window-based greedy contention management for transactional memory gokarna sharma (lsu) brett...

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Window-Based Greedy Contention Management for Transactional Memory

Gokarna Sharma (LSU)Brett Estrade (Univ. of Houston)

Costas Busch (LSU)

DISC 2010 - 24th International Symposium on Distributed Computing

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Transactional Memory - Background

• The emergence of multi-core architectures– Opportunities and challenges

• How to handle access to shared data?– Locks, Monitors, …

• Transactional memory (TM) is an alternative synchronization abstraction– Simple, composable, …

• Three types – Hardware, Software, and Hybrid TMs– Our focus is on STM Systems


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STM Systems• Progress is ensured through contention management (CM)

policy

• If transactions modify different data– everything is OK

• If transactions modify same data– conflicts arise that must be resolved - job of a contention

management policy

• Of particular interest are greedy contention managers– Transactions immediately restart after every abort


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Prior Work• Mostly empirical evaluation

• Theoretical Analysis– [Guerraoui et al., PODC’05]

• Greedy Contention Manager • Competitive ratio = O(s2) (s is the number of shared resources)

– [Attiya et al., PODC’06]• Improved to O(s)

– [Schneider & Wattenhofer, ISAAC’09]• RandomizedRounds Contention Manager• Competitive ratio = O(C . log n) (C is the maximum number of conflicting transactions and

n is the number of transactions)

– [Attiya & Milani, OPODIS’09]• Bimodal Scheduler• Competitive ratio = O(s) (for bimodal workload with equi-length transactions)DISC 2010 - 24th International Symposium on Distributed Computing

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Our Contributions• Execution window model for TM

• Makespan bound of any CM algorithm based on the contention measure C with in the window and the window parameters M and N

• Two new randomized contention management algorithms that are very close to O(s)-competitive

• An adaptive version that adapts to the amount of contention C

1 2 3 N

N

M

1 2 3

M

Transactions. . .

Threads .

..


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Roadmap

• Previous TM models and problem complexity

• Our TM model

• Our algorithms and proof ideas


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Previous TM Models

• One-shot scheduling problem– n transactions, a single transaction per thread– Best bound proven to be achievable is O(s)

• Problem Complexity: directly related to vertex coloring– Coloring problem -> One-shot scheduling problem -> One-shot

scheduling Solution -> Coloring Solution

• NP-Hard to approximate an optimal vertex coloring

• Can we do better under the limitations of coloring reduction? DISC 2010 - 24th International Symposium on Distributed Computing

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Execution Window Model

• A M N window W – M threads with a sequence of N transactions per thread,

i.e., collection of N one-shot transaction sets

1 2 3 N

N

M

1

2 3

M

Transactions

.

.

.

. . .

Threads


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Makespan Bounds• Let C denote the maximum number of conflicting transactions for any transaction

inside the window

• Trivial Makespan Bounds:– Straightforward upper bound: τ . min(CN,MN), where τ is the execution time

duration– One-shot analysis bound [Attiya et al., PODC’06]: O(sN)– Using RandomizedRounds [Schneider & Wattenhofer, ISAAC’09] N times, makespan bound:

O(τ . CN logM)

• Our Bounds: – Offline-Greedy: Makespan bound = O(τ . (C + N log(MN))) and

Competitive Ratio = O(s + log(MN)) with high probability– Online-Greedy: Makespan bound = O(τ . (C log(MN) + N log2(MN))) and

Competitive Ratio = O(s . log(MN) + log2(MN)) high probabilityDISC 2010 - 24th International Symposium on Distributed Computing

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Intuition

• The random delays help conflicting transactions shift inside the window and their execution time may not coincide

• More apparent in scenarios where conflicts are more frequent inside the same column transactions and less frequent in different column transactions

N

N’

Random interval

1 2 3 N

M

1 2 3 N

N

M

. . .


