Rich%20Transactions%20on%20Reasonable%20Hardware

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Rich Transactions onReasonable Hardware

• J. Eliot B. Moss
• Univ. of Massachusetts, Amherst
• moss_at_cs.umass.edu

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Language/Virtual Machine Challenges

• Common uses of TM simple for programmer
• Semantics simple, clean, useable
• Realize potential programming benefits
• Advanced uses possible and clean
• Overcome hardware limitations
• Size of transactions
• Semantics of whats built in

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The Simplest Case

• Setting Java (can explore other platforms)
• Credit to Harris and Fraser for simple form
• atomic statements
• Executes statements
• Either entirely or not at all
• Repeated until succeeds exactly-once effect
• A single state transition (as seen by others)
• Provides a simple model ACI of ACID

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Mapping Simple Form to Hardware

• Transaction read write set of cache lines
• Conflict if write line touched by other txn
• Abort drop set of records, buffered writes
• Commit drop records, install writes
• Issue Txns with large sets
• Need graceful overflow to memory
• Perhaps even to virtual memory (disk)

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Semantics of Nesting Actions

• Harris Fraser Aggregate into one big set
• This one big txn commits only at top level
• Closed Nesting
• Sub-txn can abort without top aborting
• Sub-txn commits into tops sets
• Retry high-conflict part w/o aborting all
• Good for libraries, composing modules
• Allows alternate strategies, optional parts, ...
• m-of-n replica commit, ...

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Semantics of Nesting Actions (2)

• No additional syntax, richer semantics
• Hardware support needed
• Tag lines with txn id (SMTs need anyway)
• Multiple copies of same line different ids
• Txn id analogous to address space ID
• Can get by with a handful of bits
• Add parent/child txn table
• More complex txn conflict detect logic
• Txn relationships can be cached (like TLB)

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Predicated (Conditional) Actions

• Harris and Fraser construct
• atomic (predicate) stmts
• stmts executed only when predicate is true
• Evaluate predicate (as part of an action)
• If true, continue to evaluate stmts
• predicate and stmts part of same action
• If false, wait until predicate is true
• In hardware, set to lowest priority, then ...
• Wait until read/write set conflict and retry

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Predicated Actions Hardware Need

• Low priority notion, so waiter aborted
• Giving up after a while, controls on how long
• Possibly maintaining state for waiting thread
• No explicit notify conflict is the indicator
• If predicate tends to be cheap, just retry
• Kicking up to s/w for more explicit handling

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Beyond Closed Nesting

• Closed nesting only builds largers sets
• Consider modifying a cache or table
• New/deleted entries touched ok, since want to
  block access to modifications
• Things along path in data structure creates
  often undesirable conflicts
• Need Distinguish physical data structure from
  its logical contents

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Beyond Closed Nesting Open Nesting

• Open nesting
• Open nested action truly commits changes
• Leaves explicit access control information
• Access control managed by software
• Abort of enclosing txn requires undo
• Retry of enclosing txn requires redo

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Open Nesting Syntax/Design

• Possible syntax
• open atomic stmts undo u redo r
• Implementation strategy
• Language run-time builds undo/redo lists
• When needed, they are run as txns
• Need access to txn ids, conflict tests, etc.
• Transactions become explicit, can be named and
  manipulated

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Some Applications of Open Nesting

• Shared table structures (already noted)
• May help support GC
• Open nested commit updates (some) data held by
  enclosing atomic blocks supports GC work done by
  executing thread
• Admittedly idea still rough ...
• GC from other threads may need reflective
  ability to see/update transactional data of
  application threads ...

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Other Desired Features

• atend actions happen at end of top-level
• May help with issue of I/O
• Connection to OS supported durability
• Specially designated pages commit call
• Lock coupling release locks on parts of data
  structures walked but not needed
• E.g. searching list linearly to remove item

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Other Desired Features (2)

• Checkpointing and rollback to checkpoints
• Concurrency spawning parallel sub-txns
• Transaction per loop iteration
• Control on ordering relationships
• H/WOS desire cheap thread spawning
• Implication multiple threads, each working
  within the same transaction hierarchy
• Again, explicit txn ids make this easier

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Non-Normal Outcomes

• Harris and Fraser exception semantics
• Throwing is a transaction success
• Usual Java semantics
• Claim More useful to undo visible effects
• Abort writes to previously existing objects
• Retain objects created in failing txn
• Allows return of reason for failure
• Example Denied credit

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Non-Normal Outcomes (2)

• Example syntax fail with new Exc(...)
• Challenge keeping some state, tossing rest
• May be able to implement this with combination of
  open nesting and run-time system support
• In open nested action, create copy of newly
  allocated reachable objects and commit the open
  nested txn