Transcript Slides

Lecture 15:
Concurring
Concurrently
CS201j: Engineering Software
University of Virginia
Computer Science
David Evans
http://www.cs.virginia.edu/evans
Our computer can only do one
instruction at a time, why would
we want to program pretending
it can do many things at once?
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Concurrent Programming
• Some problems are clearer to program
concurrently:
– Modularity
• Don’t have to explicitly interleave code for different
abstractions (especially: user interfaces)
• High-level interactions – synchronization,
communication
– Modeling
• Closer map to real world problems: things in the
real world aren’t sequential
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Concurrency in Java
public class Thread implements Runnable {
// OVERVIEW: A thread is a thread of execution in a program.
// The Java Virtual Machine allows an application to have
// multiple threads of execution running concurrently.
public Thread (Runnable target)
// Creates a new Thread object that will run the target.
public void start ()
// Starts a new thread of execution.
… many other methods
}
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Making a
Thread
public class Thread implements Runnable {
public Thread (Runnable target)
public void start ()
… many other methods
}
// from PS5 SimObject class:
final public void init(int x, int y, Grid grid)
// REQUIRES: init has not previously been called for this.
// The cell is at (row, col) on grid.
// MODIFIES: this
// EFFECTS: Initializes the cell at row, col on grid with isPaused = true.
// NOTE: This method is final, that means subtypes cannot override this method.
{
this.mx = x; this.my = y; this.grid = grid; this.isPaused = true;
Thread thread = new Thread(this);
thread.setPriority(Thread.MIN_PRIORITY);
thread.start();
//@set isInitialized = true;
}
What do you know about SimObject type?
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Runnable
public interface Runnable {
public void run()
When an object implementing interface Runnable is
used to create a thread, starting the thread causes the
object's run method to be called in that separately
executing thread. The general contract of the
method run is that it may take any action
whatsoever.
}
So, to be a subtype of Runnable, SimObject must have a method
void run () with no preconditions and any postconditions it wants.
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Making a Runnable
abstract public class SimObject implements Runnable {
…
public void run ()
// EFFECTS: Executes one turn by calling the
// executeTurn method, and sleeps for a time
// and repeats.
{
while (true) {
executeTurn ();
delay (TURN_DELAY + random.nextInt(TURN_RANDOM));
}
}
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Actually…
abstract public class SimObject implements Runnable {
…
public void run ()
// REQUIRES: this has been initialized
//@also_requires isInitialized
// EFFECTS: Executes one turn by calling the
// executeTurn method, and sleeps for a time
// and repeats.
{
We are violating the substitution principle!
…
SimObject.run() has a stronger precondition
}
than Runnable.run().
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Concurrency
• Making a concurrent Java program is
easy:
Make a subtype R of Runnable
new Thread (new R ()).start ()
• Making a concurrent Java program that
behaves correctly is really, really hard!
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Scheduling Meetings
• Alice wants to schedule a meeting with
Bob and Colleen
Bob
“When can
you meet
Friday?”
Alice
“When can
you meet
Friday?”
Colleen
“11am or 3pm”
“9am or 11am”
Picks meeting
time
Reserves 11am
for meeting
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“Let’s meet
at 11am”
“Let’s meet
at 11am”
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Reserves 11am
for meeting
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Partial Ordering of Events
• Sequential programs give use a total
ordering of events: everything happens in
a determined order
• Concurrency gives us a partial ordering of
events: we know some things happen
before other things, but not total order
Alice asks to schedule meeting before Bob replies
Alice asks to schedule meeting before Colleen replies
Bob and Colleen both reply before Alice picks meeting time
Alice picks meeting time before Bob reserves time on calendar
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Race Condition
Bob
Doug
“When can
you meet
Friday?”
“When can
you meet
Friday?”
Alice
“When can
you meet
Friday?”
Colleen
“9, 11am or 3pm”
“9am or 11am”
“9, 11am or 3pm”
“Let’s meet
at 11am”
Reserves 11am
for Doug
“Let’s meet
at 11am”
Picks meeting
time
“Let’s meet
at 11am”
“I’m busy
then…”
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Preventing Race Conditions
• Use locks to impose ordering constraints
• After responding to Alice, Bob reserves all
the times in his response until he hears
back (and then frees the other times)
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Locking
Doug
“When can
you meet
Friday?”
Bob
“When can
you meet
Friday?”
Alice
“When can
you meet
Friday?”
Colleen
“9, 11am or 3pm”
“9am or 11am”
Locks calendar
“3pm”
“Let’s meet
at 11am”
Picks meeting
time
“Let’s meet
at 11am”
“Let’s meet
at 3”
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Doug
“When can
Bob
you meet
Friday?”
“When can
you meet
Friday?”
Can’t schedule
meeting, no
response from
Bob
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Deadlocks
Alice
“When can
you meet
Friday?”
Colleen
“When can
you meet
Friday?”
Locks
calendar
for Doug,
can’t
respond to
Alice
“9, 11am or 3pm”
Locks calendar
for Alice, can’t
respond to Doug
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Can’t schedule
meeting, no
response from
Colleen
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Why are threads hard?
• Too few ordering constraints: race conditions
• Too many ordering constraints: deadlocks
• Hard/impossible to reason modularly
– If an object is accessible to multiple threads, need to
think about what any of those threads could do at any
time!
• Testing is even more impossible than it is for
sequential code
– Even if you test all the inputs, don’t know it will work if
threads run in different order
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Charge
• Computers are single-threaded machines
that provide their owner the illusion of
multiple threads.
• Brains are multi-threaded machines that
provide their owner with the illusion of a
single thread.
• Friday section: return exams, practice with
subtyping and concurrency
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