Architectural Semantics for Practical Transactional Memory Austen McDonald, et al. International Sym

1
Architectural Semantics for Practical
Transactional MemoryAusten McDonald, et
al.International Symposium on Computer
Architecture (ISCA-33), Boston, MA, June 2006

• Goal provide ISA for existing HTMs that enables
• flexible nestingsubsumption hurts performance
  when libraries use transactions
• sw contention error managemente.g. allow
  different conflict policies for different nesting
  levels, or enable softwarae to see the state for
  debug/error handling before rolling back the
  transaction
• conditional synchronizationuse transactions for
  producer/consumer barriers as well as mutual
  exclusion
• system calls, i/o, runtime exceptions within
  transactions
• Approach
• two-phase commit
• commit, violation, and abort handlers
• closed open nested transactions

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Two-Phase Commit

• Semantics
• xvalidateverify that there were no conflicts
  mark the xaction as validated
• xcommitmark the xaction as committed make its
  writes visible to shared memory
• (in between xvalidate xcommit)can see
  speculative state thread-private dataaccesses
  to shared data should be wrapped within
  open-nested xactions
• Use
• commit handler inserted in between by
  compiler/runtimee.g. finalize system calls
  I/O, coordinate with other code before committing
  (e.g. error checking code)

3
Handlers

• General
• handler stack in thread-private memoryarbitrary
  number of handlers, each with arbitrary number of
  arguments, all pushed onto the stack
• register with handler code to callresponsible
  for calling all handlers
• Commit Handlers
• executed between xvalidate xcommit
• enable sw to finalize tasks at commite.g.
  syscalls with permanent side effects execute as
  commit handlers (write to file or send msgs)
• Violation Handlers
• similar to user-level exceptioncurrent PC saved
  in xvPC register, conflict address saved in
  xvaddr register, then jump to handler
• enable sw contention management on a conflict
• enable sw to undo committed inner open xaction
• Abort handlers
• similar to violation handlers

4
Nesting

• limited levels of nesting
• each nested xaction has own TCB frame
• single stack for each type of handler
• easy to merge handlers of inner closed xactions
  with parent when committing
• commit handlers of inner open xactions are
  executed, violation/abort handlers discarded
• multilevel conflict detection
• xvcurrent register indicate which levels are
  affected
• always jump to innermost xaction violation
  handler
• problem how to track multiple active versions of
  the same data (needed by different levels of
  nesting)
• soln1 multiple R/W bits per cache location to
  track read/write set membership for all nesting
  levels
• R/W bits directly correspond to xvcurrent
• need to merge Ri/Wi into Ri1/Wi1 on
  closed-nested commit (lazily before new child
  level)
• soln2 multiple cache locations for the different
  versions, each with nesting level tag
• level of nesting limited by associativity
• need to do extra compare of nesting level tag to
  get the latest data
• need to merge entry into parents entry on closed
  nested commit (lazily before new child level)
• nested parallelism not supported

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Hardware Transactional State
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(Partial)Transaction Protocol
xcommit
COMMITTED
ld/st
run commit handlers (xchcode)
NONE
VALIDATED
commit (open/outermost)merge read/write set
(closed)erase TCB/logs
no conflicts
xbegin/xbegin_open
conflict detection(NOP if lazy closed inner)
initialize TCB
conflict
xvalidate or
ACTIVE
external ld/st(eager)
xbegin_open/ xbegin
run violation handlers (xvhcode) disable
violation reporting set xvpending if more
violations
xvpending
ld/st
! xvpending
initialize TCB
add to read/write set add to write buffer/undo log
xret enable violation reportingjump xvPC
xabort
! xvpending
ABORTED
run abort handlers (xahcode) disable violation
reporting set xvpending if more violations
xvpending
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Miscellaneous

• immediate loads/stores provided to access
  thread-private datato be used with caution (e.g.
  by compiler, system developer)
• xactions used for prod/cons described in
  atomos-carlstrom-pldi06
• I/O implemented using commit handler
• xaction writes in private buffer
• commit handler writes in shared buffer with open
  nesting