1 Process Description and Control Module 1.1. 2 When to Switch a Process ? n A process switch may occur whenever the OS has gained control of CPU. ie.

1

Process Description and ControlProcess Description and Control

Module 1.1Module 1.1

2

When to Switch a Process ?When to Switch a Process ? A process switch may occur whenever the

OS has gained control of CPU. ie when: Supervisor Call

explicit request by the program (ex: file open). The process will probably be blocked

Trap An error resulted from the last instruction. It may

cause the process to be moved to the Exit state Interrupt

the cause is external to the execution of the current instruction. Control is transferred to IH

3

Examples of interrupts Clock

process has expired his time slice and is transferred to the ready state

I/O first move the processes that were waiting for

this event to the ready (or ready suspend) state then resume the running process or choose a

process of higher priority Memory fault

memory address is in virtual memory so it must bring corresponding block into main memory

thus move this process to a blocked state (waiting for the I/O to complete)

4

Mode SwitchingMode Switching It may happen that an interrupt does not

produce a process switch The control can just return to the

interrupted program Then only the processor state information

needs to be saved on stack This is called mode switching (user to

kernel mode when going into IH) Less overhead: no need to update the PCB

like for process switching

5

Steps in Process (Context) SwitchingSteps in Process (Context) Switching

Save context of processor including program counter and other registers

Update the PCB of the running process with its new state and other associate info

Move PCB to appropriate queue - ready, blocked

Select another process for execution Update PCB of the selected process Restore CPU context from that of the

selected process

6

Execution of the Operating SystemExecution of the Operating System

Up to now, by process we were referring to “user process”

If the OS is just like any other collection of programs, is the OS a process?

If so, how it is controlled?

The answer depends on the OS design.

7

Nonprocess Kernel (old)Nonprocess Kernel (old)

The concept of process applies only to user programs

OS code is executed as a separate entity in privilege mode

OS has its own memory regions and stack OS code never gets executed within a process

8

Execution within User ProcessesExecution within User Processes

Virtually all OS code gets executed within the context of a user process

On Interrupts, Traps, System calls: the CPU switch to kernel mode to execute OS routine within the context of user process (mode switch)

Control passes to process switching functions (outside processes) only when needed

9

Execution within User ProcessesExecution within User Processes

OS code and data are in the shared address space and are shared by all user processes

Separate kernel stack for calls/returns when the process is in kernel mode

Within a user process, both user and OS programs may execute (more than 1)

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Process-based Operating SystemProcess-based Operating System

The OS is a collection of system processes major kernel functions are separate processes small amount of process switching functions is

executed outside of any process Design that easily makes use of multiprocessors

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UNIX SVR4 Process managementUNIX SVR4 Process management

Most of OS executes within user processes Uses two categories of processes:

System processes run periodically in kernel mode for administrative

and housekeeping functions (memory allocation, process swapping...)

User processes run in user mode for user programs run in kernel modes for system calls, traps, and

interrupts

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UNIX SVR4 Process StatesUNIX SVR4 Process States Similar to our 7 state model 2 running states: User and Kernel

transitions to other states (blocked, ready) must come from kernel running

Sleeping states (in memory, or swapped) correspond to our blocking states

A preempted state (timeout) is distinguished from the ready state (but they form 1 queue) In the same ready queue, there are two states

Preemption can occur only when a process is about to move from kernel to user mode Kernel code is not preemptable To simplify switching, when the process resumes

running, it runs in the user running (with empty kernel stack)

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UNIX Process State DiagramUNIX Process State Diagram

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15

Context SwitchContext Switch Context Switch Mechanism

The kernel permits it under four circumstances: When a process puts itself to sleep When it exits When it returns from a system call to user mode When it returns to user mode after interrupt handling

The kernel ensures integrity and consistency of internal data structures by prohibiting arbitrary context switches.

The procedure for a context switch is similar to the procedures for handling interrupts and system calls, except that the kernel restores the context layer of a different process instead of the previous context layer of the same process.

Steps for a Context Switch

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UNIX Process ImageUNIX Process Image

User-level context Process Text (ie: code: read-only) Process Data User Stack (calls/returns in user mode) Shared memory (for IPC)

only one physical copy exists but, with virtual memory, it appears as it is in the process’s address space

Register context

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UNIX Process ImageUNIX Process Image System-level context

Process table entry the actual entry concerning this process in the

Process Table maintained by OS• Process state, UID, PID, priority, event awaiting, signals

sent, pointers to memory holding text, data...

