Condition Variables and Transactional Memory: Problem or Opportunity?

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Condition Variables and Transactional Memory
Problem or Opportunity?

• Polina Dudnik and Michael Swift
• University of Wisconsin, Madison

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Executive Summary

• Problem thread synchronization in TM
• Goal robust synchronization primitive
• State of the art Retry/orelse
• Main point Condition Variables CVs still
  relevant
• Our contributions
• TM-compatible condition variables
• Implementation independent
• Performance comparable to original CVs

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Outline

• Lessons from the past
• New ideas for TM
• Transaction/Condition Variable Interactions
• Design of TM-safe Condition Variables
• Evaluation

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What is the problem?

• How should threads coordinate?
• How should threads wait for an event or state
  change?
• How does this problem change with transactions?

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On the Road to CVs

• Semaphores Dijkstra 65
• Condition Critical Regions (CCRs) Hoare 72

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Hoare on CCR

• I feel this proposal condition critical regions
  is not suitable for operating system
  implementation. My proposed method encourages the
  programmer to ignore the question of which of
  several outstanding requests for a resource
  should be granted.
• Sir Anthony Hoare Belfast 71

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Limitations of CCR

• Atomicity
• Excessive Context Switching
• Restrictive Scheduling

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Solution Condition Variables

• Condition variable queue of waiters
• Associated lock maintains mutual exclusion
• Signaling a CV hint that state has changed
  Mesa semantics Lampson 79
• Multiple CVs provide prioritized wakeup

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Problem Solved

• CCR
• Atomicity Concerns
• Performance Issues
• Restrictive Scheduling

• Condition Variables
• Monitors
• Precise Wakeup
• Explicit Signaling/Multiple CVs

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CVs Limitations

• Nested monitor problem
• How about nested function calls?
• Or waiting on two event queues?
• Retry/orelse solves these problems!

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TM Synchronization Today

• Retry/orelse Harris et al. 2005

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Why not retry?

• Nesting
• Composability
• Restrictive scheduling
• Potentially poor scalability
• Other proposals have the same problems
• atomic CCRs Harris 2003, atomic Wait
  Smaragdakis 2007, X10 conditional atomic blocks
  Charles 2005
• It is too early to give up on condition
  variables!

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Outline

• Lessons from the past
• New ideas
• Transaction/Condition Variable Interactions
• Design of TM-safe Condition Variables
• Evaluation

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TM Condition Variables

• Original condition variables

• int get()
• lock(l)
• getters
• while (!available)
• wait(cv, l)
• getters--
• available false
• unlock(l)
• return contents

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TM Condition Variables

• Convert locks to transactions

• int get()
• begin_tx
• getters
• while (!available)
• wait(cv)
• getters--
• available false
• end_tx
• return contents

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TM Condition Variables

• Wait outside the transaction

• int get()
• begin_tx
• getters
• while (!available)
• end_tx
• wait(cv)
• begin_tx
• getters--
• available false
• end_tx
• return contents

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Lost Wakeup

• Waiting Thread
• begin_tx
• read_state
• prepare_wait()
• end_tx
• wait()

• Signaling Thread
• begin_tx
• update_state
• signal()
• end_tx

TIME
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TM Condition Variables

• Split wait
• prepare wait within transaction
• commit
• complete wait
• restart transaction

• int get()
• begin_tx
• getters
• while (!available)
• prepare_wait(cv)
• end_tx
• complete_wait(cv)
• begin_tx
• getters--
• available false
• end_tx
• return contents

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TM Condition Variables

• What happens with concurrent signalers?

• int get()
• begin_tx
• getters
• while (!available)
• prepare_wait(cv)
• end_tx
• complete_wait(cv)
• begin_tx
• getters--
• available false
• end_tx
• return contents

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Update-Signal Order

• Waiting Thread
• begin_tx
• while (!state)
• prepare_wait()
• end_tx
• wait()
• begin_tx
• end_tx

• Signaling Thread
• begin_tx
• update_state
• signal()
• end_tx

TIME
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TM-CV Implementations
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Outline

• Lessons from the past
• New ideas for TM
• Transaction/Condition Variable Interactions
• Design of TM-safe Condition Variables
• Evaluation

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Workloads

• libMicro
• Stress-test of conditional synchronization
• FluidAnimate PARSEC
• Many critical sections, few condition variables
  ops
• StreamCluster PARSEC
• Few critical sections, few condition variable ops
• Platform Solaris GEMS/LogTM-SE, 16 threads

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Evaluation

• Questions to answer
• Does it work?
• How two versions compare in performance?
• How does performance compare to locks?

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Results
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Evaluation

• Answers to questions
• Does it work?
• YES
• How two versions compare in performance?
• Differ under stress
• How does performance compare to locks?
• Comparable performace

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Conclusions

• Condition Variables are still relevant with TM
• Two implementations of TM-CV
• Different requirements on TM system
• Performance difference subject to potential
  overlap
• Read the paper for
• Implementation independence

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QUESTIONS?
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Backup Slides
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Different Colors
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Example CV usage

• void BeginWrite()
• pthread_mutex_lock(mutex)
• while (NWriters 1 NReaders gt 0)
• WaitingWriters
• pthread_cond_wait(CanWrite,mutex)
• --WaitingWriters
• NWriters 1
• pthread_mutex_unlock(mutex

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Lost Wakeup

• Waiting Thread
• BEGIN_TX
• while (!state)
• prepare_wait()
• COMMIT_TX
• wait()
• BEGIN_TX
• COMMIT_TX

• Signaling Thread
• BEGIN_TX
• update_state
• BEGIN_ESCAPE
• signal()
• END_ESCAPE
• COMMIT_TX