Cs431-cotter1 Linux Operating System Review Chapter 10.

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Linux Operating System

Review

Chapter 10

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Overview

• System Overview• Linux Processes• CPU Scheduler• Process Synchronization• Memory Management• Input / Output• File Systems • Security

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Figure 10-1. The layers in a Linux system.

Interfaces to Linux

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Figure 10-2. A few of the common Linux utility programs required by POSIX.

Linux Utility Programs


cs431-cotter 5Figure 10-3. Structure of the Linux kernel

Kernel Structure


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Overview


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Linux Processesint pid;:pid = fork (); //create a child processif (pid < 0) // fork failed!!

errexit(); // handle errorelse if (pid == 0) {: //child process work goes here}else //pid must be > 0{: //parent process work goes here}

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Figure 10-6. Some system calls relating to processes.

Process Management System Calls in Linux


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Figure 10-8. The steps in executing the command ls typed to the shell.

Implementation of Exec


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Overview


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Linux Scheduling Algorithm

• SCHED_FIFO Designed for static priority. Soft real-time processes.

• SCHED_RR designed for real-time processes that require fair assignment of CPU time.

• SCHED_OTHER designed for normal user processes. Preemptible. Priority determined by base priority and time remaining in quantum.

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Figure 10-10. Illustration of Linux runqueue and priority arrays.

Scheduling in Linux


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Overview


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Figure 10-5. The signals required by POSIX.

Signals in Linux (1)


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Signals must be captured with signal handler

• Program must explicitly handle signals, if it does not want to use default handling (usually termination)

• sigaction (signal_to_catch, new_action, old_action)• struct sigaction {

void * sa_handler; // What function do we call?sigset_t mask; // Mask of signals to block when calledint sa_flags; // Special flags to set.};

• Will be executed automatically on reception of appropriate signal

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Semaphores

• POSIX version of semaphores– #include <semaphore.h>– classic semaphore implementation

• System V version of semaphores– #include <sys/sem.h>– Enhanced version of semaphores to include sets

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Figure 2-30. Some of the Pthreads calls relating to mutexes.

Mutexes in Pthreads


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Named Pipes

• Permanent objects• Available to processes that can access the

filespace (same system or on a shared file system)

• Processes do not have to be related.

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Message Queues

• A “linked list of messages”• Message queue has kernel persistence

– Supports asynchronous communications.– Messages are stored in the queue, independent of the

sender (sender can close queue without losing messages).

– Receiver can retrieve messages at a later time.

• Messages have a priority– Higher priority messages are retrieved first (POSIX)– Max priority is 32768

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Shared Memory

• Allows 2 or more processes to share the same main memory space– Memory can be allocated as blocks (pages) of

memory– Memory can be mapped as a file that is

available in memory to multiple processes

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Overview


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Figure 10-12. (a) Process A’s virtual address space. (b) Physical memory. (c) Process B’s virtual address space.

Memory Management in Linux (1)


(a) (b) (c)

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Figure 10-13. Two processes can share a mapped file.

Memory Management in Linux (2)


(a) (b) (c)

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Figure 10-16. Linux uses four-level page tables.

Physical Memory Management (3)


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Figure 10-17. Operation of the buddy algorithm.

Memory Allocation Mechanisms


(a) (b) (c) (d) (e) (f) (g) (h) (i)

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Overview


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Figure 10-22. The Linux I/O system showing one file system in detail.

Implementation of Input/Output


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Overview


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System Organization

• Like UNIX...

• Predefined root directory structure with preferred locations for kernel files

/ (root)

bin

boot

dev

etc

home

lib

mnt

proc

root

sbin

tmp

usr

var

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Figure 10-24. (a) Before linking. (b) After linking.

The Linux File System (2)


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Figure 10-25. (a) Separate file systems. (b) After mounting.

The Linux File System (3)


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Figure 10-30. File system abstractions supported by the VFS.

The Linux Virtual File System


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Figure 10-31. Disk layout of the Linux ext2 file system.

The Linux Ext2 File System (1)


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Figure 10-34. The relation between the file descriptor table, the open file description table, and the i-node table.

The Linux Ext2 File System (4)


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Journal File Systems

• Designed to speed file system recovery from system crashes.

• Traditional systems use an fs recovery tool (e.g. fsck) to verify system integrity

• As file system grows, recovery time grows. • Journal File Systems track changes in meta-data

to allow rapid recovery from crashes

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Journal File System

• Uses a version of a transaction log to track changes to file system directory records.

• If a system crash occurs, the transaction log can be reviewed back to a check point to verify any pending work (either undo or redo).

• For large systems, recovery time goes from hours or days to seconds.

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Journal File System - Examples

• ReiserFS– Designed originally for Linux (32 bit system)– Supports file systems to 2TB -> 16TB– Journal support of meta-data– Btree structure supports large file counts– Supports block packing for small files– No fixed inode allocation - more flexible.

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Figure 10-35. Examples of remote mounted file systems. Directories shown as squares, files shown as circles.

NFS Protocols


cs431-cotter 39Figure 10-36. The NFS layer structure

NFS Implementation


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Overview


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Figure 10-37. Some example file protection modes.

Security In Linux


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Figure 10-38. system calls relating to security.

Security System Calls in Linux


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Summary


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Questions• What command would be needed to print the last 250

characters of a file “data.txt”• What function can a parent process use to capture the

exit code from a child process?• In terms of Linux scheduling what is a runqueue?• How does Linux use the Buddy algorithm to allocate

memory for a process?• What information is kept in an open file description table

that isn’t kept anywhere else?• What is the primary advantage of a journaling file system

over earlier file systems (such as ext2fs)?

Cs431-cotter1 Linux Operating System Review Chapter 10.

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