cs2110Concurrency1.ppt

THREADS & CONCURRENCY
Lecture 23– CS2110 – Fall 2018

Prelim room assignments (not yet posted
elsewhere)
5:30. URHG01: netids lac326 to rzt4
5:30. BKL200: netids sa2229 to zz632
plus all conflicts moving from 7:30 to 5:30 exam
--------------------------------
7:30. URHG01: aa2277 to gyz2
7:30. BKL200: hcm58 to ky356
plus all conflicts moving from 5:30 to 7:30 exam
There will be a homework on concurrency.
2

CPU Central Processing Unit. Simplified
view
The CPU is the part of the
computer that executes
instructions.
Java: x= x + 2;
Suppose variable x is at
Memory location 800,
Instructions at 10
Machine language:
10: load register 1, 800
11: Add register 1, 2
12: Store register 1, 800
3
Basic uniprocessor-CPU computer.
Black lines indicate data flow, red
lines indicate control flow
From wikipedia

Part of Activity Monitor in Gries’s laptop
4
>100 processes are competing for time. Here’s some of them:

Clock rate
 Clock rate “frequency at which CPU is
running”
Higher the clock rate, the faster
instructions
are executed.
 First CPUs: 5-10 Hz
(cycles per second)
 Today MacBook Pro 3.5GHz
 Your OS can control the
clock rate, slow it down when
idle, speed up when more
5

Why multicore?
 Moore’s Law: Computer speeds and memory
densities nearly double each year
6

But a fast computer runs hot
 Power dissipation rises as square of the clock
rate
 Chips were heading toward melting down!
 Put more CPUs on a chip:
with four CPUs on one
chip, even if we run each at
half speed we can perform
more overall computations!
7

Today: Not one CPU but many
Processing Unit is called a core.
 Modern computers have “multiple cores” (processing
units)
 Instead of a single CPU (central processing unit) on
the chip 5-10 common. Intel has prototypes with 80!
 We often run many programs at the same time
 Even with a single core (processing unit), your program
may have more than one thing “to do” at a time
 Argues for having a way to do many things at once
8

Many programs. Each can have several
“threads of execution”
We often run many programs at the same time
And each program may have several “threads of
execution”
9
Example, in a Paint program,
when you click the pencil tool,
a new thread of execution is
started to call the method to
process it:
Main GUI thread
Process pencil
click

Programming a Cluster...
10
• Sometimes you want to write a
program that is executed on
many machines!
• Atlas Cluster (at Cornell):
• 768 cores
• 1536 GB RAM
• 24 TB Storage
• 96 NICs (Network Interface
Controller)

Programming a Cluster...
11
Programs like those that simulate
weather have a big array of
points,
At each step of simulation,
calculates whether at each point
based on info at surrounding
points.
Each is on a separate processor.

Many processes are executed
simultaneously on your computer
12
• Operating system provides support for multiple
“processes”
• Usually fewer processors than processes
• Processes are an abstraction:
at hardware level, lots of multitasking
–memory subsystem
–video controller
–buses
–instruction prefetching

Concurrency
 Concurrency refers to a single program in which several
processes, called threads, are running simultaneously
 Special problems arise
 They reference the same data and can interfere with
each other, e.g. one process modifies a complex
structure like a heap while another is trying to read it
 CS2110: we focus on two main issues:
 Race conditions
 Deadlock
13

Race conditions
 A “race condition” arises if two or more processes
access the same variables or objects concurrently and
at least one does updates
 Example: Processes t1 and t2 x= x + 1; for some
static global x.
Process t1 Process t2
… ...
x= x + 1; x= x + 1;
But x= x+1; is not an “atomic action”: it takes several steps
14

Race conditions
 LOAD x
 ADD 1
 STORE x
 ...
 LOAD x
 ADD 1
 STORE x
Thread t1 Thread t2
15
 Suppose x is initially 5
 ... after finishing, x = 6! We “lost” an update

Race conditions
 Typical race condition: two processes wanting to change
a stack at the same time. Or make conflicting changes
to a database at the same time.
 Race conditions are bad news
 Race conditions can cause many kinds of bugs, not
just the example we see here!
 Common cause for “blue screens”: null pointer
exceptions, damaged data structures
 Concurrency makes proving programs correct much
harder!
16

Deadlock
 To prevent race conditions, one often requires a process
to “acquire” resources before accessing them, and only
one process can “acquire” a given resource at a time.
 Examples of resources are:
 A file to be read
 An object that maintains a stack, a linked list, a hash
table, etc.
 But if processes have to acquire two or more resources
at the same time in order to do their work, deadlock can
occur. This is the subject of the next slides.
17

Dining philosopher problem
18
Five philosophers
sitting at a table.
Each repeatedly
does this:
1. think
2. eat
What do they
eat?
spaghetti.
Need TWO forks
to eat spaghetti!

19
Each does
repeatedly :
1. think
2. eat (2 forks)
eat is then:
pick up left fork
pick up right fork
pick up food, eat
put down left fork
put down rght fork
At one point,
they all pick up
their left forks
DEADLOCK!

