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The Linux Operating System
C. Blane Adcock
Bryan Knehr
Kevin Estep
Jason Niesz
Multiprocessing
• Multiprocessing available from Linux 2.0 and
above via Symmetric-Multiprocessing (SMP)
• Multiple hardware platforms supported by SMP
• System Administrator must construct their own
SMP Kernel
User Levels and Threads
• User Levels
– Single User
– Multi User
• Thread Types
– User Level
– Kernel Level
• Multitasking OS
Hardware Platforms
• X86 Based
– Intel
– AMD
– Other
• RISC, Alpha, SPARC, etc…
Command Language
• Control Statements
– Shell reads and interprets control statements.
– Commands
• E.g. “ls”, “cd”, “netstat”, etc…
• Separators, e.g. “|”
– Shell Scripts
Graphical User Interface
• Desktop Environments
– KDE/GNOME are both based off XFree86
• X Windows System
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Client/Server Interface
Platform Independent
Provides API
Implements X Protocol
X Protocol
• X Client
– Device Independent
– Displayed on X Server
– Run Local/Non-Local
• X Server
– Device Dependent
– Device Independent
• Advantages/Disadvantages
Process Control Block
• In Linux, processes have the following attributes:
– Owner’s ID
– Process Name
– Process ID
– Process State
– Process ID of the parent process
– Total time the process has been in the system
Synchronization
• Signals
– Processes put themselves on wait queues, waiting for an
event.
• Counting Semaphores
– Multiple processes can share large numbers of
semaphores
• Mutexes
– Process locks
CPU Scheduling
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Preemptive
Simple priority based algorithm
Two types of processes
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Real time (two types)
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•
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First in first out (FIFO)
Round Robin (RR)
Normal (non real time)
CPU Scheduling (Con’t)
• Real time processes always have priority over
normal processes
• When the scheduler gets invoked it begins to
calculate a “goodness” value for each process in
the run queue
• The process with the highest goodness value is
selected to run, providing it is “ready to run”
• A process is ready to run if it is not waiting for
anything, such as a keyboard input
Calculating the Goodness Value for Real
Time Processes
• All processed marked SCHED_FIFO or
SCHED_RR (real time processes) automatically
start with a value of 1000 to ensure priority over
normal processes
• The value of the priority field is then added to
make up the goodness value
Calculating the Goodness Value for
Normal Processes
• The goodness value for normal process is based on 4
factors:
– The task counter
• If the counter is zero the goodness value for the process is zero
• If not zero the goodness value is set to the counter value
– The priority value
• The task’s priority value is added to the goodness value
– A 1 point bonus for processes that share memory maps
– A 15 point bonus for tasks that ran on the current processor the last
time they ran
Calculating the Goodness Value for
Normal Processes (Con’t)
• If all the processes in the queue have a goodness
value of zero, the scheduler has the counter values
recalculated and the recalculates the goodness
values
• Problems
– Every time the scheduler is invoked all the goodness
values are recalculated
– This is a O(n) operation, n is the number of tasks in the
queue, therefore is n is large this will be costly to the
CPU (as high as 30% total CPU time)
Deadlock Detection and Resolution
• The nmi_watchdog command
– Runs non-maskable interrupts to detect
deadlock and display error messages
• Open source watchdogs
Approach to Memory Management
• Virtual to Physical Address mapping
– Fixed Size Pages
• Offset
• Virtual Page Frame Number
– Translation Process
• Page Table
– Valid Flag, Physical Page Frame Number,
Access Control Information
Page Replacement Algorithm
• Buddy algorithm
– Page Allocation
– Page Deallocation
File System Organization
• Hierarchal directory structure
• Every directory and subdirectory is a subdirectory
of the root directory, symbolized “/”
• This forms an inverted tree structure with the root
at the top and everything stemming from there
• Creates a parent-child relationship between
directories and their subdirectories
The ext3 File System
• An extension of the ext2 file system, but adds
journaling
– Journaling adds quick recovery from crash
• Customization of data protection
– data=writeback
– data=ordered
– data=journal
• ext3 is backward compatible