-consumer problem using semaphores

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Semaphores

• The producer-consumer problem using semaphores

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Monitors (1)

• Example of a monitor

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Monitors (2)

• Outline of producer-consumer problem with
  monitors
• only one monitor procedure active at one time
• buffer has N slots

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Monitors (3)

• Solution to producer-consumer problem in Java
  (part 1)

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Monitors (4)

• Solution to producer-consumer problem in Java
  (part 2)

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Monitors
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Message Passing

• The producer-consumer problem with N messages

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Barriers

• Use of a barrier
• processes approaching a barrier
• all processes but one blocked at barrier
• last process arrives, all are let through

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Dining Philosophers (1)

• Philosophers eat/think
• Eating needs 2 forks
• Pick one fork at a time
• How to prevent deadlock

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Dining Philosophers (2)

• A nonsolution to the dining philosophers problem

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Dining Philosophers (3)

• Solution to dining philosophers problem (part 1)

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Dining Philosophers (4)

• Solution to dining philosophers problem (part 2)

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The Readers and Writers Problem

• A solution to the readers and writers problem

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The Sleeping Barber Problem (1)
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The Sleeping Barber Problem (2)
Solution to sleeping barber problem.
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SchedulingIntroduction to Scheduling (1)

• Bursts of CPU usage alternate with periods of I/O
  wait
• a CPU-bound process
• an I/O bound process

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Introduction to Scheduling (2)

• Scheduling Algorithm Goals

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Histogram of CPU-burst Times
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CPU Scheduler

• Selects from among the processes in memory that
  are ready to execute, and allocates the CPU to
  one of them.
• CPU scheduling decisions may take place when a
  process
• 1. Switches from running to waiting state.
• 2. Switches from running to ready state.
• 3. Switches from waiting to ready.
• 4. Terminates.
• Scheduling under 1 and 4 is nonpreemptive.
• All other scheduling is preemptive.

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Dispatcher

• Dispatcher module gives control of the CPU to the
  process selected by the short-term scheduler
  this involves
• switching context
• switching to user mode
• jumping to the proper location in the user
  program to restart that program
• Dispatch latency time it takes for the
  dispatcher to stop one process and start another
  running.

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Scheduling Criteria

• CPU utilization keep the CPU as busy as
  possible
• Throughput of processes that complete their
  execution per time unit
• Turnaround time amount of time to execute a
  particular process
• Waiting time amount of time a process has been
  waiting in the ready queue
• Response time amount of time it takes from when
  a request was submitted until the first response
  is produced, not output (for time-sharing
  environment)

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Optimization Criteria

• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time

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First-Come, First-Served (FCFS) Scheduling

• Process Burst Time
• P1 24
• P2 3
• P3 3
• Suppose that the processes arrive in the order
  P1 , P2 , P3 The Gantt Chart for the schedule
  is
• Waiting time for P1 0 P2 24 P3 27
• Average waiting time (0 24 27)/3 17

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FCFS Scheduling (Cont.)

• Suppose that the processes arrive in the order
• P2 , P3 , P1 .
• The Gantt chart for the schedule is
• Waiting time for P1 6 P2 0 P3 3
• Average waiting time (6 0 3)/3 3
• Much better than previous case.
• Convoy effect short process behind long process

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Shortest-Job-First (SJR) Scheduling

• Associate with each process the length of its
  next CPU burst. Use these lengths to schedule
  the process with the shortest time.
• Two schemes
• nonpreemptive once CPU given to the process it
  cannot be preempted until completes its CPU
  burst.
• preemptive if a new process arrives with CPU
  burst length less than remaining time of current
  executing process, preempt. This scheme is know
  as the Shortest-Remaining-Time-First (SRTF).
• SJF is optimal gives minimum average waiting
  time for a given set of processes.

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Example of Non-Preemptive SJF

• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4
• SJF (non-preemptive)
• Average waiting time (0 6 3 7)/4 - 4

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Example of Preemptive SJF

• Process Arrival Time Burst Time
• P1 0.0 7
• P2 2.0 4
• P3 4.0 1
• P4 5.0 4
• SJF (preemptive)
• Average waiting time (9 1 0 2)/4 - 3

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Determining Length of Next CPU Burst

• Can only estimate the length.
• Can be done by using the length of previous CPU
  bursts, using exponential averaging.

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Prediction of the Length of the Next CPU Burst
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Examples of Exponential Averaging

• ? 0
• ?n1 ?n
• Recent history does not count.
• ? 1
• ?n1 tn
• Only the actual last CPU burst counts.
• If we expand the formula, we get
• ?n1 ? tn(1 - ?) ? tn -1
• (1 - ? )j ? tn -1
• (1 - ? )n1 tn ?0
• Since both ? and (1 - ?) are less than or equal
  to 1, each successive term has less weight than
  its predecessor.