U (user) area additional process info needed by the kernel when

executing in the context of this process• effective UID, timers, limit fields, files in use ...

Kernel stack (calls/returns in kernel mode) Per Process Region Table (used by memory manager)

Page tables defining code, stack, data regions

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19

UNIX System ProcessesUNIX System Processes

Process 0 is created at boot time and becomes the “swapper” after forking process 1 (the INIT process)

When a user logs in: process 1 creates a process for that user

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Processes in UNIX SystemProcesses in UNIX System User Process

most processes on typical system associated with users at a terminal

Daemon Process not associated with any users do system-wide functions administration and control of networks, execution of time-

dependent activities, line printer spooling, and so on run at user mode and make system calls to access system

service like user process Kernel Process

execute only in kernel mode process 0 spawns kernel process, such as page-reclaiming

process vhand, and then becomes the swapper process extremely powerful, not flexible

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A tree of processes on a typical A tree of processes on a typical SolarisSolaris

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UNIX Process CreationUNIX Process Creation

Every process, except process 0, is created by the fork() system call fork() allocates entry in process table and

assigns a unique PID to the child process child gets a copy of process image of parent:

both child and parent are executing the same code following fork()

but fork() returns the PID of the child to the parent process and returns 0 to the child process

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C Program Forking Separate ProcessC Program Forking Separate Process

int main(){Pid_t pid;

/* fork another process */pid = fork();if (pid < 0) { /* error occurred */

fprintf(stderr, "Fork Failed");

exit(-1);}else if (pid == 0) { /* child process */

execlp("/bin/ls", "ls", NULL);}else { /* parent process */

/* parent will wait for the child to complete */

wait (NULL);printf ("Child Complete");exit(0);

}}

Note that system(“/bin/ls”) standard C library function is equivalent to fork() and

then exec(“bin/ls”).

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Virtual Memory – ReviewVirtual Memory – Review The memory referenced by a

logical address is called virtual memory is maintained on

secondary memory (ex: disk)

pieces are brought into main memory only when needed

For better performance, the file system is often bypassed and virtual memory is stored in a special area of the disk called the swap space

larger blocks are used and file lookups are not used.

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Paging – Review Paging – Review

Typically, each process has its own page table.

Page tables are variable in length (depends on process size). Stored in main memory instead of registers. A single register holds the starting physical address of the page table of the running process.

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Address Translation in a Paging System – Address Translation in a Paging System – Review Review

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Sharing Pages of a text editor – Review Sharing Pages of a text editor – Review

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Translation Lookaside Buffer – Review Translation Lookaside Buffer – Review

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TLB Comments – Review TLB Comments – Review TLB use associative mapping hardware to simultaneously

interrogates all TLB entries to find a match on page number The TLB must be flushed each time a new process enters the

Running state The CPU uses two levels of cache on each virtual memory

reference first the TLB: to convert the logical address to the physical address TLB is a special on-chip cache (other than L1,L2, L3 caches) If no on-chip TLB, L1 will typically have it. once the physical address is formed, the CPU then looks in

the cache for the referenced word in L1, L2 and L3 Caches L1 is the fastest and the most expensive, followed by L2, followed

by L3

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L1 & L2 Caches -- Review L1 & L2 Caches -- Review

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Referencing a memory word -- ReviewReferencing a memory word -- Review

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Unix Processes & RegionUnix Processes & Region

Only 4K bytes are shared, and not the

whole region

Virtual addresses of the

regions are connected

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Layout of the Unix KernelLayout of the Unix Kernel Although the kernel executes in the context of a process, the

virtual memory mapping associated with the kernel is independent of all processes.

The code and data for the kernel reside in the system permanently, and all processes share it.

The kernel page tables are analogous to the page tables associated with a process.

In many machines, the virtual address space of a process is divided into several classes, including system and user, and each class has its own page tables.

When executing in kernel mode, the system permits access to kernel addresses, but it prohibits such access when executing in user mode.

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•Example of the virtual addresses of the kernel and a process regions•Kernel text/data (first two triples)•Kernel stack within process (third triple)•Process (text/data,stack)

35

This is logical copy. Text is shared and only reference count is copied.