20
Simple solution to
deadlock:
Number the forks. Pick
up smaller one first
1. think
2. eat (2 forks)
eat is then:
pick up smaller fork
pick up bigger fork
pick up food, eat
put down bigger fork
put down smallerfork
1
2
4
3
5

Java: What is a Thread?
 A separate “execution” that runs within a single
program and can perform a computational task
independently and concurrently with other threads
 Many applications do their work in just a single thread:
the one that called main() at startup
 But there may still be extra threads...
 ... Garbage collection runs in a “background” thread
 GUIs have a separate thread that listens for events
and “dispatches” calls to methods to process them
 Today: learn to create new threads of our own in Java
21

Thread
 A thread is an object that “independently computes”
 Needs to be created, like any object
 Then “started” --causes some method to be called. It
runs side by side with other threads in the same
program; they see the same global data
 The actual executions could occur on different CPU
cores, but but don’t have to
 We can also simulate threads by multiplexing a
smaller number of cores over a larger number of
threads
22

Java class Thread
 threads are instances of class Thread
 Can create many, but they do consume space & time
 The Java Virtual Machine creates the thread that
executes your main method.
 Threads have a priority
 Higher priority threads are executed preferentially
 By default, newly created threads have initial priority
equal to the thread that created it (but priority can be
changed)
23

Creating a new Thread (Method 1)
24
class PrimeThread extends Thread {
long a, b;
PrimeThread(long a, long b) {
this.a= a; this.b= b;
}
@Override public void run() {
//compute primes between a and b
...
}
}
PrimeThread p= new PrimeThread(143, 195);
p.start();
overrides
Thread.run()
Call run() directly?
no new thread is used:
Calling thread will run it
Do this and
Java invokes run() in new thread

Creating a new Thread (Method 2)
25
class PrimeRun implements Runnable {
long a, b;
PrimeRun(long a, long b) {
this.a= a; this.b= b;
}
public void run() {
//compute primes between a and b
...
}
}
PrimeRun p= new PrimeRun(143, 195);
new Thread(p).start();

Example
26
Thread[Thread-0,5,main] 0
Thread[main,5,main] 0
public class ThreadTest extends Thread {
public static void main(String[] args) {
new ThreadTest().start();
for (int i= 0; i < 10; i++) {
System.out.format("%s %dn",
Thread.currentThread(), i);
}
}
public void run() {
for (int i= 0; i < 10; i++) {
}
}
}
Thread name, priority, thread group

Example
27 Thread[main,5,main] 0
for (int i= 0; i < 10; i++) {
}
}
public void run() {
currentThread().setPriority(4);
for (int i= 0; i < 10; i++) {
}
}
}

Example
28 Thread[main,5,main] 0
for (int i= 0; i < 10; i++) {
}
}
public void run() {
currentThread().setPriority(6);
for (int i= 0; i < 10; i++) {
}}}

Example
29
waiting...
running...
waiting...
running...
waiting...
running...
waiting...
running...
waiting...
running...
waiting...
running...
waiting...
running...
waiting...
running...
waiting...
running...
waiting...
running...
done
static boolean ok = true;
for (int i = 0; i < 10; i++) {
System.out.println("waiting...");
yield();
}
ok = false;
}
public void run() {
while (ok) {
System.out.println("running...");
yield();
}
System.out.println("done");
}
}
If threads happen to be sharing
a CPU, yield allows other waiting
threads to run.

Terminating Threads is tricky
 Easily done... but only in certain ways
 Safe way to terminate a thread: return from method
run
 Thread throws uncaught exception? whole program
will be halted (but it can take a second or two ... )
 Some old APIs have issues: stop(), interrupt(),
suspend(),
destroy(), etc.
 Issue: Can easily leave application in a “broken”
internal state.
 Many applications have some kind of variable telling
the thread to stop itself.
30

Threads can pause
 When active, a thread is “runnable”.
 It may not actually be “running”. For that, a CPU
must schedule it. Higher priority threads could run
first.
 A thread can pause
 Call Thread.sleep(k) to sleep for k milliseconds
 Doing I/O (e.g. read file, wait for mouse input, open
file) can cause thread to pause
 Java has a form of locks associated with objects.
When threads lock an object, one succeeds at a time.
31

Background (daemon) Threads
 In many applications we have a notion of “foreground”
and “background” (daemon) threads
 Foreground threads are doing visible work, like
interacting with the user or updating the display
 Background threads do things like maintaining data
structures (rebalancing trees, garbage collection, etc.)
 On your computer, the same notion of background
workers explains why so many things are always
running in the task manager.
32

Fancier forms of locking
 Java developers have created various synchronization
abstract data types
 Semaphores: a kind of synchronized counter
(invented by Dijkstra)
 Event-driven synchronization
 The Windows and Linux and Apple O/S have kernel
locking features, like file locking
 But for Java, synchronized is the core mechanism
33

Summary
 Use of multiple processes and multiple threads within
each process can exploit concurrency
 Which may be real (multicore) or “virtual” (an
illusion)
 When using threads, beware!
 Synchronize any shared memory to avoid race
conditions
 Synchronize objects in certain order to avoid
deadlocks
 Even with proper synchronization, concurrent
programs can have other problems such as
“livelock”
 Serious treatment of concurrency is a complex topic
34

cs2110Concurrency1.ppt

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