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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 Herlihy Brown University. In many areas of CS. Systems research fought all the hard battles FORTRAN  complexity theory UDP/IP  network algorithms

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It Ain’t the Meat, it’s the Notion Why Theory is Essential to Teaching Concurrent Programming

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  1. It Ain’t the Meat, it’s the NotionWhy Theory is Essential to Teaching Concurrent Programming Maurice Herlihy Brown University

  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 Multicore Programming Education

  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 Multicore Programming Education

  4. Homework Challenge Devise Wait-Free 1-Enqueuer, 1-Dequeuer Buffer using only reads & writes Multicore Programming Education

  5. Another Homework Challenge How about two dequeuers? Multicore Programming Education

  6. Shared Memory Atomic reads and writes to individual variables write read Multicore Programming Education 6

  7. Snapshot Challenge Wait-Free algorithm to read multiple variables atomically? Read both Read both Multicore Programming Education 7

  8. Snapshot Challenge Wait-Free algorithm to write multiple variables atomically? writeboth write both Multicore Programming Education 8

  9. Bakery Algorithm class 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; } } … 6 2 n-1 0 f f t f f t f f 0 0 4 0 0 5 0 0 Classic, first-come-first served mutual exclusion algorithm Critical Section Multicore Programming Education 9

  10. Is Locked? isLocked() True means was locked at some instant, false means was free at some instant. Bakery Lock Multicore Programming Education

  11. I Can Has Lock? tryLock() Acquire lock if free & return true, otherwise immediately return false. Bakery Lock Multicore Programming Education

  12. Our Experience Teaching Concurrency • Brown (10 years) • Tel-Aviv (10 years) • Industry Multicore Programming Education

  13. Mutual Exclusion • Peterson, Bakery, space lower bounds • Teaches • Deadlock • Livelock • Fairness, first-come-first-served, … Multicore Programming Education

  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 Multicore Programming Education

  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 Multicore Programming Education

  16. 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 … Uh, Oh, Spin Locks bad Test&test&set less bad Test&set Multicore Programming Education

  17. Then What? • Implement simple T&S lock • Dreadful performance • Implement T&T&S lock • Improved performance Multicore Programming Education

  18. Homework Challenge Devise Wait-Free 1-Enqueuer, 1-Dequeuer Buffer using only reads & writes Multicore Programming Education

  19. Homework Challenge Yes, you can Multicore Programming Education

  20. Another Homework Challenge How about two dequeuers? Multicore Programming Education

  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 Multicore Programming Education

  22. Another Homework Challenge Alas, no, you can’t Multicore Programming Education

  23. What If we have Test&Set? Multicore Programming Education

  24. What If we have Test&Set? No, you still can’t Multicore Programming Education

  25. What If we have Compare&Swap? Multicore Programming Education

  26. What if We Have Compare&Sswap? Yes, you can Multicore Programming Education

  27. Snapshot Challenge Wait-Free algorithm to read multiple variables atomically? Reads from 0 and 1 Reads from 1 and 2 Multicore Programming Education 27

  28. Snapshot Challenge Wait-Free algorithm to read multiple variables atomically? Reads from 0 and 1 Yes, you can Reads from 1 and 2 Multicore Programming Education 28

  29. Multiple Assignment Challenge Wait-Free algorithm to write multiple variables atomically? Writes to 0 and 1 Writes to 1 and 2 Multicore Programming Education 29

  30. Multiple Assignment Challenge Wait-Free algorithm to write multiple variables atomically? Writes to 0 and 1 No, you can’t Writes to 1 and 2 Multicore Programming Education 30

  31. Is Locked? isLocked() True means was locked at some instant, false means was free at some instant. Bakery Lock Multicore Programming Education

  32. Is Locked? isLocked() Yes, you can True means was locked at some instant, false means was free at some instant. Bakery Lock Multicore Programming Education

  33. I Can Has Lock? tryLock() Acquire lock if free & return true, otherwise immediately return false. Bakery Lock Multicore Programming Education

  34. I Can Has Lock? tryLock() No, you can’t Acquire lock if free & return true, otherwise immediately return false. Bakery Lock Multicore Programming Education

  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 Multicore Programming Education

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