Download Objectives Process control block (1) Process Control Block (2)

Process control block (1)
Operating Systems
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Previous lecture defined process as:
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Part of E1.9 - “Principles of Computers and Software
Engineering”
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Program in execution
Consisting of
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Lecture 3: Introduction to process management
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Instructions
Data
Stack and context
Process attributes
The state of a process is defined by its current
activity (e.g. running, waiting)
Process is represented using a process control block.
E1.9 - Operating systems (Lect 3)
Objectives
Process Control Block (2)
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introduce the concepts of:
“ Process
state and process control block
“ Process scheduling
“ Threads
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Process state
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Program counter
CPU registers
CPU-scheduling information
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Memory management information
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Accounting information
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I/O status information
“ e.g list of I/O devices allocated and list of open files
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“ Process
creation
“ Process termination
“ Process switching
E1.9 - Operating systems (Lect 3)
Typical Process Control Block (PCB) includes:
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describe the steps involved in:
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e.g. running, waiting
e.g process priority
e.g values of base and limit registers
e.g process number
E1.9 - Operating systems (Lect 3)
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Process States (1)
Process States (1)
Two-state model (1)
Two-state model (3)
dispatch
Creation
Process
Creation
Not-running
running
Process
Termination
Termination
Not-running-queue
PN
PN-1
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P2
P1
dispatch
Processor
pause
pause
Process scheduling decisions:
Processes either running or not-running
“ Processes created as not-running
“ After completion, processes are terminated
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E1.9 - Operating systems (Lect 3)
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Which process to pick next from the not-running
queue?
When is a process paused?
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E1.9 - Operating systems (Lect 3)
Process States (1)
Process States (2)
Two-state model (2)
Five-state model (1)
Process management in the two-state model:
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Move process from not-running to running state
Restore its context
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Move process from running to not-running state
Save its context
E1.9 - Operating systems (Lect 3)
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e.g waiting for I/O
Dispatching of such process is a waste of processor time
Need to separate not running processes in more
categories:
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Pausing of a process
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Two state model: some processes in the not-running
queue might not actually be ready to execute
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Selecting a process for running
“ Dispatching of a process
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New: process was just created but has not yet entered the
system
Waiting: process is waiting for I/O
Ready: process is ready to execute
Result: Five state model
E1.9 - Operating systems (Lect 3)
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Process States (2)
Process States (3)
Five-state model (2)
Seven-state model (2)
Process
Creation
Five-state model:
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Process
Creation
New
dispatch
admitted
exit
ready
running
pause
I/O or event
completion
Waiting
New
Process
Termination
dispatch
admitted
exit
ready
ready-suspend I/O or event
I/O or
event
wait
running
pause
Load from disk
Terminated
Process
Termination
I/O or
event
wait
Waiting
completion
Load from
disk
Terminated
Suspend to
disk
Waiting-suspend
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Three queues: ready, new, waiting
E1.9 - Operating systems (Lect 3)
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E1.9 - Operating systems (Lect 3)
Process States (3)
Process States (3)
Seven-state model (1)
Seven-state model (3)
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All processes (including those waiting for I/O) are loaded in
memory
Problem if memory is limited
We can store (suspend) some of processes in secondary
storage
Need to add new two categories of process states:
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Five-state model:
Long term scheduling:
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Medium-term scheduling
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Short-term or CPU scheduling
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Determine whether process is admitted to system
Determine whether process is placed to ready or ready-suspend
queue
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Seven-state model takes into account these new
states
E1.9 - Operating systems (Lect 3)
Seven state model illustrates the need for three
different types of scheduling
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Ready-suspend: Process is ready to execute (not waiting for
I/O) but is suspended to secondary storage
Waiting-suspend: Process is waiting for I/O and has been
suspended to secondary storage.
Determine when to move a process to/from the suspended state
Determine when to interrupt a process currently running (note:
this is different to processes interrupted by other events (e.g.
hardware interrupt)).
Determine which process from the ready queue to run next.
E1.9 - Operating systems (Lect 3)
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Process States (4)
Process States (5)
Scheduling queues and queuing diagrams (1)
Process Tables
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OS maintains process tables
that hold the PCBs.
“ One entry per PCB
“ Pointer entry in PCB points to
the next PCB in the queue.
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Scheduling queues
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Need even more queues – apart from new, ready
and waiting, now need new, ready, waiting,
waiting-suspend, ready-suspend.
Also useful to have various I/O device queues to
place processes waiting for a specific device (one
per device).
