The Process and the Kernel

The Process and the Kernel
2017/4/21 UNIX Foundations The Process and the Kernel

Course Description The Goals for this course Prerequisites:
2017/4/21 Course Description The Goals for this course Understand UNIX Understand Operating Systems Prerequisites: COSC 2P13 : Introduction to Operating Systems COSC 2P91 : Procedural Programming

2017/4/21 Course Description An intensive study of computer operating system design Multiprogramming Time-sharing Real-time processing Job and task control Synchronization of concurrent processes and processors Resource scheduling Protection Management of hierarchical storage.

How to study this course
2017/4/21 How to study this course Read and remember Read the book, remember the concepts and commands Think Think operating systems as natural administrative agents Practice Coding with UNIX, use and understand of UNIX commands and get the results

2017/4/21 The Textbook Used Uresh Vahalia, UNIX Internals: The New Frontiers, Prentice Hall, 1996 Why we use this book? UNIX is one of the most popular operating systems of the world. If you understand UNIX, you can understand other operating systems.

2017/4/21 References William Stallings, Operating Systems,5th Ed., Prentice Hall,2005 A. Tanenbaum, Modern Operating Systems, 2nd ed. Prentice Hall 2001 Kay A. Robbins and Steven Robbins, UNIX Systems Programming, Prentice Hall,2003 W. R. Stevens, Advanced Programming in the UNIX Environment, Addison Wesley, 1992

Summary of UNIX History
2017/4/21 Summary of UNIX History XPG Digital UNIX SVID POSIX Solaris ULTRIX 4.4BSD HP-UX SVR4 4.3BSD SCO UNIX SunOS AIX SVR3 4.2BSD XENIX SVR2 4BSD 3BSD UNIX

Summary of UNIX History
2017/4/21 V1 . V6 Summary of UNIX History Xenix BSD V7 PWB 2BSD . 2.9BSD 2.10BSD 2.11BSD PWB2 32V 3BSD 4BSD 4.2BSD 4.3BSD 4.4BSD SIII V8 Xenix2 SYSV V10 SCO V.2 Plan9 Ultrix V.3 SUNOS AIX V.3.2 Mach OSF1 SVR4 Solaris LINUX

2017/4/21 Flexibility Traditional Kernel: file, scheduling, executable file formats

2017/4/21 Modern UNIX Kernel

2017/4/21 Assessment of UNIX Advantages: open software process, well designed, small and simple kernel, text files in system databases, simple, uniform interface to I/O devices, portability (written in C)

2017/4/21 Assessment of UNIX Disadvantages: expansion of the more and more complex I/O library, unfriendly user interface, building block approach to tools, but not to the kernel, too many versions and standards, monolithic, unmodular and more and more complex kernel.

UNIX Architecture Hardware is surrounded by the operating-system
2017/4/21 UNIX Architecture Hardware is surrounded by the operating-system Operating system is called the kernel Comes with a number of user services and interfaces shell C compiler

UNIX Architecture Application Programs Shell Editors, and
2017/4/21 UNIX Architecture Hardware Kernel Shell Editors, and Private User Programs Compiler Components Compiler Application Programs

The layers of a UNIX system.
2017/4/21 User Interface The layers of a UNIX system.

A few of the more common UNIX utility programs required by POSIX
2017/4/21 A few of the more common UNIX utility programs required by POSIX

Process In UNIX Process is an instance of a running program.
2017/4/21 Process In UNIX Process is an instance of a running program. Lifetime: fork/vfork->exec->exit Well-defined hierarchy: parent,child,init, init process: the top process swapper & pagedeamon Orphans: the parent process is terminated.

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Process Creation Submission of a batch job User logs on
2017/4/21 Process Creation Submission of a batch job User logs on Created to provide a service such as printing Spawned by an existing process

Process Termination Batch job issues Halt instruction User logs off
2017/4/21 Process Termination Batch job issues Halt instruction User logs off Process executes a service request to terminate Error and fault conditions

Reasons for Process Termination
2017/4/21 Normal completion Time limit exceeded Memory unavailable Bounds violation Protection error example write to read-only file Arithmetic error Time overrun process waited longer than a specified maximum for an event

Reasons for Process Termination
2017/4/21 I/O failure Invalid instruction happens when try to execute data Privileged instruction Data misuse Operating system intervention such as when deadlock occurs Parent terminates so child processes terminate Parent request

Process States The Running state The Ready state The Blocked state
2017/4/21 Process States The Running state The process that gets executed (single CPU) The Ready state any process that is ready to be executed The Blocked state when a process cannot execute until some event occurs (ex: the completion of an I/O)

