Atom-Aid: Detecting and Surviving Atomicity Violations

Brandon Lucia, Joseph Devietti, Karin Strauss and Luis Ceze

LBA Reading Group 7/3/08Slides by Michelle Goodstein

Motivation

Parallel programs are hard to writeNon-deterministic bugs emerge

Data racesDeadlocksAtomicity Violations

Non-deterministically affect program performanceLarge portion of non-deadlock concurrency bugs

Motivation

Observe that BulkSC, ASO, Implicit Transactions provide implicit atomicityImplicit Atomicity: Two accesses in the same

“chunk” appear atomicLeverage implicit atomicity to avoid

atomicity violations

Outline

MotivationImplicit AtomicityAtom-Aid DesignExperimental Setup & ResultsConclusions

Implicit Atomicity

Property of systems that:Group instructions into larger chunks Support consistency at coarser granularity

Coarse grain consistencyReduces space of possible interleavingsSoftware remains oblivious

Atom-AidWorks if system provides implicit atomicityBuilt on BulkSC

Atomicity Violation

Space of Interleavings

Serializability

Serializable iff order assumed by the programmer produces same final state as actual interleaving

Serializability

Implicit Atomicity and Atomicity Violations

Atomicity-lacking sectionPortion of code from one memory access to

another which should be atomic but is notIntuitively: If all code for an atomicity

violation lies within same “chunk”, violation will never occurNatural hiding

Implicit Atomicity and Atomicity Violations

Formallyc is the default chunk sized is the size of an atomicity-lacking section

If c < d|Atomicity-lacking section| > |default chunk|Pr(hiding violation) = 0

If c ≥ dPr(hiding violation) = (c-d+1)/c

Hiding an Atomicity Violation

d

atomicitylackingsection

Chunk

bdry

Chunk bdry

(c-d)th instr

c

1st memory operation from atomicity lacking section here, entire section atomic

Implications of natural hiding

Atomicity Violation Statistics

Typical atomicity violation: 500-750 instrIf chunk size ~ 2,000 instrs

Pr(hiding violation) around 63-75%“Naturally hiding” violations

Goal: Raise Pr(hiding violation) ~ 100%

Atom-Aid Design

Smart chunking Place chunk boundaries deliberately instead of

arbitrarily Try to increase number of hidden atomicity violations

Intuition Observe accesses to some location a Notice unserializable sequence of accesses to a Next time access a, (potentially) “do something”

“Do something” : Add a chunk boundary

Atom-Aid Design Details

Track:Type and address of memory op Read/Write Sets

RC, WC : current chunk

RP, WP : previous chunkRRC, WRC : remote ops while current chunk executingRRP, WRP : remote ops while prior chunk executing

ChunkBreakSetAddresses with past unserializable accesses

Atom-Aid Design Details

Atom-Aid Design Details

Address A is Added to ChunkBreakSetAnother access to A:

Atom-Aid is alerted Decides whether to break a chunk prematurely

Always breaking a chunk doesn’t help

Maintain two conditionsNever break a chunk twiceIf add address to ChunkBreakSet during a

chunk, cannot break that chunk

Chunk Breaking Flowchart

Atom-Aid Implementation

Built on BulkSCThree sets of signatures added

RC, WC : part of BulkSC RP, WP : copy from RC, WC after commitRRC,WRC : remote downgrades, copy from other

processors sigsRRP,WRP : copy from RCP,WCP after commit

ChunkBreakSet: Signature, or cache augmentation

Experimental Setup

Model BulkSC like system using PINResults average over several runs

95% confidence intervals added around average

Explicitly annotate code Begin/end of atomicity-lacking section

Dynamically check if markers within same chunkYes: atomicity violation hidden

Observations

None of the “natural” violations exceeded 1000 instructions

BankAccount2, CircularList2, LogProc&Sweep2 modified to evaluate larger sizes

Workloads

Natural Hiding (no Atom-Aid)

Atom-Aid: Bug Kernels

Atom-Aid: Applications

Atom-Aid: Signature vs Exact

Atom-Aid as a debugger

Record the PC when Atom-Aid inserts a chunk boundary

Group PCs into line of code, function where it appears

Examine functions in order of highestlowest frequency to look for bugs

Time to find bugs

Conclusions

Implicit atomicity can help programmers by preventing bugs from becoming visible

Atom-Aid leverages implicit atomicity via smart chunk boundaries

Atom-Aid can hide almost 100% of atomicity violations