Introduction to Monitors

Monitors :
- Consist of private data and operations on that data.
- It can contain types, constants, variables and procedures.
- Only the procedures explicitly marked can be seen outside the monitor.
- The monitor body allows the private data to be initialized.
- The compiler enforces mutual exclusion on a particular monitor.
- Each monitor has a boundary queue, and processes wanting to call a monitor routine
join this queue if the monitor is already in use.
- Monitors are an improvement over conditional critical regions because all the code
that accesses the shared data is localized.

To allow a process to wait within the monitor, a condition variable must be declared, as: condition x,y.
Condition variable can only be used with the operations wait and signal.
The operation x.wait()means that the process invoking this operation is suspended until another process invokes. Also called delay function.
The operation x.signal()resumes exactly one suspended process. If no process is suspended, then the signal operation has no effect.Also called resume function.

Monitor Implementation using Semaphores
- 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--;
- The operation x.signal can be implemented as:
if (x-count > 0)
{
next-count++;
signal(x-sem);
wait(next);
next-count--;
}

Critical Regions

A critical region is a section of code that is always executed under mutual exclusion. Critical regions shift the responsibility for enforcing mutual exclusion from the programmer(where it resides when semaphores are used) to the compiler.
They consist of two parts:
- Variables that must be accessed under mutual exclusion.
- A new language statement that identifies a critical region in which the variables
are accessed.
var
v : shared T;
...
region v do
begin
...
end;
All critical regions that are ‘tagged’ with the same variable have compiler-enforced mutual exclusion so that only one of them can be executed at a time:
Process A:
region V1 do
begin
{Do some stuff.}
end;
region V2 do
begin
{Do more stuff.}
end;

Process B:
region V1 do
begin
{Do other stuff.}
end;
Here process A can be executing inside its V2 region while process Bis executing inside its V1 region, but if they both want to execute inside their respective V1 regions only one will be permitted to proceed.

Semaphores

Semaphores can best be described as counters used to control access to shared resources by multiple processes. They are most often used as a locking mechanism to prevent processes from accessing a particular resource while another process is performing operations on it.
A semaphore is a protected variable or abstract data type which constitutes the classic method for restricting access to shared resources such as shared memory in a parallel programming environment. A counting semaphore is a counter for a set of available resources, rather than a locked/unlocked flag of a single resource.
A semaphore S is an integer variable that, apart from initialization, is accessed only through two standard atomic operations : wait and signal. These operations were originally termed P(for wait) and V(for signal). The classical definitions of wait and signal are :
wait(S) : while S<=0 do no-op;
S=S-1;

signal(S) : S:= S+1;

Modifications to the integer value of the semaphore in the wait and signal operations must be executed indivisibly.
- Semaphores can be used to deal with the n-process critical- section problem.
- Semaphores can be used to solve various synchronization problems.

A mutex is a binary semaphore (a semaphore with capacity of 1), usually including extra features like ownership, priority inversion protection or recursivity. The differences between mutexes and semaphores are operating system dependent, though mutexes are implemented by specialized and faster routines. Mutexes are meant to be used for mutual exclusion (post/release operation is restricted to thread which called pend/acquire) only and binary semaphores are meant to be used for event notification (post-ability from any thread) and mutual exclusion.