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Priority Scheduling

• A priority number (integer) is associated with
  each process
• The CPU is allocated to the process with the
  highest priority (smallest integer ? highest
  priority).
• Preemptive
• nonpreemptive
• SJF is a priority scheduling where priority is
  the predicted next CPU burst time.
• Problem ? Starvation low priority processes may
  never execute.
• Solution ? Aging as time progresses increase
  the priority of the process.

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Scheduling in Interactive Systems (1)

• Round Robin Scheduling
• list of runnable processes
• list of runnable processes after B uses up its
  quantum

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Round Robin (RR)

• Each process gets a small unit of CPU time (time
  quantum), usually 10-100 milliseconds. After
  this time has elapsed, the process is preempted
  and added to the end of the ready queue.
• If there are n processes in the ready queue and
  the time quantum is q, then each process gets 1/n
  of the CPU time in chunks of at most q time units
  at once. No process waits more than (n-1)q time
  units.
• Performance
• q large ? FIFO
• q small ? q must be large with respect to context
  switch, otherwise overhead is too high.

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Example of RR with Time Quantum 20

• Process Burst Time
• P1 53
• P2 17
• P3 68
• P4 24
• The Gantt chart is
• Typically, higher average turnaround than SJF,
  but better response.

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Time Quantum and Context Switch Time
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Turnaround Time Varies With The Time Quantum
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Multilevel Queue

• Ready queue is partitioned into separate
  queuesforeground (interactive)background
  (batch)
• Each queue has its own scheduling algorithm,
  foreground RRbackground FCFS
• Scheduling must be done between the queues.
• Fixed priority scheduling (i.e., serve all from
  foreground then from background). Possibility of
  starvation.
• Time slice each queue gets a certain amount of
  CPU time which it can schedule amongst its
  processes i.e., 80 to foreground in RR, 20 to
  background in FCFS

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Scheduling in Interactive Systems (2)

• A scheduling algorithm with four priority classes

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Multilevel Queue Scheduling
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Multilevel Feedback Queue

• A process can move between the various queues
  aging can be implemented this way.
• Multilevel-feedback-queue scheduler defined by
  the following parameters
• number of queues
• scheduling algorithms for each queue
• method used to determine when to upgrade a
  process
• method used to determine when to demote a process
• method used to determine which queue a process
  will enter when that process needs service

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Example of Multilevel Feedback Queue

• Three queues
• Q0 time quantum 8 milliseconds
• Q1 time quantum 16 milliseconds
• Q2 FCFS
• Scheduling
• A new job enters queue Q0 which is served FCFS.
  When it gains CPU, job receives 8 milliseconds.
  If it does not finish in 8 milliseconds, job is
  moved to queue Q1.
• At Q1 job is again served FCFS and receives 16
  additional milliseconds. If it still does not
  complete, it is preempted and moved to queue Q2.

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Multilevel Feedback Queues
89
Lottery Scheduling
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Multiple-Processor Scheduling

• CPU scheduling more complex when multiple CPUs
  are available.
• Homogeneous processors within a multiprocessor.
• Load sharing
• Asymmetric multiprocessing only one processor
  accesses the system data structures, alleviating
  the need for data sharing.

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Real-Time Scheduling

• Hard real-time systems required to complete a
  critical task within a guaranteed amount of time.
• Soft real-time computing requires that critical
  processes receive priority over less fortunate
  ones.

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Dispatch Latency
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Algorithm Evaluation

• Deterministic modeling takes a particular
  predetermined workload and defines the
  performance of each algorithm for that workload.
• Queueing models
• Implementation

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Evaluation of CPU Schedulers by Simulation
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Scheduling in Batch Systems (1)

• An example of shortest job first scheduling

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Scheduling in Batch Systems (2)

• Three level scheduling

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Scheduling in Real-Time Systems

• Schedulable real-time system
• Given
• m periodic events
• event i occurs within period Pi and requires Ci
  seconds
• Then the load can only be handled if

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Policy versus Mechanism

• Separate what is allowed to be done with how it
  is done
• a process knows which of its children threads are
  important and need priority
• Scheduling algorithm parameterized
• mechanism in the kernel
• Parameters filled in by user processes
• policy set by user process

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Thread Scheduling (1)

• Possible scheduling of user-level threads
• 50-msec process quantum
• threads run 5 msec/CPU burst

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Thread Scheduling (2)

• Possible scheduling of kernel-level threads
• 50-msec process quantum
• threads run 5 msec/CPU burst