Process States The New state The Exit state
2017/4/21 Process States The New state OS has performed the necessary actions to create the process has created a process identifier has created tables needed to manage the process but has not yet committed to execute the process (not yet admitted) because resources are limited The Exit state Termination moves the process to this state It is no longer eligible for execution Tables and other info are temporarily preserved for auxiliary program

Five-State Process Model
2017/4/21 Five-State Process Model Dispatch Release Admit New Ready Running Exit Time-out Event Wait Event Occurs Blocked

Single Blocked Queue Ready Queue Release Dispatch Admit Processor
2017/4/21 Single Blocked Queue Ready Queue Release Dispatch Admit Processor Time-out Event Wait Event Occurs Blocked Queue

Multiple Blocked Queues
2017/4/21 Multiple Blocked Queues Ready Queue Release Dispatch Admit Processor Time-out Event 1 Wait Event 1 Queue Event 1 Occurs Event 2 Wait Event 2 Queue Event 2 Occurs

2017/4/21 Suspended Processes Processor is faster than I/O so all processes could be waiting for I/O Swap these processes to disk to free up more memory Blocked state becomes suspend state when swapped to disk Two new states Blocked, suspend Ready, suspend

Process State Transition Diagram with Two Suspend States
2017/4/21 Process State Transition Diagram with Two Suspend States Ready Admit Suspend Admit Dispatch Activate Ready, suspend Ready Running Exit Suspend Time out Event Wait Event Occurs Event Occurs Activate Blocked, suspend Blocked

Processor State Information
2017/4/21 Processor State Information Contents of processor registers User-visible registers Control and status registers Stack pointers Program status word (PSW) contains status information

Process Control Information
2017/4/21 Process Control Information Additional information needed by the operating system to control and coordinate the various active processes scheduling and state information data structuring interprocess communication process privileges memory management resource ownership and utilization

Process Creation Assign a unique process identifier
2017/4/21 Process Creation Assign a unique process identifier Allocate space for the process Initialize process control block Set up appropriate linkages Ex: add new process to linked list used for scheduling queue Other maintain an accounting file

When to Switch a Process
2017/4/21 When to Switch a Process Interrupts Clock process has executed for the maximum allowable time slice I/O Memory fault memory address is in virtual memory so it must be brought into main memory Trap error occurred may cause process to be moved to Exit state Supervisor call such as file open

Change of Process State
2017/4/21 Change of Process State Save context of processor including program counter and other registers Update the process control block with the new state and any accounting information Move process control block to appropriate queue - ready, blocked Select another process for execution Update the process control block of the process selected Update memory-management data structures Restore context of the selected process

UNIX Process State Initial (idle) Ready to run Kernel/User running
2017/4/21 UNIX Process State Initial (idle) Ready to run Kernel/User running Zombie Asleep for 4BSD: stopped/suspend

Process states and state transitions
2017/4/21 Process states and state transitions

Process Context User address space: Control information:
2017/4/21 Process Context User address space: code, data, stack, shared memory regions, Control information: u area, proc, kernel stack, ATM Credentials: UID & GID Environment variables: inherited from the parent Hardware context(in PCB of u area): PC, SP, PSW, MMR, FPU

User Credentials Superuser: UID=0, GID=1
2017/4/21 User Credentials Superuser: UID=0, GID=1 Real IDs: log in, send signals Effective IDs: file creation and access exec: suid mode: that of the owner; sgid mode: that of the calling process setuid / setgid: SV & BSD are different with these saved UID, saved GID in SV setgroup in BSD

The u Area part of the process space, needed only when running. PCB
2017/4/21 The u Area part of the process space, needed only when running. PCB proc pointer UID s Arguments, results, error status from system calls Signal handlers and related information Info from the program header, text, data, stack size, MM info Open file descriptor Vnodes & controlling terminal pointers CPU usage statistics, profiling info, disk quotas, & resource limits Per-process kernel stack

The proc Process table Id
2017/4/21 The proc Process table Id Location of the kernel address map for the u area Current process state Forward and backward pointers in scheduled queue Sleep channel (7.2.3) Scheduling priority and related (Chapter 5) Signal handling info(Chapter 4). MM info Pointers to link active, free, zombie processes in lists Flags Pointers to keep structures on a hash queue by PID Hierarchy info

A typical process hierarchy in 4.3BSD UNIX
2017/4/21 Process ID A typical process hierarchy in 4.3BSD UNIX Parent Process ID pointer to parent’s proc Pointer to the younger sibling Pointer to the oldest child

