Concurrency is a fundamental concept in computer science that allows multiple processes to execute simultaneously, improving the efficiency and performance of programs. In this section, we will explore how concurrency is handled in ALGOL, including key concepts, practical examples, and exercises to solidify your understanding.
Key Concepts
- Concurrency: The ability of a program to execute multiple parts simultaneously.
- Processes: Independent units of execution within a program.
- Synchronization: Mechanisms to control the order of execution among concurrent processes.
- Mutual Exclusion: Ensuring that multiple processes do not access shared resources simultaneously in a conflicting manner.
- Deadlock: A situation where two or more processes are unable to proceed because each is waiting for the other to release resources.
Concurrency in ALGOL
ALGOL, being one of the earlier programming languages, has limited built-in support for concurrency. However, it provides mechanisms to implement basic concurrent processes and synchronization.
Example: Basic Concurrent Processes
Let's start with a simple example of creating two concurrent processes in ALGOL.
begin
integer i, j;
process P1;
begin
for i := 1 step 1 until 10 do
print(i);
end;
process P2;
begin
for j := 1 step 1 until 10 do
print(j);
end;
activate P1;
activate P2;
end;Explanation
- Processes P1 and P2: Defined using the
processkeyword. - Activation: The
activatestatement is used to start the execution of the processes. - Output: The output will interleave the values of
iandjas both processes run concurrently.
Synchronization Example
To avoid conflicts when accessing shared resources, we need synchronization mechanisms. Here’s an example using a simple lock mechanism.
begin
integer i, j;
boolean lock;
lock := false;
process P1;
begin
for i := 1 step 1 until 10 do
begin
while lock do skip;
lock := true;
print(i);
lock := false;
end;
end;
process P2;
begin
for j := 1 step 1 until 10 do
begin
while lock do skip;
lock := true;
print(j);
lock := false;
end;
end;
activate P1;
activate P2;
end;Explanation
- Lock Variable: A boolean variable
lockis used to control access to the shared resource (in this case, the print statement). - While Loop: Each process waits (
while lock do skip) until the lock is released. - Locking and Unlocking: The lock is set to
truebefore accessing the shared resource and set tofalseafter the access is complete.
Practical Exercises
Exercise 1: Implementing a Counter
Create a program with two processes that increment a shared counter variable. Ensure that the counter is updated correctly using synchronization.
begin
integer counter;
boolean lock;
counter := 0;
lock := false;
process Incrementer1;
begin
for i := 1 step 1 until 5 do
begin
while lock do skip;
lock := true;
counter := counter + 1;
print(counter);
lock := false;
end;
end;
process Incrementer2;
begin
for j := 1 step 1 until 5 do
begin
while lock do skip;
lock := true;
counter := counter + 1;
print(counter);
lock := false;
end;
end;
activate Incrementer1;
activate Incrementer2;
end;Solution Explanation
- Shared Counter: The
countervariable is shared between the two processes. - Synchronization: The
lockvariable ensures that only one process can update the counter at a time.
Exercise 2: Avoiding Deadlock
Modify the previous example to avoid potential deadlock situations by ensuring that the processes do not wait indefinitely.
begin
integer counter;
boolean lock;
counter := 0;
lock := false;
process Incrementer1;
begin
for i := 1 step 1 until 5 do
begin
while lock do skip;
lock := true;
counter := counter + 1;
print(counter);
lock := false;
end;
end;
process Incrementer2;
begin
for j := 1 step 1 until 5 do
begin
while lock do skip;
lock := true;
counter := counter + 1;
print(counter);
lock := false;
end;
end;
activate Incrementer1;
activate Incrementer2;
end;Solution Explanation
- Avoiding Deadlock: The same locking mechanism is used, but care is taken to ensure that the lock is always released, preventing deadlock.
Common Mistakes and Tips
- Forgetting to Release Locks: Always ensure that locks are released after the critical section to avoid deadlocks.
- Improper Synchronization: Ensure that all access to shared resources is properly synchronized to avoid race conditions.
- Testing Concurrent Programs: Thoroughly test concurrent programs as they can have non-deterministic behavior, making bugs harder to reproduce and fix.
Conclusion
In this section, we explored the basics of concurrency in ALGOL, including creating concurrent processes and implementing synchronization mechanisms. By understanding these concepts, you can write more efficient and robust programs that can handle multiple tasks simultaneously. In the next module, we will delve into file handling in ALGOL, which will further enhance your programming skills.