Process scheduling moves processes
between queues
“ Queuing diagrams
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Example: The ready queue and various I/O device queues
[Silberschatz et al 2001]
E1.9 - Operating systems (Lect 3)
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E1.9 - Operating systems (Lect 3)
Process States (4)
Operations on Processes(1)
Scheduling queues and queuing diagrams (2)
Overview
Creation
PN
I/O1
I/O2
PN-1
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B2
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P2
P1
Bm-1
Bm
Blocked-waiting-for-I/O2 queue
C1
C2
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Major process operations used in the diagrams:
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Blocked-waiting-for-I/O1 queue
B1
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Termination
Ready-queue
Cm-1 Cm
dispatch
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Processor
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pause
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Process creation
Process termination
Process switch
Request I/O1
Request I/O2
(Example of a (partial)
queuing diagram)
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Operations on Processes(2)
Operations on Processes(3)
Process creation (1)
Process termination
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Process creation
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Create new PCB
Allocate memory for instructions, data, stack
Load instructions and data into allocated memory
Initialise stack and context
Initialise PCB
Process termination
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When?
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When?
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System initialisation
Execution of a process-creation system call by a running
process (e.g. POSIX fork, Win32 CreateProcess)
User creates a new process (e.g. through the shell)
Initiation of a batch job
E1.9 - Operating systems (Lect 3)
Release resources allocated to process
Delete PCB entries
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Completion of task (normal, voluntary exit)
Exit due to error, e.g division by zero, I/O error, protection
error.
Exit due to termination of parent process (cascading
termination)
Termination by the OS, e.g if a resource is unavailable or its
use has exceeded accounting limits.
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E1.9 - Operating systems (Lect 3)
Operations on Processes(2)
Operations on Processes(4)
Process creation (2)
Process switch
The creating process is called parent process;
new processes are called children of this
process
“ Possibilities after creation:
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Parent continues execution concurrently with the children
Parent waits for some or all of children to terminate in
order to continue.
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Child process is a duplicate of the parent process
Child process has a new program loaded into it
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Kernel saves context (e.g registers, pointers,
memory management information etc) into the
process’ PCB
Loads context of the new process scheduled to
run
Context switch time is pure overhead
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In terms of address space of the new process:
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Process or context switch
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In terms of execution
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can become substantial depending on whether
parts of memory must be saved too.
Hardware support can reduce this overhead
(e.g by providing multiple sets of registers).
E1.9 - Operating systems (Lect 3)
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Threads(1)
Threads(2)
Need for
How? (2)
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Processes encompass two concepts:
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Kernel-level threads
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Kernel performs creation, scheduling and
management in kernel space.
Disadvantages:
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Advantages
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Threads:
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Code execution
Resource grouping
Can be thought at lightweight processes
Share code section, data section and OS resources (e.g
open files) with other threads within the process
Have own program counter, register set and stack.
Allow several threads of execution within the same process
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Avoid context switching overheads
But no protection (e.g. memory) between threads
E1.9 - Operating systems (Lect 3)
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Slower to create and manage than user-level threads
Can easily handle threads with blocking system calls
In multiprocessor environments, kernel can schedule
threads on different processors
Recycling threads
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E1.9 - Operating systems (Lect 3)
Threads(2)
Threads(4)
How? (1)
Benefits
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User-level threads: Threads library (e.g POSIX
pthreads)
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Have to write own scheduling algorithm
Difficult to handle with blocking system calls
E1.9 - Operating systems (Lect 3)
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Interactive processes can perform a lengthy
operation in a separate thread, while they handle
user’s input in another thread
Example: Multithreaded Web browser
Economy
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Fast to create and manage
Disadvantages
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Responsiveness
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Each process starts with a single thread present.
A thread can create new threads by calling library
procedures, e.g thread_create.
Parent thread typically specifies the name of a procedure for
the new thread to run
Parent thread gets identifier for the new thread.
Advantages
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Allocating memory and resources for processes is
costly
Utilization of multiprocessor architectures
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Process management
Recommended Reading
What next?
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Next three lectures will examine issues introduced today
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First: Process scheduling
Second: Process synchronization
Third: Deadlocks
E1.9 - Operating systems (Lect 3)
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Read chapter 4 and section 5.1 of Silberschatz’s book
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Read chapter 2, sections 2.1-2.2.5 of Tanenbaum’s book
Rest of chapter 2 will be covered in later lectures
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Read chapter 3, and section 4.1 of Stallings’ book
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Summary
Process management
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Process state models include two-, five- and seven-state models
The steps involved in process creation, termination, and switch
were presented.
Threads are used to allow for multiple executions to take place
within the same process.
Next lecture:
Process scheduling
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