SLIDE 1
Concurrency
SLIDE 2 Concurrency
Definition: Several scripts are executing simultaneously and potentially interacting with each other
This is how we assign grades! Based on the Birkahni Theorem, we usually get the grades to average to a B+, though due to the size of the class this semester, the average will be a C+ (jk)
SLIDE 3
Race Condition
Concurrency Issue
SLIDE 4 Race Condition
Definition: when events of a program don't happen in the
- rder that the programmer intended.
SLIDE 5 Function Definitions
- read value: reads in a value from user input
- increments value: increments the value, but does not set it
- sets value: sets the value to the incremented version of it.
SLIDE 6
Race Condition Example
We have two programs, program 1 and program 2, and a global variable ‘num’. Ideally, we want the script in program 1 to run before the script in program 2, but this won’t always be the case. We’ll look at two scenarios, the first where they run in order (serial), and the second where they don’t (race condition).
SLIDE 7 Serial - Example
(Global Integer Value)
‘num’ starts out with value 0
SLIDE 8 Serial - Example
(Global Integer Value)
read value (reads the value of ‘num’, and sets ‘temp1’ to that value)
SLIDE 9 Serial - Example
(Global Integer Value)
increments value (increases the value of ‘temp1’ by 1)
SLIDE 10 Serial - Example
(Global Integer Value)
1
sets value (sets ‘num’ to the value of ‘temp1’, which is 1)
SLIDE 11 Serial - Example
(Global Integer Value)
1 1
read value (reads the value of ‘num’, and sets ‘temp2’ to that value)
SLIDE 12 Serial - Example
(Global Integer Value)
1 1 1
increments value (increases the value of ‘temp2’ by 2)
SLIDE 13 Serial - Example
(Global Integer Value)
1 1 1 3
sets value (sets ‘num’ to the value of ‘temp2’, which is 3)
SLIDE 14 Serial - Example
(Global Integer Value)
1 1 1 3
This is the expected output. We’re good here!
SLIDE 15
What if we interleaved the commands?
SLIDE 16 Race Condition - Example
(Global Integer Value)
‘num’ starts out with value 0
SLIDE 17 Race Condition - Example
(Global Integer Value)
read value (reads the value of ‘num’, and sets ‘temp1’ to that value)
SLIDE 18 Race Condition - Example
(Global Integer Value)
read value (reads the value of ‘num’, and sets ‘temp2’ to that value)
SLIDE 19 Race Condition - Example
(Global Integer Value)
increments value (increases the value of ‘temp1’ by 1)
SLIDE 20 Race Condition - Example
(Global Integer Value)
increments value (increases the value of ‘temp2’ by 2)
SLIDE 21 Race Condition - Example
(Global Integer Value)
1
sets value (sets ‘num’ to the value of ‘temp1’, which is 1)
SLIDE 22 Race Condition - Example
(Global Integer Value)
1 2
sets value (sets ‘num’ to the value of ‘temp2’, which is 2)
SLIDE 23 Race Condition - Example
(Global Integer Value)
1 2
This is the NOT the expected output. ‘num’ is only 2!
SLIDE 24
Takeaway
Concurrency is great because it allows for tasks to be broken up and completed almost simultaneously. However, you have to be careful how you break up the tasks so you don’t get erroneous behavior.
SLIDE 25
Race Condition Example from Lecture
SLIDE 26
Winky Face Problem
SLIDE 27
SLIDE 28
SLIDE 29
Deadlock
Concurrency Issue
SLIDE 30
Deadlock
Definition: a situation in which two or more competing actions are each waiting for the other(s) to finish, and thus no one ever finishes.
SLIDE 31
Deadlock - Example
SLIDE 32
Dining Philosopher Problem
SLIDE 33
SLIDE 34
SLIDE 35
Recursion
Base Case(s): Recursive Case(s):
SLIDE 36 Recursion
Base Case(s):
- Simplest form of the problem
Recursive Case(s):
SLIDE 37 Recursion
Base Case(s):
- Simplest form of the problem
Recursive Case(s):
- Divide problem into smaller instances
SLIDE 38 Recursion
Base Case(s):
- Simplest form of the problem
Recursive Case(s):
- Divide problem into smaller instances
- Invoke function (recursively)
SLIDE 39 Recursion
Base Case(s):
- Simplest form of the problem
Recursive Case(s):
- Divide problem into smaller instances
- Invoke function (recursively)
- Work towards base case
SLIDE 40
Ladder Fractal
SLIDE 41
Ladder Fractal Solution
SLIDE 42
Rectangle Fractal
SLIDE 43
Rectangle Fractal Solution
