04~Chapter_04 - Companionslidesfor...

Foundations of Shared Memory
Companion slides for
The Art of Multiprocessor Programming
by Maurice Herlihy & Nir Shavit

© 2007 Herlihy & Shavit 2
Last Lecture
•
Defined concurrent objects using linearizability
and sequential consistency
•
Fact : implemented linearizable objects (Two
thread FIFO Queue) in read-write memory
without mutual exclusion
•
Fact : hardware does not provide linearizable read-
write memory

© 2007 Herlihy & Shavit 3
Fundamentals
•
What is the weakest form of communication that
supports mutual exclusion?
•
What is the weakest shared object that allows
shared-memory computation?

© 2007 Herlihy & Shavit 4
Alan Turing
•
Helped us  understand what is and is not computable
on a sequential machine.
•
Still best model available

© 2007 Herlihy & Shavit 5
0 1 1 0 1 0
1
Turing Machine
Reads and Writes
Infinite tape
Finite State
Controller

© 2007 Herlihy & Shavit 6
Turing Computability
•
Mathematical model of computation
•
What is (and is not) computable
•
Efficiency (mostly) irrelevant
0 1 1 0 1 0
1

© 2007 Herlihy & Shavit 7
Shared-Memory Computability?
•
Mathematical model of concurrent computation
•
What is (and is not) concurrently computable
•
Efficiency (mostly) irrelevant
10011
Shared Memory

© 2007 Herlihy & Shavit 8
Foundations of Shared Memory
To understand modern multiprocessors we need
to ask some basic questions …

© 2007 Herlihy & Shavit 9
Foundations of Shared Memory
To understand modern multiprocessors we need
to ask some basic questions …
What is the weakest useful form of shared
memory?

© 2007 Herlihy & Shavit 10
Foundations of Shared Memory
To understand modern multiprocessors we need
to ask some basic questions …
What is the weakest useful form of shared
memory?
What can it do?

© 2007 Herlihy & Shavit 11
Foundations of Shared Memory
To understand modern multiprocessors we need
to ask some basic questions …
What is the weakest useful form of shared
memory?
What can it do?
What can’t it do?

© 2007 Herlihy & Shavit 12
Register
10011
Holds a (binary)
value
*
* A memory location: name is historical

© 2007 Herlihy & Shavit 13
Register
Can be read
10011
10011

© 2007 Herlihy & Shavit 14
Register
Can be written
10011
01100

© 2007 Herlihy & Shavit 15
public interface Register<T> {
public T read();
public void write(T v);
}
Registers

© 2007 Herlihy & Shavit 16
public interface Register <T> {
public T read();
public void write( T v );
}
Registers
Type of register
(usually Boolean or m-bit Integer)

© 2007 Herlihy & Shavit 17
10011
Single-Reader/Single-Writer Register
01100
10011

© 2007 Herlihy & Shavit 18
10011
Multi-Reader/Single-Writer Register
01100
10011

© 2007 Herlihy & Shavit 19
mumble
mumble
11011
Multi-Reader/Multi-Writer Register
mumble
10011
10011
10011
01010