Transcript Chapter 2.3 : Interprocess Communication • Process concept • Process scheduling

Chapter 2.3 : Interprocess
Communication
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Process concept 
Process scheduling 
Interprocess communication
Deadlocks
Threads
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Producer - Consumer Problem
Producer Process
Consumer Process
Produce
Get from buffer
Put in buffer
Consume
BUFFER
• Buffer is shared (ie., it is a shared variable)
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Progress in time…..
Producer
p1
1
p2
2
p3
3
Buffer
p4
4
3 instead of 2!
Consumer
1
c1
2
c2
• Both processes are started at the same time
and consumer uses some old value initially
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t
A Race Condition
• Because of the timing and which process
starts first
• There is a chance that different executions
may end up with different results
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Critical Sections
• Critical Section
– A section of code in which the process accesses
and modifies shared variables
• Mutual Exclusion
– A method of preventing for ensuring that one
(or a specified number) of processes are in a
critical section
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Why Processes Need to
Communicate?
• To synchronize their executions
• To exchange data and information
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Rules to Form Critical Sections
1. No two processes may be simultaneously
inside their CS (mutual exclusion)
2. No assumptions are made about relative
process speeds or number of CPUs
3. A process outside a CS should not block
other processes
4. No process should wait forever before
entering its CS
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Mutual Exclusion Problem :
Starvation
• Also known as Indefinite
Postponement
• Definition
– Indefinitely delaying the scheduling of a process in
favour of other processes
• Cause
– Usually a bias in a systems scheduling policies (a bad
scheduling algorithm)
• Solution
– Implement some form of aging
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Another Problem : Deadlocks
– Two (or more) processes are blocked waiting
for an event that will never occur
– Generally, A waits for B to do something and B
is waiting for A
– Both are not doing anything so both events
never occur
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How to Implement Mutual
Exclusion
• Three possibilities
– Application: programmer builds some method
into the program
– Hardware: special h/w instructions provided
to implement ME
– OS:
provides some services that can
be used by the programmer
• All schemes rely on some code for
– enter_critical_section, and
– exit_critical_section
• These "functions" enclose the critical
section
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Application Mutual Exclusion
• Application Mutual Exclusion is
– implemented by the programmer
– hard to get correct, and
– very inefficient
• All rely on some form of busy waiting
(process tests a condition, say a flag, and
loops while the condition remains the same)
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Example
• Producer
produce
If lock = 1 loop until lock = 0
lock=1
put in buffer
lock=0
• Consumer
If lock = 1 loop until lock = 0
lock=1
get from buffer
lock=0
consume
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Hardware ME :
Test and Set Instruction
• Perform an indivisible x:=r and r:=1
• x is a local variable
• r is a global register set to 0 initially
• repeat (test&set(x)) until x = 0;
< critical section >
r:= 0;
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Hardware ME :
Exchange Instruction
• Exchange: swap the values of x and r
• x is a local variable
• r is a global register set to 1 initially
• x:= 0;
repeat exchange(r, x) until x = 1;
< critical section >
exchange(r, x);
Note: r:= 0 and x:= 1 when the process is
in CS
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Hardware ME Characteristics
• Advantages
– can be used by a single or multiple processes
(with shared memory)
– simple and therefore easy to verify
– can support multiple critical sections
• Disadvantages
– busy waiting is used
– starvation is possible
– deadlock is possible (especially with priorities)
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Another Hardware ME :
Disabling Interrupts
• On a single CPU only one process is
executed
• Concurrency is achieved by interleaving
execution (usually done using interrupts)
• If you disable interrupts then you can be
sure only one process will ever execute
• One process can lock a system or degrade
performance greatly
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Mutual Exclusion Through OS
• Semaphores
• Message passing
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Semaphores
• Major advance incorporated into many
modern operating systems (Unix, OS/2)
• A semaphore is
– a non-negative integer
– that has two indivisible, valid operations
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Semaphore Operations
• Wait(s)
If s > 0
then s:= s - 1
else block this process
• Signal(s)
If there is a blocked process on this
semaphore then wake it up
else s:= s + 1
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More on Semaphores
• The other valid operation is initialisation
• Two types of semaphores
– binary semaphores can only be 0 or 1
– counting semaphores can be any non-negative
integer
• Semaphores are an OS service implemented
using one of the methods shown already
– usually by disabling interrupts for a very short
time
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Producer - Consumer Problem:
Solution by Semaphores
Produce
CS
Wait(mutex)
Put in buffer
Signal(mutex)
Wait(mutex)
Get from buffer
Signal(mutex)
Consume
• Initially semaphore mutex is 1
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Another Example
• Three processes all share a resource on which
– one draws an A
– one draws a B
– one draws a C
• Implement a form of synchronization so that the output
appears ABC
Process A
Process B
Process C
think();
think();
think();
draw_A();
draw_B();
draw_C();
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• Semaphore b = 0, c = 0;
Process A
Process B
Process C
wait(b);
think();
wait(c);
think();
draw_A();
think();
draw_B();
signal(b);
draw_C();
signal(c);
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Message Passing
• Provides synchronization and information
exchange
• Message Operations:
– send(destination, &message)
– receive (source, &message)
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Producer - Consumer Problem
Using Messages
#define N 100 /*number of message slots*/
producer( )
{int item; message m;
while (TRUE) {
produce_item(&item);
receive(consumer,&m);
build_message(&m, item);
send(consumer,&m);
}}
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Consumer( )
{int item; message m;
for (i=0; i<N; i++) send(producer,&m);
while (TRUE) {
receive(producer,&m);
extract_item(&m,&item);
send(producer,&m);
consume_item(item);
}}
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