cse 5343/7343 fall 2002 case studies

CSE 5343/7343 UNIX Case Study 1

CSE 5343/7343Fall 2002

Case Studies

UNIX


UNIX Case Study Outline• History• Design Philosophy• Process Management

– Processes– PCB/Process Table/U Area– Process States– Process Scheduling/Priorities– IPC

• Memory Management• File Systems


History ([2],[3])• 1965 – Bell and GE joined project MAC at MIT to develop

Multics. Goal: multicomputing and data sharing for a large group of users.

• Ken Thompson (Bell) developed a game called “Space Travel” and found an unused PDP-7.

• 1969 – Thompson and Ken Ritchie implemented an earlier

designed file system on PDP-7. – This grew into UNIX. – File system design and idea of command interpreter (shell)

as a user process came from Multics– Fork idea came from Berkeley’s GENIE OS.


History (cont’d)• 1971 – UNIX used in first real project: text processing for

BELL labs patent department.• 1971 – UNIX required 16K bytes for system, 8K bytes for user

programs, 512K bytes of disk, and limit of 64K disk bytes per file.

• Thompson set out to write a new Fortran compiler but developed a new programming language: B. B was based on BCPL (a tool for compiler writing and systems programming). B was interpretive. He later improved on B and called it C.

• 1973 – UNIX rewrittten in C (unheard of at the time). AT&T offered UNIX free to universities.

• 1974 – Ritchie and Thompson paper describing UNIX in CACM.


History (cont’d)• 1977 - Bell combined several versions into UNIX System II and

marketed. History (cont’d)• 1978 – After distribution of Ver7 in 1978, the Unix Support Group

(USG) at AT&T took over responsibilities from the research for distribution within AT&T.

• 1982 – First external distribution from USG – System III.• UC Berkeley developed their version of UNIX (Berkeley software

Distributions). – BSD introduced vi in 2BSD, demand-pages virtual memory in

3BSD, TCP/IP networking protocol in 4.2BSD.– Less than 3% of BSD written in assembly.

• 1991 – Finnish student Linus Torvalds wrote Linux for 80386, 32 bit processor


Design Philosophy ([2],[3])

• Smplicity• C• Time-sharing• Simple user interface (modular and may be

replaced)• Device independence (treat files and devices in

same manner)• Aimed for programming environment• Flexibility


Design (cont’d)

• Fig 1.1 p5 [2]

• Programs such as shell and vi interact with kernel using well defined system call procedures.

• Cc built on top of c preprocessor, two-pass compiler.

• Fig 2.1 p20 [2]


Processes• Process is program in execution• Consists of machine instructions (text), data, and

stack regions.• Separate stack for user and kernel mode.• PID (Process ID)• Processes are either user processes, daemon

processes, or kernel processes.• Daemons are not associated with user, but do system

wide functions. Init may create daemons that exist throughout the life of the system or as needed.


Solaris Threads ([3])

• Kernel and user level threads• Fig 5.6 p141 [3]• Only kernel level threads are scheduled• Implements Pthread API• LWP between kernel and user level threads• Each LWP associated with a kernel thread.• User processes have at least one LWP.• User threads may be bound or unbound to a LWP.• May have kernel thread without LWP.• Pool of LWPs for a process


Solaris Threads (cont’d)• User level thread

– Thread ID– Registers– Stack pointer/stack– Priority

• Process– Process ID– Memory map– Open files– Priority– LWPs

•Kernel thread–Copy of kernel registers–Pointer to LWP–Priority–Scheduling information–Stack

•LWP–Registers

–Memory

–Accounting information


PCB ([1],[2],[4])• Process Table:

– State– UID for owner – Memory status (swapped or in memory)– Parent PID– Child PID– Event descriptor if suspended

• Fig 2.5 p28 [2]• Entry in kernel process table that points to process region table.

These in turn point to entries in the region table and to the regions for that process. This allows independent processes to share regions.

• Fig 6.2 p153 [2]


U (User) Area ([2])

• Information needed when process is executing.• Process table slot• Information about current system call• File descriptors for open files• Current directory• Current root• Login terminal• Kernel has direct access to U area of currently

executing process.


