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

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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

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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

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Homework Challenge

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

& writes

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Another Homework Challenge

How about two dequeuers?

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Shared Memory

Atomic reads and writes to individual variables

read

write

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Snapshot Challenge

Wait-Free algorithm to read multiple

variables atomically?

Read both

Read both

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Snapshot Challenge

Wait-Free algorithm to

write multiple variables

atomically?writeboth

write both

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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

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Is Locked?isLocked(

)

Bakery Lock

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

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tryLock()

Bakery Lock

Acquire lock if free & return true, otherwise

immediately return false.

I Can Has Lock?

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Our Experience Teaching Concurrency

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

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Mutual Exclusion

• Peterson, Bakery, space lower bounds

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

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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

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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

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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

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Then What?

• Implement simple T&S lock– Dreadful performance

• Implement T&T&S lock– Improved performance

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Homework Challenge

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

& writes

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Yes,

you ca

n

Homework Challenge

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Another Homework Challenge

How about two dequeuers?

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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

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Alas, no, you can’t

Another Homework Challenge

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What If we have Test&Set?

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What If we have Test&Set?

No, you still can’t

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What If we have Compare&Swap?

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What if We Have Compare&Sswap?

Yes,

you ca

n

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Snapshot Challenge

Reads from 0 and

1

Reads from 1 and

2

Wait-Free algorithm to read multiple

variables atomically?

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Snapshot Challenge

Reads from 0 and

1

Reads from 1 and

2

Yes,

you ca

n

Wait-Free algorithm to read multiple

variables atomically?

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Multiple Assignment Challenge

Writes to 0 and 1

Writes to 1 and 2

Wait-Free algorithm to

write multiple variables

atomically?

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Writes to 0 and 1

Writes to 1 and 2

Wait-Free algorithm to

write multiple variables

atomically?

Multiple Assignment Challenge

No, you can’t

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Is Locked?isLocked(

)

Bakery Lock

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

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isLocked()

Bakery Lock

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

Yes,

you ca

n

Is Locked?

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I Can Has Lock?

tryLock()

Bakery Lock

Acquire lock if free & return true, otherwise

immediately return false.

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tryLock()

Bakery Lock

Acquire lock if free & return true, otherwise

immediately return false.

I Can Has Lock?

No, you can’t

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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