Transactional Memory
for Smalltalk
Software Composition Group
University of Bern
Lukas Renggli
Oscar Nierstrasz

Concurrent Programming
in Smalltalk
Semaphore forMutualExclusion
RecursionLock new
Mutex new
SharedQueue new

Problems
Deadlocks
Starvation
Priority Inversion
Complexity

Software
Transactional Memory

Programming
with Transactions

Lock Based
tree := BTree new.
lock := Semaphore forMutualExclusion.
\" writing \"
lock critical: [ tree at: #a put: 1 ].
\" reading \"
lock critical: [ tree at: #a ].

Transactional
tree := BTree new.
\" writing \"
[ tree at: #a put: 1 ] atomic.
\" reading \"
tree at: #a.

Inside Transactions

Our Approach
Full implementation in Smalltalk
Lazy code transformation
Method annotations
Context dependent code execution

Static Model

Class
name CompiledMethod
superclass * selector 1
selector
subclasses methods parseTree method
instanceVariables
AtomicMethod
/selector 1
/parseTree atomicMethod

Code Transformation
Instance Variables
Variable Bindings
Message Sends

Original Source Code
BTree>>at: aKey put: anObject
| leaf |
leaf := root leafForKey: aKey.
leaf insertKey: aKey value: anObject.
root := leaf root.
^ anObject

Transformed Source Code
BTree>>__atomic__at: aKey put: anObject
| leaf |
leaf := (self atomicInstVarAt: 1)
__atomic__leafForKey: aKey.
leaf __atomic__insertKey: aKey value: anObject.
self atomicInstVarAt: 1 put: leaf __atomic__root.
^ anObject

Transformed Source Code
BTree>>__atomic__at: aKey put: anObject
| leaf |
leaf := (self atomicInstVarAt: 1)
__atomic__leafForKey: aKey.
leaf __atomic__insertKey: aKey value: anObject.
self atomicInstVarAt: 1 put: leaf __atomic__root.
^ anObject

Transformed Source Code
BTree>>__atomic__at: aKey put: anObject
| leaf |
leaf := (self atomicInstVarAt: 1)
__atomic__leafForKey: aKey.
leaf __atomic__insertKey: aKey value: anObject.
self atomicInstVarAt: 1 put: leaf __atomic__root.
^ anObject

Annotate methods
that need special treatment
Infrastructural code
Exception handling
Execution contexts
Many primitives
Variable sized objects

Dynamic Model

Transaction
Change
escapeContext
0..1 do: aBlock *
Process apply
currentTransaction retry: aBlock changes
hasChanged
checkpoint object hasConflict
abort: anObject
*
ObjectChange CustomChange
previousCopy applyBlock
workingCopy conflictTestBlock

Validation

Speed Comparison
Method Invocation 1×
Special Method Invocation 2×
Instance Variable Read 20×
Instance Variable Write 19×
Indexed Variable Read 18×
Indexed Variable Write 17×
5×
10×
15×
20×
25×

Performance of n concurrent
t [ms]
800.0
edit operations in Pier
700.0 1 CPU
600.0
500.0
400.0
2 CPUs
300.0
200.0 4 CPUs
100.0 8 CPUs
0.0 n
0 10 20 30 40 50 60 70 80 90 100
non-synchronized lock-based transactional

Future Work

Implement within a
multi-core environment

Improve speed

Applications
Concurrency Contol
Source Code Loading
Context Oriented Programming