Typical Functions of an Operating-System Kernel
2017/4/21 Typical Functions of an Operating-System Kernel Process Management Process creation and termination Process scheduling and dispatching Process switching Process synchronization and support for inter-process communication Management of process control blocks Memory Management Allocation of address space to processes Swapping Page and segment management

Typical Functions of an Operating-System Kernel
2017/4/21 Typical Functions of an Operating-System Kernel I/O Management Buffer management Allocation of I/O channels and devices to processes Support Functions Interrupt handling Accounting Monitoring

Operating System Control Structures
2017/4/21 Operating System Control Structures An OS maintains the following tables for managing processes and resources: Memory tables I/O tables File tables Process tables

Memory Tables Allocation of main memory to processes
2017/4/21 Memory Tables Allocation of main memory to processes Allocation of secondary memory to processes Protection attributes for access to shared memory regions Information needed to manage virtual memory

I/O Tables I/O device is available or assigned Status of I/O operation
2017/4/21 I/O Tables I/O device is available or assigned Status of I/O operation Location in main memory being used as the source or destination of the I/O transfer

File Tables Existence of files Location on secondary memory
2017/4/21 File Tables Existence of files Location on secondary memory Current Status Attributes Sometimes this information is maintained by a file-management system

2017/4/21 Process Table Process image consists of program, data, stack, and attributes Attributes process control block

Process Control Block Process Identification
2017/4/21 Process Control Block Process Identification Unique numeric identifier may be an index into the primary process table User identifier who is responsible for the job

Execution of the Operating System
2017/4/21 Execution of the Operating System Nonprocess Kernel execute kernel outside of any process operating system code is executed as a separate entity that operates in privileged mode Execution Within User Processes operating system software within context of a user process process executes in privileged mode when executing operating system code

Execution of the Operating System
2017/4/21 Execution of the Operating System Process-Based Operating System major kernel functions are separate processes a process is invoked by the operating system

The UNIX kernel A special program that runs directly on the hardware.
2017/4/21 The UNIX kernel A special program that runs directly on the hardware. Implements the process model and services. Resides on disk, in a file /vmunix or /unix. Bootstrapping: loads the kernel. Initializes the system and sets up the environment, remains in memory before shut down

UNIX Services System call interface Hardware exceptions Interrupts
2017/4/21 UNIX Services System call interface Hardware exceptions Divide by 0, overflowing user stack Interrupts Devices Swapper, pagedaemon

The Kernel interacts with processes and devices
2017/4/21 The Kernel interacts with processes and devices BACK

2017/4/21 Mode,Space & Context By modes: some critical resources can be protected. Kernel Mode: More privileged, kernel functions User Mode: Less privileged, user functions Virtual Memory VM space Address Translation Maps Memory Management Unit

Kernel data Current process & Context switch
2017/4/21 Kernel data Current process & Context switch One instance of the kernel Global data structure Pre-process objects System call, Mode Switch User area: info. about a process Kernel stack:

2017/4/21 Context UNIX kernel is re-entrant: several processes may be involved in kernel activities concurrently. Execution context Process context: System context (Interrupt context):

Execution mode and Context
2017/4/21 Execution mode and Context

Executing in Kernel Mode
2017/4/21 Executing in Kernel Mode 3 types of events: Device interrupts Exceptions Traps or software interrupts Dispatch table System context: interrupts Process context: traps, exceptions & software interrupts

The System Call Interface
2017/4/21 The System Call Interface syscall(): the starting point in kernel mode, but in process context. Copy arguments , save hardware context on the kernel stack. Use system call number to index dispatch vector Return results in registers, restore hardware context, to user mode, control back to the library routine.

UNIX Interrupt Handling
2017/4/21 UNIX Interrupt Handling Interrupt handler(interrupt service routine): runs in kernel mode and system context, not permitted to block. the time used to service an interrupt charged to the interrupted process The clock interrupt handler charges the clock tick to the current process ipl(interrupt priority level)- specified for each interrupt and saved in interrupt register of the processor status word Interrupts are preemptive

Setting the interrupt priority in 4.3BSD and SVR4
2017/4/21 Setting the interrupt priority in 4.3BSD and SVR4

2017/4/21 Interrupt handling

Synchronization UNIX is re-entrant
2017/4/21 Synchronization UNIX is re-entrant UNIX is non-preemptive, keeping the kernel states consistent. Relinquish CPU voluntarily.

Blocking Operations Blocks the process (make it sleep).
2017/4/21 Blocking Operations Blocks the process (make it sleep). Lock: a single bit flag wanted(flag): sleep(): wake(): wake all the waiting processes. upon waking up: check once again to make sure.

