Distributed Transactional Memory Presented by Gala Yadgar

2 Model A network of nodes Transactions are immobile Objects move from node to node

3 Model Cache coherence protocol Locate the current copy Move and invalidate Metric Location aware A C B D

4 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

5 Motivation The contention manager guarantees atomicity Should be obstruction free Performance goals Makespan Competitive ratio Makespan of optimal

6 Transactional memory proxy Local request: Local object – return copy Remote object – locate with Ballistic Remote request: Object not in use – invalidate copies and send Object in use – abort or postpone response Commit: No invalidations – commit Invalidations – abort

7 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

8 Hierarchical clustering L=3

9 Hierarchical clustering L=3 Level 0 Physical nodes are leaf nodes x and y are connected iff d(x,y) < 2 1 Leader 0 is the maximal independent set

10 Hierarchical clustering L=3 Level l Only nodes from leader l-1 x and y are connected iff d(x,y) < 2 l+1 Leader l is the maximal independent set Level L Root L ≤ log 2 Diam + 1

11 Hierarchical clustering L=3 Level l, node x Lookup parent set Level l+1 nodes within distance 10*2 l+1 from x Home parent Closest lookup parent Move parent set Level l+1 nodes within distance 4*2 l+1 from x x

12 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

13 Publish() L=3 Object at node p Create a single directed path from root to p Home i (p).link = Home i-1 (p) * We deal with a single object

14 lookup() L=3 Request at node q Up phase Home i-1 (q) initiates a search for a non-null link at lookupProbe i (q) Down phase Follow links to a leaf Obtain copy or wait with leaf

15 move() L=3 Request at node q Up phase Home i-1 (q) initiates a search for a non-null link at moveProbe i (q) Home i (q).link = Home i-1 (q) Redirect if found Down phase Follow links to a leaf Erase links Wait in queue

16 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

17 Overtaking L=3 Object at node p a - 1 st request b - 2 nd request b enqueued first. a b

18 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

19 Finite write response time Every move request is satisfied within time n * T E + n * T O from when it is generated T E – maximum enqueue delay T O – maximum time to reach a successor n – number of nodes Equivalent – finite read response time a b

20 Proof By time at most t + n * T E, either 1. All successor links between r and its n predecessors r 1, r 2, …, r n have been established 2. There is k≤n-1, r k is p, the publish request. 1. At least two requests r i, r j come from the same node They are different (Lemma 1) One was satisfied The object reached a predecessor

21 Proof Let x be the location of the object at time t + n * T E r is at most n steps away from x by taking the successor links r will have the object by time at most t + n * T E + n * T O

22 Bounded overtaking (corollary) (Every move request is satisfied within time n * T E + n * T O from when it is generated) Request r is generated at time t All requests generated after time t + n * T E will be ordered after r All requests generated prior to time t - n * T E will be ordered before r

23 Lemma 1 There exists no set of finite number of requests R={r 1, r 2, …, r f } whose successor links form a cycle r’s arrow: a downward link added by r’s visit Outside arrows: established by requests outside R P C

24 Invariants 1. The root always has an arrow 2. Requests see an arrow at the peak level before the down phase 3. During the down phase, requests see an arrow until they reach a leaf 4. r’s arrow at level i points to C=home i-1 (r)

25 Invariants 5. r adds and arrow P  C at time t At time t –, r added an arrow to a grandchild C  At time t + that arrow will be erased by r’ r’ reached C from P r’ erased r’s arrow P  C During [t -,t + ], C always has an arrow May be redirected from one grandchild to another P C

26 Proof H: the highest peak level reached by requests in R The first request to reach H sees an outside arrow We show: in any level l<H some request from R sees an outside arrow That request is queued behind an outside request

27 Proof (by induction ) Base: at level H Step: At time t, r in R sees an outside arrow at level k in node P. The arrow was established by x not in R. P  C, C is x’s home directory x also established C  at level k-1, at time t – At time t +, r reaches C. Either 1. Another request from R sees C  during [t -,t + ] 2. r sees C  at time t + P C

28 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

29 Overtaking revisited L=3 Object at node p a - 1 st request b - 2 nd request b enqueued first. What if a’s priority is higher? a b

30 Intuitively… Optimal schedule Minimum cost Hamiltonian path Visit each node once Greedy schedule worst case Tx aborted by all higher priority Txs Each abort requires a move() Node with timestamp k visited k times

31 Performance Work An operation’s communication overhead Distance The cost of communicating directly from the requesting node to its destination Stretch work/distance Executions can be sequential or concurrent

32 Performance Publish cost The publish operation has work O(Diam) Move cost If an object has moved a combined distance d since its initial publication, the amortized move stretch is O(min{log 2 d,L})

