Process Synchronization
Process Synchronization
Background
The Critical-Section Problem
Peterson’s Solution
Synchronization Hardware
Mutex Locks
Semaphores
Classic Problems of Synchronization
Monitors
Background
Processes can execute concurrently
May be interrupted at any time, partially completing
execution
Concurrent access to shared data may result in data
inconsistency
Maintaining data consistency requires mechanisms to ensure
the orderly execution of cooperating processes
Illustration of the problem:
Suppose that we wanted to provide a solution to the
consumer-producer problem that fills all the buffers. We can
do so by having an integer counter that keeps track of the
number of full buffers. Initially, counter is set to 0. It is
incremented by the producer after it produces a new buffer
and is decremented by the consumer after it consumes a
buffer.
Producer
while (true) {
/* produce an item in next produced */
while (counter == BUFFER_SIZE) ;
/* do nothing */
buffer[in] = next_produced;
in = (in + 1) % BUFFER_SIZE;
counter++;
}
Consumer
while (true) {
while (counter == 0)
; /* do nothing */
next_consumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
counter--;
/* consume the item in next consumed */
}
Race Condition
counter++ could be implemented as
register1 = counter
register1 = register1 + 1
counter = register1
counter-- could be implemented as
register2 = counter
register2 = register2 - 1
counter = register2
Consider this execution interleaving with “count = 5” initially:
S0: producer execute register1 = counter {register1 = 5}
S1: producer execute register1 = register1 + 1 {register1 = 6}
S2: consumer execute register2 = counter {register2 = 5}
S3: consumer execute register2 = register2 – 1 {register2 = 4}
S4: producer execute counter = register1 {counter = 6 }
S5: consumer execute counter = register2 {counter = 4}
Critical Section Problem
Consider system of n processes {p0, p1, … pn-1}
Each process has critical section segment of code
Process may be changing common variables, updating table, writing file,
etc
When one process in critical section, no other may be in its critical
section
Critical section problem is to design protocol to solve this
Each process must ask permission to enter critical section in entry section,
may follow critical section with exit section, then remainder section
General structure of process Pi Algorithm for process Pi
do {
while (turn == j);
critical section
turn = j;
remainder section
} while (true);
Solution to Critical-Section Problem
1. Mutual Exclusion - If process Pi is executing in its critical section, then no
other processes can be executing in their critical sections
2. Progress - If no process is executing in its critical section and there exist some
processes that wish to enter their critical section, then the selection of the
processes that will enter the critical section next cannot be postponed
indefinitely
3. Bounded Waiting - A bound must exist on the number of times that other
processes are allowed to enter their critical sections after a process has made
a request to enter its critical section and before that request is granted
Assume that each process executes at a nonzero speed
No assumption concerning relative speed of the n processes
Critical-Section Handling in OS
Two approaches depending on if kernel is preemptive or non- preemptive
Preemptive – allows preemption of process when running in kernel mode
Non-preemptive – runs until exits kernel mode, blocks, or voluntarily yields
CPU
Essentially free of race conditions in kernel mode
Peterson’s Solution
Good algorithmic description of solving the problem
Two process solution
Assume that the load and store machine-language instructions are atomic;
that is, cannot be interrupted
The two processes share two variables:
int turn;
Boolean flag[2]
The variable turn indicates whose turn it is to enter the critical section
The flag array is used to indicate if a process is ready to enter the critical
section. flag[i] = true implies that process Pi is ready!
do {
Algorithm for Process Pi
flag[i] = true;
Provable that the three CS requirement are met:
turn = j;
1. Mutual exclusion is preserved
while (flag[j] && turn = = j);
Pi enters CS only if:
either flag[j] = false or turn = i critical section
2. Progress requirement is satisfied flag[i] = false;
3. Bounded-waiting requirement is met remainder section
} while (true);
Synchronization Hardware
Many systems provide hardware support for implementing the critical section
code.
All solutions below based on idea of locking
Protecting critical regions via locks
Uniprocessors – could disable interrupts
Currently running code would execute without preemption
Generally too inefficient on multiprocessor systems
Operating systems using this not broadly scalable
Modern machines provide special atomic hardware instructions
Atomic = non-interruptible
Either test memory word and set value
Or swap contents of two memory words
do {
acquire lock
critical section
Solution to Critical-section release lock
Problem Using Locks remainder section
} while (TRUE);
Mutex Locks
Previous solutions are complicated and generally inaccessible
to application programmers
OS designers build software tools to solve critical section
problem
Simplest is mutex lock
Protect a critical section by first acquire() a lock then
release() the lock
Boolean variable indicating if lock is available or not
Calls to acquire() and release() must be atomic
Usually implemented via hardware atomic instructions
But this solution requires busy waiting
This lock therefore called a spinlock
acquire() and release()
acquire() {
while (!available)
; /* busy wait */
available = false;
}
release() {
available = true;
}
do {
acquire lock
critical section
release lock
remainder section
} while (true);
Semaphore
Synchronization tool that provides more sophisticated ways (than Mutex locks)
for process to synchronize their activities.