Algorithm for resource locking
2017/4/21 Algorithm for resource locking

2017/4/21 Blocking Interrupts Blocking interrupts while accessing critical sections. int x = splbio();/* raise ipl*/ modify disk buffer cache; splx(x); /*restore ipl*/ Critical region:few & brief. Blocked interrupts may access the critical region. Different interrupts may have the same ipl Blocking an interrupt may block others.

UNIX Process Implementation
2017/4/21 UNIX Process Implementation

New Processes & Programs
2017/4/21 New Processes & Programs fork: creates a new process. returns 0 to the child, PID to the parent exec: begins to execute a new program

Using fork & exec if ((result = fork()==0){ /* child code*/ … …
2017/4/21 Using fork & exec if ((result = fork()==0){ /* child code*/ … … if (execve(“new program”),…)<0) perror(“execve failed!”); } else if (result<0) { perror(“fork”);/*fork failed*/ } /*parent continures here*/

Shell creating a process
2017/4/21 Shell creating a process A highly simplified shell

Steps in executing the command ls typed to the shell
2017/4/21 The ls Command Steps in executing the command ls typed to the shell

Process Creation Almost an exact clone of the parent.
2017/4/21 Process Creation Almost an exact clone of the parent. Reserve swap space for the child Allocate a new PID and proc structure for the child Initialize proc structure Allocate address translation map (ATM) Allocate u area and copy Update the u area to refer to the new ATM & Swap space Add the child to the set of processes sharing the text region of the program Duplicate the parent’s data and stack regions update ATM to refer to these new pages. Acquire references to shared resources inherited by the child Initialize the hardware context Make the child runnable and put it on a scheduler queue Arrange to return with 0 Return the PID to the parent

2017/4/21 fork Optimization It is wasteful to make an actual copy of the address space of the parent Copy-on-write (SV) : only the pages that are modified must be copied vfork (BSD): The parent loans the address space and blocks until the child returns to it. dangerous! csh: exploits it.

Invoking a New Program Process address space Text: code
2017/4/21 Invoking a New Program Process address space Text: code Initialized data: Uninitialized data(bss): Shared memory(SV): Shared libraries: Heap: dynamic space User stack: space allocated by the kernel

exec System Call Parse & access Verify the permission
2017/4/21 exec System Call Parse & access Verify the permission Read the header and check if valid executable If the file has SUID or SGID bits set in its mode, change the caller’s effective UID or GID to that of the owner Copy the arguments to exec and the env. variables into kernel. Allocate swap space for the data and stack region Set up the new address space. Copy the arguments and env. variables back onto the new user stack. Reset all signal handlers to default actions. Initialize the hardware context.

Process Termination (exit())
2017/4/21 Process Termination (exit()) Turns off all signals. Closes all open files. Releases the text file and other resources such as the current directory. Writes to the accounting log. Saves resource usage statistics and exit status in the proc structure. Changes state to SZOMB, and puts the proc on the zombie process list. Makes the init process inherit any live children of the exiting process. Releases the address space, u area, ATM, and swap space. Notifies the parent by sending it a SIGCHLD signal. Wakes up the parent if it is asleep. Calls swtch() to schedule a new process to run.

Awaiting Process Termination
2017/4/21 Awaiting Process Termination wait(stat_loc);/* SV, BSD & POSIX*/ wait3(statusp, options, rusagep); /*BSD*/ waitpid(pid, stat_loc, options);/*POSIX*/ waitid(idtype, id, infop, options);/*SVR4*/

2017/4/21 Zombie Processes after exit(), process holds only proc structure and is in zombie state until cleaned completely. wait() (issued by the parent or the init process)frees the proc structure and completes process exit, If the child dies before the parent which does not wait for it, the proc is not released until the system is rebooted. Parent can specify that it will not wait for its children

2017/4/21 LINUX Modular structure – collection of modules, some of them loadable and unloadable on demand, Module – object file whose code be linked and unlinked from the kernel at runtime, executed in kernel mode on behalf of the current process Loadable module characteristic – Dynamic linking, Stackable modules – may serve as libraries when they are referenced by client modules higher up in the hierarchy, and as clients when they reference modules further down the hierarchy.

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2017/4/21 Linux components At user level – tasks (specific to Linux – combine features of processes and threads); At kernel level – interacting collection of components; Hardware components – in this case IA-64 Intel Itanium architecture

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Linux Task Data Structure
2017/4/21 Linux Task Data Structure State Scheduling information Identifiers Interprocess communication Links Times and timers File system Address space Processor-specific context

Linux States of a Process/Thread
2017/4/21 Linux States of a Process/Thread Running Interruptable Uninterruptable Stopped Zombie

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The Process and the Kernel

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