Ref count of processes accessing the current directly. Used when opening a local filenename, not starting with “/”.

In case parent process or one of its ancestors

did chroot

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Fork Creating a New Process ContextFork Creating a New Process Context

ParentData

Per ProcessRegion Table Open Files

Current Directory

Changed Root

.

.

.

.

.Kernel Stack

U Area

Per ProcessRegion Table Open Files

Current Directory

Changed Root

.

.

.

.

.Kernel Stack

U Area

FileTable

InodeTable

Parent Process

Child Process

ParentUser Stack

SharedText

ChildData

ChildUser Stack

37

38

39

Inode of the executable file, with region type “Data”.

Done also for code and stack.

40

41

42

If the shell recognizes the input string as a built-in command (e.g., cd, for, while, …), it executes the command internally without creating new process; otherwise, it assumes the command is the name of an executable file.

For commands with &, the shell does not wait but immediately restarts the loops and reads the next command line.

43

We need first to understand system calls for the file systems.

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Introduction to Unix file systemIntroduction to Unix file system

Unix is an indexed file system

Every file on a UNIX system has a unique indode that contains the information necessary to for a process to access a file by a well defines system calls.

Processes specify the file by its path name.

Internally, the algorithm namei converts a user-level path to inode number.

Inodes are stored in a linear array on disk.

Contains also link counts

and reference counts

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46

Byte Capacity of a FileByte Capacity of a File System V UNIX. Assume that

Run with 13 entries 1 logical block : 1K bytes Block number address : a 32 bit (4byte) integer

1 block can hold up to 256 block number (1024byte / 4byte) 10 direct blocks with 1K bytes each=10K bytes 1 indirect block with 256 direct blocks= 1K*256=256K bytes 1 double indirect block with 256 indirect blocks=256K*256=64M

bytes 1 triple indirect block with 256 double indirect

blocks=64M*256=16G

Size of a file : 4G (232), if file size field in inode is 32bits

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A directory is a file whose data is a sequence of entries, each consisting an inode number and the name of a file.

If inode number is 0, it indicates that the entry is empty (i.e., available for use again).

48

nameinamei

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link count – hard links counts representing file names in directory hierarchy reference count (will see shortly) -- for opening and referencing files by processes

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51

•Kernel maintains two data structures: file table and user fd table

•File table is a global kernel structure

•User fd table is allocated per process

•The file table keeps track of the byte offset in the file where the user’s next read or write will start, and the access rights allows to opening process

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54

So what happens when we do close(stdout)?How about dup(fd)?

55

DUPDUP

The dup system call copies a fd into the first free slot of the user fd table, returning the new fd to the user.

Syntax newfd = dup(fd);

Why is this needed? Servers in building sophisticated system programs as

that of the shell Assume

A process opened the file “/etc/passwd” fd3, then opened “local” fd4, then opened the file “/etc/passwd” fd5, and finally duped fd3 returning fd6.

See illustration of this on next slide

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57

So what really happened?

How redirecting input??

58

How about piping?

59

PipingPiping

Pipe fifo(first-in-first-out) Synchronization of process execution

Its data is transient: once data is read from a pipe, it cannot be read again

Use only direct block (not the indirect block)

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61

The ShellThe Shell

1. reads a command line from its standard input and interprets it2. built in command

cd, for, while …executes command internally without creating process

3. simple command line(program and parameters)who, grep –n include *.c, ls -l forks and creates a child processexecs the program user specifiedshell waits until the child process exits

4. run a process asynchronously(in the background)nroff –mm bigdocument &sets an internal variable ampershell does not execute wait

5. redirect standard output to a filenroff –mm bigdocument &the child creates the output filecloses its standard outputdup its file descriptor to standard outputredirects standard input and standard error in similar way

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The Shell(Cont.)The Shell(Cont.)

6. pipe ls –l | wc parent process forks and creates a child process child creates a pipe child process forks and creates a grandchild process grandchild process handles the first command(ls)

close its standard output file descriptor dups the pipe write descriptor close original pipe write descriptor

child process handles the second command(wc) close its standard input file descriptor dups the pipe read descriptor close original pipe read descriptor

output of grandchild(ls) goes to the input of child(wc) parent shell waits its child(wc) to exit

Shell

wc

ls -lwrite

read

exit

wait