DREADLOCKS : Efficient Deadlock Detection Maurice Herlihy Joint - - PowerPoint PPT Presentation

DREADLOCKS: Efficient Deadlock Detection

Maurice Herlihy Joint work with Eric Koskinen

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Spin-Locks

• Good for short pauses
• Multiprocessors
• Can be made cache-friendly

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Deadlock

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Waits-For Graph

waiting

• wned

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Deadlock

waiting

• wned

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Dealing with Deadlock

• Avoidance

– Not practical if demands not known in advance

• Detection

– Existing algs require complex probing and poking

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Can Use Timeouts

• Abort if you don’t get lock in time
• Plus

– Easy to implement

• Minus

– How do you choose timeout? – Is choice robust?

• Different platforms/apps?

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Dreadlocks

• Fast, incremental deadlock detection
• Low overhead
• Cache-friendly
• Algorithm …

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Digest

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Digest

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Digest

Uh-oh!

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Algorithm

• Each thread publishes digest

– Initially contains only itself – Always contains itself

• Spins on lock owner’s digest

– Propagates changes (union with own) – Aborts if finds itself in owner’s digest

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publ i c voi d publ i c voi d l ock( ) { l ock( ) { whi l e whi l e ( t r ue t r ue) { ) { whi l e whi l e ( ( owner = st at e. get ( ) ) ! = ( ( owner = st at e. get ( ) ) ! = nul l nul l ) { ) { i f i f ( owner . cont ai ns( m e) ) { ( owner . cont ai ns( m e) ) { t hr ow new t hr ow new Abor t edExcept i on( ) ; Abor t edExcept i on( ) ; } } el se i f el se i f ( owner . changed( ) ) { ( owner . changed( ) ) { m yDi gest . set Uni on( owner , m e) ; m yDi gest . set Uni on( owner , m e) ; } } i f i f ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { m yDi gest . set Si ngl e( m e) ; m yDi gest . set Si ngl e( m e) ; r et ur n r et ur n; } } }

T&T&Set Spin Lock

(1)

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publ i c voi d l ock( ) { publ i c voi d l ock( ) { whi l e ( t r ue) { whi l e ( t r ue) { whi l e whi l e ( ( owner = st at e. get ( ) ) ! = ( ( owner = st at e. get ( ) ) ! = nul l nul l ) { ) { i f ( owner . cont ai ns( m e) ) { i f ( owner . cont ai ns( m e) ) { t hr ow new Abor t edExcept i on( ) ; t hr ow new Abor t edExcept i on( ) ; } el se i f ( owner . changed( ) ) { } el se i f ( owner . changed( ) ) { m yDi gest . set Uni on( owner , m e) ; m yDi gest . set Uni on( owner , m e) ; } } i f ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { i f ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { m yDi gest . set Si ngl e( m e) ; m yDi gest . set Si ngl e( m e) ; r et ur n; r et ur n; } } }

T&T&Set Spin Lock

(1)

Busy lock points to owner digest

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publ i c voi d l ock( ) { publ i c voi d l ock( ) { whi l e ( t r ue) { whi l e ( t r ue) { whi l e ( ( owner = st at e. get ( ) ) ! = nul l ) { whi l e ( ( owner = st at e. get ( ) ) ! = nul l ) { i f i f ( owner . cont ai ns( m e) ) { ( owner . cont ai ns( m e) ) { t hr ow new t hr ow new Abor t edExcept i on( ) ; Abor t edExcept i on( ) ; } el se i f ( owner . changed( ) ) { } el se i f ( owner . changed( ) ) { m yDi gest . set Uni on( owner , m e) ; m yDi gest . set Uni on( owner , m e) ; } } i f ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { i f ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { m yDi gest . set Si ngl e( m e) ; m yDi gest . set Si ngl e( m e) ; r et ur n; r et ur n; } } }

T&T&Set Spin Lock

(1)

Abort if I’m in owner’s digest

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publ i c voi d l ock( ) { publ i c voi d l ock( ) { whi l e ( t r ue) { whi l e ( t r ue) { whi l e ( ( owner = st at e. get ( ) ) ! = nul l ) { whi l e ( ( owner = st at e. get ( ) ) ! = nul l ) { i f ( owner . cont ai ns( m e) ) { i f ( owner . cont ai ns( m e) ) { t hr ow new Abor t edExcept i on( ) ; t hr ow new Abor t edExcept i on( ) ; } } el se i f el se i f ( owner . changed( ) ) { ( owner . changed( ) ) { m yDi gest . set Uni on( owner , m e) ; m yDi gest . set Uni on( owner , m e) ; } } i f ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { i f ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { m yDi gest . set Si ngl e( m e) ; m yDi gest . set Si ngl e( m e) ; r et ur n; r et ur n; } } }

T&T&Set Spin Lock

(1)

Back-propagate changes from

• wner’s digest

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publ i c voi d l ock( ) { publ i c voi d l ock( ) { whi l e ( t r ue) { whi l e ( t r ue) { whi l e ( ( owner = st at e. get ( ) ) ! = nul l ) { whi l e ( ( owner = st at e. get ( ) ) ! = nul l ) { i f ( owner . cont ai ns( m e) ) { i f ( owner . cont ai ns( m e) ) { t hr ow new Abor t edExcept i on( ) ; t hr ow new Abor t edExcept i on( ) ; } el se i f ( owner . changed( ) ) { } el se i f ( owner . changed( ) ) { m yDi gest . set Uni on( owner , m e) ; m yDi gest . set Uni on( owner , m e) ; } } i f i f ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { ( st at e. com par eAndSet ( nul l , m yDi gest ) ) { m yDi gest . set Si ngl e( m e) ; m yDi gest . set Si ngl e( m e) ; r et ur n r et ur n; } } }

T&T&Set Spin Lock

(1)

Clear own digest when lock acquired

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Concurrency

• Digest methods don’t have to be

atomic

– False positives OK if rare – False negatives OK if transient

• Means we can use multi-word bitmaps

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Bit Maps

• 32 (or 64) bit array
• Good for small sets
• Exact membership tests
• Manipulated by shifting & masking

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Bloom Filter

h2 h1

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Testing Membership

member

Yes, maybe…

h2 h1

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No False Negatives

member

No

h2 h1

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False Positivies

member

Yes, maybe

h2 h1

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Experiments

• Dreadlock TTAS lock
• Used for Abstract Locking in Boosting
• Implemented TTAS lock in C
• Built upon Boosting / TL2
• Synthetic benchmark: boosted array

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Throughput Small # Threads

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Throughput Large # Threads

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No Deadlock No Cry

• Can use locks with reckless abandon
• Dreadlocks will detect deadlocks
• So far

– Slightly higher overhead