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How it works? (1/2)• Random intervals: Assume each thread Pi knows Ci and each

transaction has same duration τ (this assumption can be removed)

• Conflicts: Divide time steps into frames [each time step is of size τ]

– Frame size depends on the conflict resolution strategy of the algorithm

• Number of frames in random intervals: Each thread chooses a random number qi independently, uniformly, and randomly from the range [0, αi -1], where αi = Ci / log(MN)

• Handling conflicts: Use prioritiesDISC 2010 - 24th International Symposium on Distributed Computing

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How it works? (2/2)

1 2 3 N

M

N

q1 ϵ [0, α1 -1], α1 = C1 / log(MN)

Frames

C=maxi Ci, 1 ≤ i ≤ M

F11 F3N

Thread 1

Thread 2

Thread 3

Thread M

F1N F12

Makespan = (C / log(MN) + Number of frames) Frame Size ( =C / log(MN) + N )Frame Size

First frame of Thread 1 where T11 executesSecond frame of Thread 1 where T12 executes


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Offline-Greedy Algorithm (1/2)• Initialization:

– Frames are of size Φ = Θ(τ . ln(MN)) time steps – Each thread Pi is assigned initially a random period of qi ϵ [0, αi-1] frames, αi

= Ci / log(MN)

– Each transaction Tij is assigned to frame Fij = qi + (j-1)

• Priority assignment: each transaction has two priorities: low or high– Transaction Tij is initially in low priority

– Tij switches to high priority in the first time step of frame Fij and remains in high priority thereafter

• Conflict resolution: uses conflict graph explicitly to resolve conflicts– Conflict graph is dynamic and evolves while the execution of the

transactions progresses DISC 2010 - 24th International Symposium on Distributed Computing

Offline-Greedy Algorithm (2/2)• Proof Intuition: With high probability each transaction commits in its

assigned frame– Let A’ A denote the subset of conflicting transactions with Tij in frame Fij

• |A’| log(MN) – 1, then Tij commits in frame Fij

• |A’| log(MN) with probability at most (1/MN)2

• Makespan: O( (C + N log(MN))) with high probability– Pro: For C N log(MN) makespan is log(MN) factor far from optimal, since N is a

trivial lower bound– Con: Need to know dependency graph to resolve conflicts

• Competitive ratio: O(s + log(MN)) with high probability– Pro: Independent with any choice of C 14DISC 2010 - 24th International Symposium on Distributed Computing

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Online-Greedy Algorithm (1/2)• Online in the sense that it does not depend on knowing the

dependency graph to resolve conflicts

• Similar to Offline-Greedy except the conflict resolution strategy

• Priority assignment– Two different priorities associated with each transaction as a vector – π(1) represent the Boolean priority as in Offline-Greedy– π(2) [1, M] represent random priorities: A transaction chooses π(2) uniformly

at random on the start of frame Fij and after every abort [Idea from Schneider &

Wattenhofer, ISAAC’09]

• Conflict resolution– On conflict of Tij with Tkl: if πij

(2) < πkl(2) then abort(Tij, Tkl) otherwise abort(Tkl,

Tij) DISC 2010 - 24th International Symposium on Distributed Computing

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Online-Greedy Algorithm (2/2)• Proof Intuition: frame duration is now Φ’ = O( log2(MN))

– Analysis is similar to Offline-Greedy

• Makespan: O((C log(MN) + N log2(MN))) with high probability– Pro: no need to know dependency graph to resolve conflicts– Con: makespan is worse in comparison to Offline-Greedy

• Competitive ratio: O(s log(MN) + log2(MN)) with high probability

• Pro: Independent of the contention measure CDISC 2010 - 24th International Symposium on Distributed Computing

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Adaptive-Greedy Algorithm• Limitations of Offline-Greedy and Online-Greedy

algorithms– The values of Ci need to be known in advance

• Adaptive-Greedy: each thread starts with guessing Ci = 1– Similar to the exponential back-off strategy used by Polka– Based on current Ci estimate, the thread attempts to execute Online-

Greedy algorithm– If a thread Pi is unable to commit transactions (bad event) then Pi

assumes choice of Ci is incorrect and starts over again by assuming Ci’ = 2 Ci for remaining transactions

• Correct choice of Ci is reached in logCi iterationsDISC 2010 - 24th International Symposium on Distributed Computing

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Discussions• For variable length transactions

– on makespan bounds is replaced with max, which is the maximum duration of any transaction in the window

– max / min factor in competitive ratio bounds, where min is the minimum duration of any transaction in the window

• Future extensions– Instead of one randomization interval at the beginning of window, random

periods of low priority between subsequent transactions

– Dynamic expansion and contraction of the execution window to preserve the contention measure C


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Conclusions

• Execution window model for TM

• Two new randomized greedy CM algorithms that are very close to O(s)-competitive

• Adaptive version of the previous algorithms for better performance by avoiding the limitations of the known value of C


window-based greedy contention management for transactional memory gokarna sharma (lsu) brett...

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