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Operating Systems CSE 411. CPU Management Sept. 25 2006 - Lecture 9 Instructor: Bhuvan Urgaonkar. Last time Pre-emptive scheduling Lottery, Reservation, … Today Signals Introduction to accounting CPU accounting Threads. Interrupts/Traps and Signals Compared. Recap interrupt/trap

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operating systems cse 411

Operating SystemsCSE 411

CPU Management

Sept. 25 2006 - Lecture 9

Instructor: Bhuvan Urgaonkar

Last time
    • Pre-emptive scheduling
    • Lottery, Reservation, …
  • Today
    • Signals
    • Introduction to accounting
      • CPU accounting
    • Threads
interrupts traps and signals compared
Interrupts/Traps and Signals Compared
  • Recap interrupt/trap
    • Used to notify the OS that something has happened that it needs to attend to
      • E.g. 1: Network packet arrived at the Ethernet card
      • E.g. 2: Process made a system call
      • E.g. 3: Process performed division by zero
      • E.g. 4: CPU about to melt!
    • Interrupt/trap handlers implemented by the OS; their addresses stored at fixed locations indexed by their numbers
    • Usually handled right away
      • Linux: Top-half right away, bottom-half at leisure
      • Interrupts: Asynchronous generation, synchronous handling
      • Traps: Synchronous generation and handling
interrupts traps versus signals
Interrupts/Traps versus Signals
  • Used to notify a process that something has happened that it needs to attend to
  • Signal handlers may be implemented by the process
  • Handled when the process is scheduled next
  • Generated and handled synchronously
    • May be due to an asynch. interrupt, but the signal will be generated synch. WHY?
  • Used to notify the OS that something has happened that it needs to attend to
  • Interrupt/trap handlers are implemented by the OS
  • Usually handled right away
  • Interrupts asynch. generated, traps synch. generated
  • Fixed number of signals defined by the OS and made known to the processes
    • UNIX: signal.h
  • A process may implement its own handler for one or more signals
    • Allowed for most signals, not allowed for SIGKILL
    • Each PCB has indicators for which signals were received and are due
    • Upon getting scheduled, the handler for signals received are executed in some order
      • Okay from the process point of view since it is unaware of when it is being scheduled or taken off the CPU
more on signals
More on signals
  • Each signal has a default action which is one of the following:
    • The signal is discarded after being received
    • The process is terminated after the signal is received
    • A core file is written, then the process is terminated
    • Stop the process after the signal is received
  • Each signal defined by the system falls into one of five classes:
    • Hardware conditions
    • Software conditions
    • Input/output notification
    • Process control
    • Resource control
examples of signals
Examples of signals
  • SIGHUP 1 /* hangup */
  • SIGINT 2 /* interrupt */
  • SIGQUIT 3 /* quit */
  • SIGILL 4 /* illegal instruction */
  • SIGABRT 6 /* used by abort */
  • SIGKILL 9 /* hard kill */
  • SIGALRM 14 /* alarm clock */
  • SIGCONT 19 /* continue a stopped process */
  • SIGCHLD 20 /* to parent on child stop or exit */
system calls related to signals
System calls related to signals
  • kill(signal_num, pid) - to send a signal
  • signal(signal_num, handler) - to handle it
signal handling visual
Signal Handling (Visual)

Signal due indicators

PCB of P

Signal is not due

Signal is due


handler; dumps core

and exits


Parent of P

ISR run; assume

P scheduled again

Sys call done;

Par scheduled


Timer interrupt

System call


Timer interrupt

Calls kill() to

send a signal to P


Accesses illegal

memory location


These are the events that P sees (its view of what is going on)

cpu accounting1
CPU Accounting
  • OS keeps track of each process’s CPU usage
    • Scheduler needs this information
    • Billing in commercial settings
      • E.g., Sun’s Grid: $1 per CPU-hour
    • Prevent resource exhaustion due to malicious or erroneous processes
      • E.g., fork bomb!
        • Solution: Limit the number of processes a process can fork
  • Lets look at the top utility
    • OS provides syscalls for getting certain usage information
      • Look at getrusage()
  • Issue: Who should be charged for CPU usage of the OS?
    • Consider the example of an I/O-intensive process
    • Suggested reading: “resource containers” paper (not part of the syllabus)
what is a thread
What is a Thread?
  • A basic unit of CPU utilization like a process (not necessarily known to the OS though)
  • “Smaller” than a process
    • Part of a process
    • Shares code + data + some other OS resources with other threads that belong to the same process
      • Files and signal handlers
user threads
User Threads
  • Thread management done by user-level threads library
  • OS doesn’t know about the existence of these threads
  • Three primary thread libraries:
    • POSIX Pthreads
    • Win32 threads
    • Java threads
kernel threads
Kernel Threads
  • OS sees and manages these threads
  • OS provides system calls to create, terminate, etc. (just like the system calls it provides for processes)
  • Examples
    • Windows XP/2000
    • Solaris
    • Linux
    • Tru64 UNIX
    • Mac OS X
  • Responsiveness
  • Resource Sharing
  • Economy
  • Utilization of MP Architectures
multithreading models
Multithreading Models
  • Many-to-One
  • One-to-One
  • Many-to-Many
many to one
  • Many user-level threads mapped to single kernel thread
  • Examples:
    • Solaris Green Threads
    • GNU Portable Threads
one to one
  • Each user-level thread maps to kernel thread
  • Examples
    • Windows NT/XP/2000
    • Linux
    • Solaris 9 and later
many to many model
Many-to-Many Model
  • Allows many user level threads to be mapped to many kernel threads
  • Allows the operating system to create a sufficient number of kernel threads
  • Solaris prior to version 9
  • Windows NT/2000 with the ThreadFiber package
two level model
Two-level Model
  • Similar to M:M, except that it allows a user thread to be bound to kernel thread
  • Examples
    • IRIX
    • HP-UX
    • Tru64 UNIX
    • Solaris 8 and earlier