33 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

34 Performance Publish cost The publish operation has work O(Diam)

35 1. Bounded link property The metric distance between a level-l child and its level-(l+1) parent is less than or equal to c b * 2 l, for some constant c b. x and y are connected iff d(x,y) < 2 l+1

36 2. Constant expansion property Any node has no more than a constant number of lookup parents and lookup children (c e ) * This property requires a constant doubling metric L=3

37 Constant doubling dimension Metric: distances between all pairs, non-negative, triangle inequality Ball B u (r) = { v | d(u,v) ≤ r } 2 α balls of radius r/2 cover ball of radius r Doubling dimension: α is constant Based on “Ad Hoc Sensor Networks” by Roger Wattenhofer

38 3. Lookup property For any two leaves p,q p l : any of p’s level-l ancestors by following move parents only. If p l is not in lookupProbe l (q)  d(p,q) ≥ c l * 2 l, for some constant c l 0 L p q plpl plpl … c l * 2 l

39 4. Move property For any two leaves p,q p l : p’s level-l home directory If p l is not in moveProbe l (q)  d(p,q) ≥ c m * 2 l, for some constant c m 0 L p q plpl … c m * 2 l

40 Lemma 3 There exists a constant c w such that for any operation that peaks at level l, the work it performs is at most c w * 2 l Proof Bounded link  link cost c b * 2 l Constant expansion  number of steps in each level

41 Publish performance The publish operation has work O(Diam) Publish operations peak at level L The work is ≤ c w * 2 L (Lemma 3) L ≤ log 2 Diam + 1

42 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

43 Performance Move cost If an object has moved a combined distance d since its initial publication, the amortized move stretch is O(min{log 2 d,L})

44 Lemma 4 q move request sequential execution Discovers a non-null link at node P at level l p move/publish request last to visit P d(p,q) ≥ c m * 2 l-1 0 L p q P … c m * 2 l-1

45 Proof The non null link at P points to home l-1 (p) home l-1 (p).link is non null at least until q removes its link (Invariant 5) It was there during q’s up phase q did not visit home l-1 (p) going up level l-1 Move property:d(p,q) ≥ c m * 2 l-1 0 L p q P … home l-1(p) c m * 2 l-1

46 Lemma 5 Distance of a sequential execution Sum of distances for all move operations In a sequential execution with distance d, the maximum level reached by any move request does not exceed min(log 2 d+c,L) c is a constant d log 2 d

47 Proof q 0 - the initial publish request l – highest level reached by a move request q – the request that peaked at level l (first) l ≤ L q saw a non null link at level l established by q 0 d(q,q 0 ) ≥ c m * 2 l-1 (Lemma 4) d ≥ d(q,q 0 ) l ≤ Log 2 (d/c m )+1 q0q0 q d l

48 Move performance Lemma 6 For any sequential execution α work(α) ≤ (c w /c m ) * l(α) * distance(α) l(α) – the maximum level reached by a move request of execution α By Lemma 5 distance(α) = d  l(α) ≤ min(log 2 d+c,L) The amortized move stretch is O(min{log 2 d,L}) No proof here…

49 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

50 Additional results Move cost Amortized move stretch is O(min{log 2 d,L}) for concurrent executions as well Idea “Lock” critical section in each level Prevent neighbors from “stealing” links

51 Additional results Lookup cost The stretch for a lookup operation is constant Idea An operation peaks at level h The work is at most c w * 2 h (Lemma 3) d(p,q) ≥ c l * 2 h (lookup property) Redefine distance with overlapping move requests

52 Additional results Multiple objects Require multiple directories Storage and request handling load on each node is O(log Diam). Idea Hash each node to multiple parallel directory nodes

53 Outline Ballistic protocol Hierarchical clustering Operations Requirements Finite response time Performance Publish Move Additional results Summary

54 Summary Distributed transactional memory Cache coherence protocol Hierarchical clustering: L ≤ log 2 Diam + 1 Results Finite write response time: n * T E + n * T O Publish cost: O(Diam) Move cost: O(min{log 2 d,L}) Constant lookup cost Multiple objects with O(log Diam) load

55 References Maurice Herlihy, Ye Sun. Distributed transactional memory for metric-space networks. Distributed Computing 20(3): (2007). Ye Sun. The Ballistic Protocol: Location-Aware Distributed Cache Coherence in Metric-Space Networks. Doctoral Thesis, Brown University Bo Zhang, Binoy Ravindran. Location-Aware Cache-Coherence Protocols for Distributed Transactional Contention Management in Metric-Space Networks