Post on 18-Dec-2015
Coarse-Grained Transactions
Maurice HerlihyBrown University
Joint work with Eric Koskinen and Matthew Parkinson (POPL 10)
TMW April 2010
Skew Heaps
0
1
Tree with “heap”
property2
43
TMW April 2010
Concurrent Skew Heap
0
1 3
54
2
Insert me!
6
Insert me!
TMW April 2010
Concurrent Skew Heap
0
1 3
54
2 6 Lock parent
Swap R & L
TMW April 2010
Concurrent Skew Heap
0
1 3
54
6
Lock right child
Unlock parent2
TMW April 2010
Concurrent Skew Heap
0
1 3
54
6
2
No global rebalancingNo global rebalancing
Good amortized performanceGood amortized performance
Good concurrencyGood concurrency
TMW April 2010
Transactional Skew Heap
0
1 3
54
2
Insert me!
6
Insert me!
TMW April 2010
Transactional Skew Heap
0
1 3
54
2 6
I wrote
0
Write-write
conflict!
Good concurrency with lockingGood concurrency with locking
Not with transactions …Not with transactions …
Confusion betweenthread-level & transaction-level synchronization
Confusion betweenthread-level & transaction-level synchronization
TMW April 2010
Coarse-Grained Synchronization
Synchronize on high-level operations,Like add(), remove(), etc. …
Not low-level reads and writes
Synchronize on high-level operations,Like add(), remove(), etc. …
Not low-level reads and writesPessimistic: update in place, undo on abortPessimistic: update in place, undo on abort
Optimistic: update private copy, apply changes on commit
Optimistic: update private copy, apply changes on commit
But what is the meaning of conflict?
But what is the meaning of conflict?
TMW April 2010
Pessimistic Boosting
transactions
Abstract locks Black-box linearizable data
object
Undo Logs
TMW April 2010
Pessimistic Boosting
transactions
Abstract locks Black-box linearizable data
object
Undo Logs
add(x)
TMW April 2010
Pessimistic Boosting
transactions
Abstract locks Black-box linearizable data
object
Undo Logs
add(x)
TMW April 2010
Pessimistic Boosting
transactions
Abstract locks Black-box linearizable data
object
Undo Logs
add(x)
TMW April 2010
Pessimistic Boosting
transactions
Abstract locks Black-box linearizable data
object
rem(x)
Undo Logs
add(x)
x
TMW April 2010
Pessimistic Boosting
transactions
Abstract locks Black-box linearizable data
object
rem(x)
Undo Logs
add(x)
x
add(y)
y
TMW April 2010
Pessimistic Boosting
transactions
Abstract locks Black-box linearizable data
object
rem(x)
Undo Logs
add(x)
x
member(x)
TMW April 2010
Pessimistic BoostingThread-safe base objectThread-safe base object
Updated in placeUpdated in placeLog InversesLog InversesConflicting operations blocked by abstract locksConflicting operations blocked by abstract locks
What does it mean for operations to conflict?What does it mean for operations to conflict?
