# Concurrency

Lecture Notes for CS 140
Spring 2019
John Ousterhout

• Readings for this topic from Operating Systems: Principles and Practice: Chapter 5 up through Section 5.1.

## Independent and Cooperating Threads

• Independent thread: one that can't affect or be affected by the rest of the universe.
• Its state isn't shared in any way by any other thread.
• Deterministic: input state alone determines results.
• Reproducible.
• Can stop and continue with no impact on behavior (only time varies).
• For independent threads, the scheduling order doesn't matter.
• Cooperating threads: those that share state.
• Behavior is nondeterministic: depends on relative execution sequence and cannot be predicted in advance.
• Behavior may be irreproducible.
• Example: one thread writes "ABC" to a console window, another writes "CBA" concurrently.
• Why permit threads to cooperate?
• Basic assumption for cooperating threads is that the order of some operations is irrelevant; certain operations are independent of certain other operations. Examples:
• Thread 1: A = 1;
Thread 2: B = 2;
• Thread 1: A = B+1;
Thread 2: B = 2*B;

## Atomic Operations

• Before we can say ANYTHING about cooperating threads, we must know that some operation is atomic: it either happens in its entirety without interruption, or not at all. Cannot be interrupted in the middle.
• References and assignments are atomic in almost all systems. A=B will always read a clean value for B and set a clean value for A (but not necessarily true for arrays or records).
• If you don't have an atomic operation, you can't make one. Fortunately, hardware designers give us atomic ops.
• If you have any atomic operation, you can use it to generate higher-level constructs and make parallel programs work correctly. This is the approach we'll take in this class.

## The "Too Much Milk" Problem

• The basic problem:
```          Person A                       Person B
3:00      Look in fridge: no milk
3:05      Leave for store
3:10      Arrive at store                Look in fridge: no milk
3:15      Leave store                    Leave home
3:20      Arrive home, put milk away     Arrive at store
3:25                                     Leave store
3:30                                     Arrive home: too much milk!
```
• What is the correct behavior?
• More definitions:
• Synchronization: using atomic operations to ensure correct operation of cooperating threads.
• Critical section: a section of code, or collection of operations, in which only one thread may be executing at a given time. E.g. shopping.
• Mutual exclusion: mechanisms used to create critical sections.
• Typically, mutual exclusion achieved with a locking mechanism: prevent others from doing something. For example, before shopping, leave a note on the refrigerator: don't shop if there is a note.
• First attempt at computerized milk buying (assume atomic reads and writes):
```1 if (milk == 0) {
2   if (note == 0) {
3     note = 1;
5     note = 0;
6   }
7 }
```
• Second attempt: change meaning of note. A buys if no note, B buys if there is a note.
```Thread A
1 if (note == 0) {
2   if (milk == 0) {
4   }
5   note = 1;
6 }

1 if (note == 1) {
2   if (milk == 0) {
4   }
5   note = 0;
6 }
```
• Third attempt: use separate notes for A and B.
```Thread A
1 noteA = 1;
2 if (noteB == 0) {
3   if (milk == 0) {
5   }
6 }
7 noteA = 0;

1 noteB = 1;
2 if (noteA == 0) {
3   if (milk == 0) {
5   }
6 }
7 noteB = 0;
```
• Fourth attempt: just need a way to decide who will buy milk when both leave notes (somebody has to hang around to make sure that the job gets done):
```Thread B
1 noteB = 1;
2 while (noteA == 1) {
3   // do nothing;
4 }
5 if (milk == 0) {