Transcript Processes - Kent State University

Lecture 4: Process Management
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process
process creation/termination
context
process control block (PCB)
context switch
5-state process model
process scheduling
• short/medium/long term
Unix process model
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Process
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process (also called task, or job) is a program in
execution
process
• program code (or text) sequence of instructions to
be executed
 note: multiple processes may be running the
same code (editor, web-browser, etc)
• context (execution state)
prompt% ps -efl
UID PID PPID C STIME TTY
root
0
0
0 Jan 19 ?
root
1
0
0 Jan 19 ?
root
2
0
0 Jan 19 ?
…
mikhail 1546 1321 0 15:38:45 pts/17
TIME COMMAND
10:53 swapper
0:17
init
7:39
telnetd
3:31
netscape
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Unix Process Creation
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one process can create another process, perhaps to do
some work for it
• child – the new process
 an (almost) identical copy of parent (same code, same data, etc.)
• parent – the original process
 parent can either wait for the child to complete, or continue
executing in parallel (concurrently) with the child
 parent does not terminate until the child does
In UNIX, a process creates a child process using the system call fork( )
• In child process, fork() returns 0
• In parent process, fork() returns process id of new child
• child often uses exec( ) to start another completely different program
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Example of Unix Process Creation
#include <sys/types.h>
#include <stdio.h>
int a = 6; /* global (external) variable */
int main(void) {
int b;
/* local variable */
pid_t pid;
/* process id */
b = 88;
printf("..before fork\n");
pid = fork();
if (pid == 0) {
a++; b++;
} else
wait(pid);
/* child */
example execution
prompt% forkprogram
..before fork
..after fork, a = 7, b = 89
..after fork, a = 6, b = 88
/* parent */
printf("..after fork, a = %d, b = %d\n", a, b);
exit(0);
}
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Unix Process Tree
as one processes creates another, they form a tree
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Process Termination
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after completing work a process may execute exit()
system call – asking the OS to delete it
• parent is notified
• process’ resources are deallocated by operating system
parent may terminate execution of a child process - abort()
system call
• possible reasons
 task assigned to child is no longer required
 child exceeded allocated resources
• if parent exits (some) OS require all children to terminate –
cascading termination
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Context
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context – information about running process
• program code (?)
• static data (bss)
• heap (dynamic data)
• procedure call stack - contains temporary data subroutine parameters, return addresses, local
variables
• register contents
 general purpose registers
 program counter (PC) — address of next
instruction to be executed
 stack pointer (SP)
 program status word (PSW) — interrupt status,
condition codes, etc.
• OS resources in use - open files, connections to other
programs
• accounting information
note that for each process there are two contexts
• for user mode and for kernel mode
program memory
allocation
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Process Control Block
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process control block (PCB) – data structure
maintained by OS to keep track of process’ state
contains
• process id number
• user id of the owner
• process state (running, not-running, etc.)
• PC, SP, PSW, general purpose registers
• memory limits (static, dynamic) - base/limit register
contents
• list of open files
• CPU scheduling information (e.g., priority)
• I/O states, I/O in progress - list of I/O devices
allocated to process, list of open files, etc. status
of I/O requests
• …
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Context Switch
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context switch - stopping
one process and
restarting another
sequence of actions
 OS takes control
(through interrupt)
 saves old context in the
process PCB
 reloads new context
from the new process
PCB
• this amounts
to changing
the execution of processes (as soon as the PC is reloaded)
 returns control to app. program
 how is context switch different from mode switch? How are they similar?
how many mode switches are there in a context switch?
 context switch requires several thousand instructions
• time-sharing OS may do 100–1000 context switches a second
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Interleaving of Process Execution in
Time-Sharing OS
from the user’s standpoint
multiple processes
are executed concurrently
in reality OS interleaves
process execution
time
process
process
process
A
B
C
process
D
process A
process B
process C
process A
process B
process C
process A
process C
process A
process D
process C
process D
process C
time
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Five State Process Model
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OS maintains the state of a process.
process is
• new – when it is being created
• running - executing on the CPU
• waiting (blocked) – waiting on input/output even to occur
• ready – ready to run on the CPU
• terminated – finished execution, is being destroyed
how many processes in the system can be in a particular state?
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OS Process Queues
OS queues PCBs according to their state
 ready queue –
processes in state
ready (waiting to
be run on CPU)
 for each device there
is an I/O queue –
processes in state
waiting, blocked
on particular device
waiting for their
request to be
submitted
to the device
controller
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Short Term
Scheduler
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selects process
from the ready
queue to run on
the CPU
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runs when
• process is created
or terminated
• process switches
from running to blocked
• interrupt occurs
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goals:
• minimize response time (e.g., program execution, character to screen)
• minimize variance of average response time — predictability may be
important
• maximize throughput
 minimize overhead (OS overhead, context switching, etc.)
 efficient use of resources
• fairness — share CPU in an equitable fashion
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Other Types
of Schedulers
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medium-term
scheduler
• temporarily
swaps processes in/out of main memory
• may suspend/resume processes
• goal: balance load for better throughput
long-term scheduler (job scheduler) – does not exist on time-sharing OS
• selects job from the spool, and loads it into memory
• executes infrequently, maybe only when process leaves system
• controls degree of multiprogramming
• goal: good mix of CPU-bound and I/O-bound processes
 I/O-bound process – spends more time doing I/O than
computations, many short CPU bursts
 CPU-bound process – spends more time doing computations; few
very long CPU bursts
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Unix Process States
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enhanced 5-state model
blocked and ready states has to be split depending on
whether a process is swapped out on disk or in memory
running state is also split depending on the mode: kernel or user
Unix process states:
• created - just created not yet ready to run
• ready (memory) - ready as soon as kernel schedules it
• ready (disk) - ready, but needs to be swapped to memory
• asleep - blocked (memory) - waiting on event in memory
• asleep - blocked (disk) - waiting on event on disk
• running (kernel) - executing in kernel mode
• running (user) - executing in user mode
• zombie - process terminated but left a record for parent to collect
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Unix Process Scheduling
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process is running in user mode until an interrupt occurs or
it executes a system call
if time slice expires the process is preempted and another is scheduled
a process goes to sleep if it needs to wait for some event to occur and is woken
up when this event occurs
when process is created decision is made whether to put it in memory or disk
fork
user mode
kernel mode
interrupt
user
running
return
enough
memory
kernel schedule
running
interrupt,
system call
preempt
ready
created
swap out
swap in
zombie
ready
wakeup
wakeup
exit
not enough
memory
sleep
asleep
swap out
asleep
disk
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Lecture Review
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the execution of a certain task from OS standpoint is
a process
• OS maintains (creates/destroys) processes for user
• OS schedules processes for user
 there are three types of schedulers: short/medium/long term
OS saves process states in a process control block
OS context switches between processes
in 5-state process model a process can be in one of five states: created,
running, ready, blocked, terminated
• UNIX adds states for being swapped out to disk
• depending on process state its PCB is attached to either ready queue
or device queue
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