TMW April 2010
Optimistic Boosting
Black-box linearizable data
object
TMW April 2010
Optimistic Boosting
private copies
Black-box linearizable data
object
TMW April 2010
Optimistic Boostingredo logs
private copies
Black-box linearizable data
object
TMW April 2010
Optimistic Boostingredo logs
private copies
Black-box linearizable data
object
add(x)
TMW April 2010
Optimistic Boostingadd(x)
redo logs
private copies
Black-box linearizable data
object
add(x)
x
TMW April 2010
Optimistic Boostingadd(x) add(y)
redo logs
private copies
Black-box linearizable data
object
add(x)
x
add(y)
y
TMW April 2010
On Commitadd(x) add(y)
redo logs
private copies
Black-box linearizable data
object
x y
add(x)
add(x)
TMW April 2010
On Commitadd(x) add(y)
redo logs
private copies
Black-box linearizable data
object
x y
add(x)
add(x)
No conflict, apply
updates to my copy
x
x
TMW April 2010
On Commitadd(x) add(y)
x y
add(x)
add(x)
x
x
Different physical values,Same logical values
TMW April 2010
On Commitadd(x) rem(x)
redo logs
private copies
Black-box linearizable data
object
x
add(x)
add(x)
Conflict! Abort & restore my copy
x
TMW April 2010
Optimistic BoostingThread-local object copiesThread-local object copies
Deferred operatons kept in redo logDeferred operatons kept in redo log
No inversesNo inversesOn commit, broadcast deferred operations
To other transactions, public copy
On commit, broadcast deferred operationsTo other transactions, public copyTransactions snoop on broadcast,
Abort if conflict detected
Transactions snoop on broadcast,Abort if conflict detected
What does it mean for operations to conflict?What does it mean for operations to conflict?
TMW April 2010
Left-Movers
time
legal history
TMW April 2010
Left-Movers
time
If and are adjacent,Ok to move earlier
TMW April 2010
Left-Mover Example: Semaphore
time
inc()dec() dec()
1 0 1 0
TMW April 2010
Inc() is Left-mover WRT Dec()
time
inc() dec() dec()
1 10 01 02 11
TMW April 2010
Same sequence of calls(results unaffected)
Left-Mover Example: Semaphore
time
inc() dec() dec()
1 10 01 02 11
TMW April 2010
Same final state
Left-Mover Example: Semaphore
time
inc() dec() dec()
3 23 23 24 33
TMW April 2010
Left-Mover Counter-example
time
1
dec()
0
inc()
1
dec()
0
TMW April 2010
Dec() not Left-Mover WRT Inc()
time
1
dec()
0
inc()
1
dec()
0-1
TMW April 2010
Right-Movers
time
legal history
TMW April 2010
Right-Movers
time
If and are adjacent,Ok to move later
TMW April 2010
Commutativity
time
If and are adjacent,Ok to swap
What Theory? Sept 2009 40
Pessimistic Semantics(modify shared state in place)
time
Txn B beg
Txn A beg
Pessimistic Commit: Move Left of pendingPessimistic Commit: Move Left of pending
cmt
What Theory? Sept 2009 41
Pessimistic Semantics(modify shared state in place)
time
Txn B beg
Txn A beg abt
What Theory? Sept 2009 42
Pessimistic Semantics(modify shared state in place)
time
3
Txn B beg
Txn A beg 1 2 4
Pessimistic Abort: Move Right of pendingPessimistic Abort: Move Right of pending
24 3 1
Pessimistic Abort: Pending ops move LeftPessimistic Abort: Pending ops move Left
What Theory? Sept 2009 43
Pessimistic Semantics(modify shared state in place)
• Operations must be LEFT moversw.r.t. other pending operations
• On commit, move operations LEFT
• On abort, move operations RIGHT (free)and append inverses
What Theory? Sept 2009 44
Optimistic Semantics(modify local copies; merge)
time
Txn B
Txn A beg
Optimistic Commit: Move Right of committedOptimistic Commit: Move Right of committed
cmt
cmt
What Theory? Sept 2009 45
Optimistic Semantics(modify local copies; merge)
time
Txn B
Txn A beg
Optimistic Abort: Discard OperationsOptimistic Abort: Discard Operations
abt
cmt
What Theory? Sept 2009 46
• Operations must be RIGHT moversw.r.t. concurrently committed operations
• On commit, move operations RIGHT
• Abort if concurrent non-LM op• On abort, discard local changes
Optimistic Semantics(modify local copies; merge)
Pessimistic Optimistic
Two Execution Models
What Theory? Sept 2009 47
Left-Moverness Right-MovernessAlgebraic Abstract
Properties
TMW April 2010
Challenges
• Automation?– Theorem proving– Model checking
• Compiler & Language support?• Implementation …