Why STMs Need Compilers (a war story)

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Why STMs Need Compilers(a war story)

• Maurice Herlihy
• Brown University

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STM

• Interested in managed languages
• Java, C
• Not C, C
• Synchronization based on objects, not addresses
• Strong isolation between transactional
  non-transactional access

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STM Libraries DSTM 2003

• Transactional wrapper for objects
• Cumbersome error-prone

// wrapper for node object TMObjectltRBNodegt xnode
// explicit open RBNode node
xnode.openRead() // next field is
wrapper TMObjectltRBNodegt next node.next
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STM Libraries SXM 2005

• Mark data types as atomic
• Use reflection run-time code generation
• Not efficient (no global optimizations)

Atomic // Atomic Red-Black Tree node public
class RBNode public int value public
Color color public bool marked public
RBNode parent, left, right public RBNode()
value 0 color Color.RED
parent null left null
right null
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Current work

• Mark data types as atomic
• Compiler does the rest

Atomic // Atomic Red-Black Tree node public
class RBNode public int value public
Color color public bool marked public
RBNode parent, left, right public RBNode()
value 0 color Color.RED
parent null left null
right null // other methods
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Optimizations

• Whole object dataflow analysis
• call tree for non-virtual methods
• Pass context from method to method
• Every method is analyzed for
• Local objects (not shared across transactions)
• Read only or read/write objects
• Promotes openReads to openWrites for RW objects
• Early opens for objects used across basic blocks
• Partial Redundancy Elimination (PRE)

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Optimizations (cont.)

• First vs. subsequent read/writes
• Inline subsequent reads/writes with fast-path
  code sequence
• Subsequent reads/writes are lock-free, even in
  the STM mode that uses short locks

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Progress Conditions

• Short-lived locks
• Blocking
• Efficient library implementation
• Obstruction-free
• No locks, non-blocking
• Inefficient library implemenetation
• After compiler optimizations,
• obstruction-free performs as well as lock-based

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Performance Comparison
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Performance Comparison
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Moral of the story

• Libraries are either
• Low-overhead but hard to use
• Inefficient but easy to use
• Compiler support
• Combines best of both worlds
• Non-local optimization