it ain’t the meat, it’s the notion why theory is essential to teaching concurrent programming...

It Ain’t the Meat, it’s the Notion

Why Theory is Essential to Teaching

Concurrent Programming

Maurice HerlihyBrown University

Multicore Programming Education

2

In many areas of CS

• Systems research fought all the hard battles– FORTRAN complexity theory– UDP/IP network algorithms

• Theory researchers arrived later to shoot the wounded

• But concurrency is different


3

Vive la Différence

• Concurrent programmers are made, not born (mostly)

• Students have little experience with concurrency (today)

• Develop intuition by– Idealized examples first– Logical reasoning


4

Homework Challenge

Devise Wait-Free 1-Enqueuer, 1-Dequeuer Buffer using only reads

& writes


5

Another Homework Challenge

How about two dequeuers?


66

Shared Memory

Atomic reads and writes to individual variables

read

write


77

Snapshot Challenge

Wait-Free algorithm to read multiple

variables atomically?

Read both

Read both


88

Snapshot Challenge

Wait-Free algorithm to

write multiple variables

atomically?writeboth

write both


99

Bakery Algorithmclass Bakery implements Lock { boolean[] flag; Label[] label; public Bakery (int n) { flag = new boolean[n]; label = new Label[n]; for (int i = 0; i < n; i++) { flag[i] = false; label[i] = 0; } } …

n-10

f f f f t ft

2

f

0 0 0 0 5 04 0

6

Critical Section

Classic, first-come-first served mutual

exclusion algorithm


10

Is Locked?isLocked(

)

Bakery Lock

True means was locked at some instant, false means was free at some instant.


11

tryLock()

Bakery Lock

Acquire lock if free & return true, otherwise

immediately return false.

I Can Has Lock?


12

Our Experience Teaching Concurrency

• Brown (10 years)• Tel-Aviv (10 years)• Industry


13

Mutual Exclusion

• Peterson, Bakery, space lower bounds

• Teaches– Deadlock– Livelock– Fairness, first-come-first-served, …


14

Linearizability

• How to describe a concurrent object?– By “equivalence” to sequential

• What is a concurrent API?• How do we reason about correctness?

– Even informally!

• Learn the rules before you break them


15

Consensus Numbers

• Certain synchronization primitives are mathematically stronger than others

• No, you can’t build lock-free X from Y– So do not waste your time trying

• Yes, you can build lock-free X from Z– The rest is optimization


16

Uh, Oh, Spin Locks

• Inexplicable in idealized model

• Well, kids …– We didn’t actually

lie to you– But the architects

gave us caches– And you have to

outwit them …

bad

less bad

Test&set

Test&test&set


17

Then What?

• Implement simple T&S lock– Dreadful performance

• Implement T&T&S lock– Improved performance


18

Homework Challenge

Devise Wait-Free 1-Enqueuer, 1-Dequeuer Buffer using only reads

& writes


19

Yes,

you ca

n

Homework Challenge


20


How about two dequeuers?


21

Maybe?

“hence [any wait-free object] can be written without a CAS/LL/SC. Instead LL/SC is used to make it more efficient but LL/SC

isn't *required* to make it wait-free.”

Found in a blog


22

Alas, no, you can’t



23

What If we have Test&Set?


24

What If we have Test&Set?

No, you still can’t


25

What If we have Compare&Swap?


26

What if We Have Compare&Sswap?

Yes,

you ca

n


2727

Snapshot Challenge

Reads from 0 and

1

Reads from 1 and

2




2828

Snapshot Challenge

Reads from 0 and

1

Reads from 1 and

2

Yes,

you ca

n




2929

Multiple Assignment Challenge

Writes to 0 and 1

Writes to 1 and 2



atomically?


3030

Writes to 0 and 1

Writes to 1 and 2



atomically?

Multiple Assignment Challenge

No, you can’t


31

Is Locked?isLocked(

)

Bakery Lock



32

isLocked()

Bakery Lock


Yes,

you ca

n

Is Locked?


33

I Can Has Lock?

tryLock()

Bakery Lock




34

tryLock()

Bakery Lock



I Can Has Lock?

No, you can’t


35

My Very Last Slide

• Teaching theory first– Develops intuition– Imposes order on chaos

• Idealized problems– Prepare for realistic, complicated

ones

• Impossibility results– Save time and effort

it ain’t the meat, it’s the notion why theory is essential to teaching concurrent programming...

Documents