Process States ([1],[2])

• Fig 6.1 p148 [2]• No context switch is needed to go from

state 2 to state 1.• States 3 and 7 are really the same.• I/O request puts process to sleep.• Zombie state – Child has finished

execution, but parent wants to get information about it from the PCB.


Process Hierarchy ([1])

• Initial boot process (0) forks a child (process 1) and process 0 becomes the swapper. Process 1 is init process and is ancestor of all other processes.

• Parent/Child/Sibling relationship indicated by pointers in process table.

• Execve usually called to replace memory portion of new process.

• Exit – process termination• Wait – Parent waits for child to exit. Reclaims process

resources. • Process group ID (GID) in process table• Execute: ps -alx


Process Scheduling([2],[3],[5])• Typical time slice values:

– 4.3BSD: 1/10 second (p 86 [2])– System V: 50-100 times per second (p247 [3])

• Round robin multilevel feedback queue• At end of context switch, kernel executes

algorithm to schedule a process.• Highest priority process which is ready is

scheduled. On tie, picks the one which has waited the longest.


Process Scheduling (cont’d)

• Higher number – Lower Priority• User and kernel priorities; user below

kernel.• User mode priority is function of recent

CPU usage. Lower priority if recently used CPU.

• Fig 8.2 p250 [2]• Priority assigned based on process group


Process Scheduling (cont’d)• Kernel to user mode: change to user mode and calculate based

on kernel resources just used.• Clock Handler adjusts all user mode process priorities at 1 second

(System V) intervals and causes kernel to reschedule.• Decay function adjusts recent CPU usage values at this time:

decay(CPU) = CPU/2• Priority is recalculated at these 1 second intervals plus when a

process is in the preempted but ready to run state: priority = CPU/2 + base level priority

• Base level priority is the threshold between user and kernel mode.• Processes move up in queue but can not change to kernel mode.• Fig 8.4 p 253 [2]


IPC ([2],[3])• Pipes – Only used from descendents of process that created the pipe.• Named Pipes- Used by unrelated processes. Has a directory entry

and is accessed as a file.• Signals

– Inform process of occurrence of asynchronous events. – Handling of signal by user process. Address of this routine is

located in uarea next to signal number.• Socket

– Introduced by 4.2BSD– Transient object; Exists only as long as some process holds a

descriptor referring to it.– Created by socket system call


IPC (cont’d)• Messages (System V)

– Fig 11.5 p363 [2]

– On sending a message, a new entry is added to the associated linked list and the message is copied form the uarea. Message headers arranged in FIFO order.

– Kernel awakens processes waiting for a message from this queue.

– On receiving a message, process indicates what should be done if no message on the queue.


IPC (cont’d)• Shared Memory (System V)

– System call creates a new region of shared memory or returns a current one.

– Reading and writing is done as normal – no system calls.

– Fig 11.9 p 368 [2]

– Shared memory remains in tact even if no processes include it in their virtual address space.


IPC (cont’d)

• Semaphores (System V)

– System calls create and use semaphore.

– Semaphore array where each entry is the count value.

– If process is put to sleep, it sleeps at an interruptable priority and wakes up on receipt of a signal.

– Handled similar to messages with id being the entry in the semaphore table.


References1. Samuel J. Leffler, Marshall Kirk McKusick, Michael J. Karels,

and John S. Quarterman, The Design and Implementation of the 4.3 BSD UNIX Operating System, Addison-Wesley, 1989.

2. Maurice J. Bach, The Design of the UNIX Operating System, Prentice Hall, 1990.

3. Abraham Silberschatz, Peter Baer Galvin, and Greg Gagne, Operating System Concepts Sixth Edition, John Wiley & Sons, 2002.

4. Milan Milenkovic, Operating Systems Concepts and Design Second Edition, McGraw Hill, 1992.

5. Andrew S. Tanenbaum, Modern Operating Systems, Prentice Hall, 1992.

cse 5343/7343 fall 2002 case studies

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