1. CSE 5343/7343 Fall 2002 Case Studies UNIX CSE 5343/7343
UNIX Case Study

2. 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
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UNIX Case Study

3. 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.
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UNIX Case Study

4. 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.
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UNIX Case Study

5. History (cont’d)
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
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UNIX Case Study

6. 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
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7. 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]
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8. 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.
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UNIX Case Study

9. 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
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10. Solaris Threads (cont’d)
User level thread
Thread ID
Registers
Stack pointer/stack
Priority
Process
Process ID
Memory map
Open files
LWPs
Kernel thread
Copy of kernel registers
Pointer to LWP
Priority
Scheduling information
Stack
LWP
Registers
Memory
Accounting information
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11. 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]
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12. 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.
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13. 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.
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14. 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
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15. Process Scheduling([2],[3],[5])
Typical time slice values:
4.3BSD: 1/10 second (p 86 [2])
System V: 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.
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16. 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
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17. 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]
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18. 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
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19. 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.
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20. 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.
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21. 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.
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UNIX Case Study

22. References
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.
Maurice J. Bach, The Design of the UNIX Operating System, Prentice Hall, 1990.
Abraham Silberschatz, Peter Baer Galvin, and Greg Gagne, Operating System Concepts Sixth Edition, John Wiley & Sons, 2002.
Milan Milenkovic, Operating Systems Concepts and Design Second Edition, McGraw Hill, 1992.
Andrew S. Tanenbaum, Modern Operating Systems, Prentice Hall, 1992.
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UNIX Case Study