Concurrency

Actor Model

Is a computational model that describes everything as an Actor. These actors have 3 possible tasks:

1. Create another Actor
2. Send messages to other Actors
3. Determine the behavior/state for the next message it receives.

Some applications

• Email with accounts, messages and addresses.
• REST APIs, with endpoints working as addresses.

Interrupt Handlers

Are one of the first concurrent programs ¹ . At the lowest level, these interrupts come in the form of hardware signals with high-priority. This interaction of the hardware telling something to the CPU, is what we refer to as interrupts².

The implemented interrupt handler then requires a specific interrupt flag to react accordingly.

Mutual Exclusion Solutions

First of all, a mutex is just a programming concept referring to how a thread, and only that thread, has access to a specific set of resources at a time.

• Dekker’s Algorithm ³

 1begin integer c1, c2 turn;
 2  c1:= 1; c2:= 1; turn = 1;
 3  parbegin
 4  process 1: begin A1: c1:= 0;
 5				   L1: if c2 = 0 then
 6						   begin if turn = 1 then goto L1;
 7								 c1:= 1;
 8							 B1: if turn = 2 then goto B1;
 9								 goto A1
10						   end;
11					   critical section 1;
12					   turn:= 2; c1:= 1;
13					   remainder of cycle 1; goto A1
14			 end;
15  process 2: begin A2: c2:= 0;
16				   L2: if c1 = 0 then
17						  begin if turn = 2 then goto L2;
18								c2:= 1;
19							B2: if turn = 1 then goto B2;
20								goto A2
21						  end;
22					   critical section 2;
23					   turn:= 1; c2:= 1;
24					   remainder of cycle 2; goto A2
25			 end
26  parend
27end .

• Peterson’s Algorithm⁴

 1bool flag[2] = {false, false};
 2int turn;
 3
 4P0:      flag[0] = true;
 5P0_gate: turn = 1;
 6         while (flag[1] && turn == 1)
 7         {
 8             // busy wait
 9         }
10         // critical section
11         ...
12         // end of critical section
13         flag[0] = false;
14
15P1:      flag[1] = true;
16P1_gate: turn = 0;
17         while (flag[0] && turn == 0)
18         {
19             // busy wait
20         }
21         // critical section
22         ...
23         // end of critical section
24         flag[1] = false;

Types of Mutex Devices

Semaphores

Semaphores “hold” the resources available and lends them to threads so they can access them for a limited period of time, while other threads wait for their required resources to be available ⁵.

A good read about semaphores is Julie Zelenski’s “Threads and Semaphores Example”⁶, outlining different examples on how semaphores are used, and the separation between a general and a binary semaphore.