Semaphore S – integer variable
Can only be accessed via two indivisible (atomic) operations
wait() and signal()
Originally called P() and V()
Definition of the wait() operation
wait(S) {
while (S <= 0)
; // busy wait
S--;
}
Definition of the signal() operation
signal(S) {
S++;
}
Semaphore Usage
Counting semaphore – integer value can range over an unrestricted
domain
Binary semaphore – integer value can range only between 0 and 1
Same as a mutex lock
Can solve various synchronization problems
Consider P1 and P2 that require S1 to happen before S2
Create a semaphore “synch” initialized to 0
P1:
S1;
signal(synch);
P2:
wait(synch);
S2;
Can implement a counting semaphore S as a binary semaphore
Semaphore Implementation
Must guarantee that no two processes can execute the wait()
and signal() on the same semaphore at the same time
Thus, the implementation becomes the critical section problem
where the wait and signal code are placed in the critical
section
Could now have busy waiting in critical section
implementation
But implementation code is short
Little busy waiting if critical section rarely occupied
Note that applications may spend lots of time in critical sections
and therefore this is not a good solution
Semaphore Implementation with no Busy waiting
With each semaphore there is an associated waiting queue
Each entry in a waiting queue has two data items:
value (of type integer)
pointer to next record in the list
Two operations:
block – place the process invoking the operation on the
appropriate waiting queue
wakeup – remove one of processes in the waiting queue
and place it in the ready queue
typedef struct{
int value;
struct process *list;
} semaphore;
Implementation with no Busy waiting (Cont.)
wait(semaphore *S) {
S->value--;
if (S->value < 0) {
add this process to S->list;
block();
}
}
signal(semaphore *S) {
S->value++;
if (S->value <= 0) {
remove a process P from S->list;
wakeup(P);
}
}
Deadlock and Starvation
Deadlock – two or more processes are waiting indefinitely for an
event that can be caused by only one of the waiting processes
Let S and Q be two semaphores initialized to 1
P0 P1
wait(S); wait(Q);
wait(Q); wait(S);
... ...
signal(S); signal(Q);
signal(Q); signal(S);
Starvation – indefinite blocking
A process may never be removed from the semaphore queue in which it is
suspended
Priority Inversion – Scheduling problem when lower-priority process
holds a lock needed by higher-priority process
Solved via priority-inheritance protocol
Classical Problems of Synchronization
Classical problems used to test newly-proposed synchronization
schemes
Bounded-Buffer Problem
Readers and Writers Problem
Dining-Philosophers Problem
Bounded-Buffer Problem
n buffers, each can hold one item
Semaphore mutex initialized to the value 1
Semaphore full initialized to the value 0
Semaphore empty initialized to the value n
Bounded Buffer Problem (Cont.)
The structure of the producer process
do {
...
/* produce an item in next_produced */
...
wait(empty);
wait(mutex);
...
/* add next produced to the buffer */
...
signal(mutex);
signal(full);
} while (true);
Bounded Buffer Problem (Cont.)
The structure of the consumer process
Do {
wait(full);
wait(mutex);
...
/* remove an item from buffer to next_consumed */
...
signal(mutex);
signal(empty);
...
/* consume the item in next consumed */
...
} while (true);
Readers-Writers Problem
A data set is shared among a number of concurrent processes
Readers – only read the data set; they do not perform any updates
Writers – can both read and write
Problem – allow multiple readers to read at the same time
Only one single writer can access the shared data at the same time
Several variations of how readers and writers are considered – all
involve some form of priorities
Shared Data
Data set
Semaphore rw_mutex initialized to 1
Semaphore mutex initialized to 1
Integer read_count initialized to 0
Readers-Writers Problem (Cont.)
The structure of a writer process
do {
wait(rw_mutex);
...
/* writing is performed */
...
signal(rw_mutex);
} while (true);
Readers-Writers Problem (Cont.)
The structure of a reader process
do {
wait(mutex);
read_count++;
if (read_count == 1)
wait(rw_mutex);
signal(mutex);
...
/* reading is performed */
...
wait(mutex);
read count--;
if (read_count == 0)
signal(rw_mutex);
signal(mutex);
} while (true);
Readers-Writers Problem Variations
First variation – no reader kept waiting unless writer has
permission to use shared object
Second variation – once writer is ready, it performs the
write ASAP
Both may have starvation leading to even more variations
Problem is solved on some systems by kernel providing
reader-writer locks
Dining-Philosophers Problem
Philosophers spend their lives alternating thinking and eating
Don’t interact with their neighbors, occasionally try to pick up 2
chopsticks (one at a time) to eat from bowl
Need both to eat, then release both when done
In the case of 5 philosophers
Shared data
Bowl of rice (data set)
Semaphore chopstick [5] initialized to 1
Dining-Philosophers Problem Algorithm
The structure of Philosopher i:
do {
wait (chopstick[i] );
wait (chopStick[ (i + 1) % 5] );
// eat
signal (chopstick[i] );
signal (chopstick[ (i + 1) % 5] );
// think
} while (TRUE);
What is the problem with this algorithm?
