Introduction to Concurrent Programming
Software & Hardware Basics
Slides by Ofer Givoli
Based on:
The art of multiprocessor programming
Maurice Herlihy and Nir Shavit, 2008
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Appendix A – Software Basics
Appendix B – Hardware Basics
1
Software Basics
2
Threads in Java
• Executes a single, sequential program
• Subclass of: java.lang.Thread
3
…
Taken from: The art of multiprocessor programming, by Maurice Herlihy and Nir Shavit, 2008 (modified)
4
Monitors
• lock + waiting set
• every object is a monitor
• Critical section: using the synchronized keyword.
• Waiting: using the wait()method
• Waking-up waiting threads, using the methods:
• notify()
• notifyAll()
5
public class ConcurrentStack<T> {
private Stack<T> innerStack = new Stack<T>();
public void push(T obj) {
innerStack.push(obj);
}
public T pop() {
return innerStack.pop();
}
}
ConcurrentStack<Integer> s = ...
s.push(1);

... = s.pop();
... = s.pop();
6
public class ConcurrentStack<T> {
private Stack<T> innerStack = new Stack<T>();
private Object monitor = new Object();
public void push(T obj) {
synchronized(monitor) {
innerStack.push(obj);
}
}
public T pop() {
synchronized(monitor) {
return innerStack.pop();
}
}
}
... = s.pop();
... = s.pop();
BLOCKED
7
public class ConcurrentStack<T> {
private Stack<T> innerStack = new
Stack<T>();
private Object monitor = new Object();
public void push(T obj) {
synchronized(monitor) {
innerStack.push(obj);
}
}
public T pop() {
synchronized(monitor) {
return innerStack.pop();
}
}
}
8
public class ConcurrentStack<T> {
private Stack<T> innerStack = new Stack<T>();
public void push(T obj) {
synchronized(this) {
innerStack.push(obj);
}
}
public T pop() {
synchronized(this) {
return innerStack.pop();
}
}
}
9
public class ConcurrentStack<T> {
private Stack<T> innerStack = new Stack<T>();
public synchronized void push(T obj) {
innerStack.push(obj);
}
public synchronized T pop() {
return innerStack.pop();
}
}
New feature: waiting for pop()
10
public class ConcurrentStack<T> {
private Stack<T> innerStack = new Stack<T>();
public synchronized void push(T obj) {
innerStack.push(obj);
}
public synchronized T pop() {
while (innerStack.empty()) {}
return innerStack.pop();
}
}
Problem?
11
public class ConcurrentStack<T> {
private Stack<T> innerStack = new Stack<T>();
public synchronized void push(T obj) {
innerStack.push(obj);
}
public synchronized T pop() {
while (innerStack.empty()) {}
return innerStack.pop();
}
}
deadlock

s.push(1);
... = s.pop();
BLOCKED
12
public class ConcurrentStack<T> {
private Stack<T> innerStack = new Stack<T>();
public synchronized void push(T obj) {
innerStack.push(obj);
}
public synchronized T pop() {
while (innerStack.empty()) {}
return innerStack.pop();
}
}
13
public class ConcurrentStack<T> {
private Stack<T> innerStack = new
Stack<T>();
public synchronized void push(T obj) {
if (innerStack.empty())
notifyAll();
innerStack.push(obj);
}
public synchronized T pop() {
while (innerStack.empty()) {wait();}
return innerStack.pop();
}
}
14
public class ConcurrentStack<T> {
private Stack<T> innerStack = new Stack<T>();
public synchronized void push(T obj) {
if (innerStack.empty())
notifyAll();
innerStack.push(obj);
}
public synchronized T pop() {
while (innerStack.empty()) {wait();}
return innerStack.pop();
}
}
... = s.pop();
s.push(1);
BLOCKED
WAITING
BLOCKED
15
public class ConcurrentStack<T> {
private Stack<T> innerStack = new Stack<T>();
public synchronized void push(T obj) {
if (innerStack.empty())
notify();
innerStack.push(obj);
}
public synchronized T pop() {
while (innerStack.empty()) {wait();}
return innerStack.pop();
}
}
lost wakeup

... = s.pop();
... = s.pop();
WAITING
WAITING
16
s.push(1);
s.push(2);
Thread.sleep(t);
Thread.yield();
17
Thread-Local Objects
class ThreadLocallD extends ThreadLocal<Integer> {
protected Integer initialValue() {
return …;
}
}
ThreadLocallD id = …;
id.set(…);
…
… = id.get();
id.set(…);
…
… = id.get();
18
• Synchronization in C#
• Pthreads
19
Hardware Basics
20
Taken from: https://software.intel.com/en-us/articles/optimizing-applications-for-numa
21
CPU
Speed:
Size:
Cost:
Power:
L1
Cache
L2
Cache
Fastest
Smallest
Highest
Highest
L3
Cache
Memory
(DRAM)
Slowest
Biggest
Lowest
Lowest
Taken from: Computer Structure 2014 slides, by Lihu Rappoport and Adi Yoaz (modified)
22
Processor 1
Processor 2
L1 cache
L1 cache
L2 cache (shared)
Memory
Taken from: Computer Structure 2014 slides, by Lihu Rappoport and Adi Yoaz
23
SMP (symmetric multiprocessing)
not scalable
NUMA (Non-uniform memory access)
Taken from: The art of multiprocessor programming, by Maurice Herlihy and Nir Shavit, 2008 (modified)
24
Cache Coherence
Cache-line states:
• Modified
• Exclusive
• Shared
• Invalid
false sharing
Taken from: The art of multiprocessor programming, by Maurice Herlihy and Nir Shavit, 2008 (modified)
25
Spinning
SMP
NUMA
Taken from: The art of multiprocessor programming, by Maurice Herlihy and Nir Shavit, 2008 (modified)
26
Multi-Core and Multi-Threaded Architectures
Taken from: The art of multiprocessor programming, by Maurice Herlihy and Nir Shavit, 2008 (modified)
• Execute instructions out-of-order/in parallel/speculatively.
• write buffer
• reordering of reads-writes by compiler
• memory barrier instruction (expensive)
• reads-writes reorder in Java
• Volatile variables in Java
27
Hardware Synchronization Instructions
• compare-and-swap/set (CAS)
• load-linked & store-conditional (LL/SC)
28
Thanks!
29