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State Teleportation How Hardware Transactional Memory can Improve Legacy Data Structures Maurice Herlihy and Eli Wald Brown University
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Transactional Memory
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Concurrent Data Structure = State Machine
atomic step memory barrier
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Concurrent Data Structure = State Machine
atomic step memory barrier critical section, CAS, load/store
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“as if” sequence of transitions
State Teleportation atomic step atomic step atomic step memory barrier memory barrier memory barrier Hardware Transaction “as if” sequence of transitions
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Three Concurrent Lists
lazy wait-free traversal + locks lock-free lock-free everything locking hand-over-hand locking
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Hand-Over-Hand Locking
b c d remove(b)
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Hand-Over-Hand Locking
b c d remove(b)
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Hand-Over-Hand Locking
b c d remove(b) Found it!
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Hand-Over-Hand Locking
b c d remove(b)
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Lazy List a b c remove(b)
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Lazy List a b c remove(b)
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Lazy List a b c remove(b)
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Lazy List a b c remove(b)
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Lazy List a b c validate … remove(b)
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Lazy List a b c Logical delete remove(b)
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Lazy List a b c Physical delete
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Lock-Free List CAS a b c d remove(c)
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seconds to finish 100,000 operations 80% writes, 20% modifications
Benchmarks median of 6 runs higher = worse seconds to finish 100,000 operations 80% writes, 20% modifications
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Benchmarks every thread has a core 1-4
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Benchmarks every thread has a hyperthread 5-8
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Benchmarks 9-up
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No Memory Management lazy lock-free locking
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Memory Management? Locking: Lazy: Lock-Free: immediate re-use
hazard pointers
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Hazard Pointers Node* hazardRead(Node** object) { while (true) {
Node* read = *object; hazard[myIndex] = read; membar(); Node* reread = *object; if (read == reread) return read; }
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Hazard Pointers read pointer to object
Node* hazardRead(Node** object) { while (true) { Node* read = *object; hazard[myIndex] = read; membar(); Node* reread = *object; if (read == reread) return read; } read pointer to object
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Hazard Pointers announce hazard Node* hazardRead(Node** object) {
while (true) { Node* read = *object; hazard[myIndex] = read; membar(); Node* reread = *object; if (read == reread) return read; } announce hazard
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Hazard Pointers make announcement visible (expensive)
Node* hazardRead(Node** object) { while (true) { Node* read = *object; hazard[myIndex] = read; membar(); Node* reread = *object; if (read == reread) return read; } make announcement visible (expensive)
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Hazard Pointers reread and validate Node* hazardRead(Node** object) {
while (true) { Node* read = *object; hazard[myIndex] = read; membar(); Node* reread = *object; if (read == reread) return read; } reread and validate
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Without Memory Management
lazy lock-free locking
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With Memory Management
lazy lock-free locking
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Lock Teleportation a b c d
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Lock Teleportation read transaction a b c d
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Lock Teleportation read transaction a b c d
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Lock Teleportation no locks acquired a b c d
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Hazard PointerTeleportation
read transaction a b c d
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Hazard PointerTeleportation
b c d
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Hazard Pointer Teleportation
read transaction a b c d
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Hazard Pointer Teleportation
no memory barriers a b c d
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Without Teleportation
lazy lock-free locking
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With Teleportation lazy lock-free locking
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Speedup lazy lock-free locking
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Adaptive Jumps If transaction commits … Add 1 to next distance
If transaction aborts… Cut next distance in half
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Average Teleport Distance
lazy lock-free locking
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Average Commit Rate lazy lock-free locking
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Abort Reasons 4 threads 8 threads capacity explicit conflict unknown
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Conclusions True cost of concurrent data structures
should include memory management Cost of hazard pointers can equalize lock-based and lock-free structures Adaptive Teleportation can substantially improve memory management costs for both lock-based and lock-free structures
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