Dining-Philosophers Problem Algorithm (Cont.)
Deadlock handling
Allow at most 4 philosophers to be sitting
simultaneously at the table.
Allow a philosopher to pick up the forks only if both
are available (picking must be done in a critical
section.
Use an asymmetric solution -- an odd-numbered
philosopher picks up first the left chopstick and then
the right chopstick. Even-numbered philosopher picks
up first the right chopstick and then the left chopstick.
Problems with Semaphores
Incorrect use of semaphore operations:
signal (mutex) …. wait (mutex)
wait (mutex) … wait (mutex)
Omitting of wait (mutex) or signal (mutex) (or both)
Deadlock and starvation are possible.
Monitors
A high-level abstraction that provides a convenient and effective
mechanism for process synchronization
Abstract data type, internal variables only accessible by code within the
procedure
Only one process may be active within the monitor at a time
But not powerful enough to model some synchronization schemes
monitor monitor-name
{
// shared variable declarations
procedure P1 (…) { …. }
procedure Pn (…) {……}
Initialization code (…) { … }
}
}
Schematic view of a Monitor
Condition Variables
condition x, y;
Two operations are allowed on a condition variable:
x.wait() – a process that invokes the operation is
suspended until x.signal()
x.signal() – resumes one of processes (if any) that
invoked x.wait()
If no x.wait() on the variable, then it has no effect on
the variable
Monitor with Condition Variables
Condition Variables Choices
If process P invokes x.signal(), and process Q is suspended in
x.wait(), what should happen next?
Both Q and P cannot execute in paralel. If Q is resumed, then P
must wait
Options include
Signal and wait – P waits until Q either leaves the monitor or it
waits for another condition
Signal and continue – Q waits until P either leaves the monitor or it
waits for another condition
Both have pros and cons – language implementer can decide
Monitors implemented in Concurrent Pascal compromise
P executing signal immediately leaves the monitor, Q is
resumed
Implemented in other languages including Mesa, C#, Java
Monitor Solution to Dining Philosophers
monitor DiningPhilosophers
{
enum { THINKING; HUNGRY, EATING) state [5] ;
condition self [5];
void pickup (int i) {
state[i] = HUNGRY;
test(i);
if (state[i] != EATING) self[i].wait;
}
void putdown (int i) {
state[i] = THINKING;
// test left and right neighbors
test((i + 4) % 5);
test((i + 1) % 5);
}
Solution to Dining Philosophers (Cont.)
void test (int i) {
if ((state[(i + 4) % 5] != EATING) &&
(state[i] == HUNGRY) &&
(state[(i + 1) % 5] != EATING) ) {
state[i] = EATING ;
self[i].signal () ;
}
}
initialization_code() {
for (int i = 0; i < 5; i++)
state[i] = THINKING;
}
}
Solution to Dining Philosophers (Cont.)
Each philosopher i invokes the operations pickup() and
putdown() in the following sequence:
DiningPhilosophers.pickup(i);
EAT
DiningPhilosophers.putdown(i);
No deadlock, but starvation is possible
Monitor Implementation – Condition Variables
For each condition variable x, we have:
semaphore x_sem; // (initially = 0)
int x_count = 0;
The operation x.wait can be implemented as:
x_count++;
if (next_count > 0)
signal(next);
else
signal(mutex);
wait(x_sem);
x_count--;
Monitor Implementation (Cont.)
The operation x.signal can be implemented as:
if (x_count > 0) {
next_count++;
signal(x_sem);
wait(next);
next_count--;
}
Resuming Processes within a Monitor
If several processes queued on condition x, and x.signal()
executed, which should be resumed?
FCFS frequently not adequate
conditional-wait construct of the form x.wait(c)
Where c is priority number
Process with lowest number (highest priority) is
scheduled next
Single Resource allocation
Allocate a single resource among competing processes using
priority numbers that specify the maximum time a process
plans to use the resource
R.acquire(t);
...
access the resurce;
...
R.release;
Where R is an instance of type ResourceAllocator
A Monitor to Allocate Single Resource
monitor ResourceAllocator
{
boolean busy;
condition x;
void acquire(int time) {
if (busy)
x.wait(time);
busy = TRUE;
}
void release() {
busy = FALSE;
x.signal();
}
initialization code() {
busy = FALSE;
}
}
End of Chapter 5
