NB-FEB: A Universal Scalable Easy-to-Use Synchronization Primitive forManycore Architectures

Phuong H. Ha (Univ. of Tromsø, Norway) Philippas Tsigas (Chalmers Univ., Sweden)Otto J. Anshus (Univ. of Tromsø, Norway)

Presentation at OPODIS ’09

December 15-18, 2009, Nimes, France

Problem Manycores require scalable strong synchronization

primitives. Conventional strong primitives do not scale well enough

for manycores [UCB Landscape]. Contention on a synchronization variable increases with

the number of processing cores.

OPODIS '09, Nimes, France

2 cores 16 cores 1000 cores

Desired features New synch. primitives for manycores should

be: Scalable

1000s of cores

Universal powerful enough to support any kind of synchronization

(like CAS, LL/SC)

Feasible able to implement in hardware

Easy-to-use

OPODIS '09, Nimes, France

Our main contributions

A novel synch. primitve with all these features

OPODIS '09, Nimes, France

• Non-blocking Full/Empty Bit (NB-FEB)• NBFEB-STM: a non-blocking STM

Road-map

NB-FEB Feasible Universal Scalable Easy-to-use

NBFEB-STM: a non-blocking STM

OPODIS '09, Nimes, France

Feasibility Key idea: slight modifications of a widely deployed primitive

A variant of the original FEB that always returns a value instead of waiting for a conditional flag

OPODIS '09, Nimes, France

Test-Flag-and-SetTFAS( x, v) { (o, flago) (x, flagx); if flagx = false then (x, flagx) (v, true); end if return (o, flago);}

Store-And-ClearSAC( x, v) { (o, flago) (x, flagx); (x, flagx) (v, false); return (o, flago);}Store-And-SetSAS( x, v) { (o, flago) (x, flagx); (x, flagx) (v, true); return (o, flago);}

LoadLoad( x) { return (x, flagx);}

Original FEB: Store-if-Clear-and-SetSICAS(x,v) { Wait for flagx to be false; (x, flagx) (v, true);}

Universality Key idea: write-once objects with 3+ states

TFAS Wait-free consensus, n

OPODIS '09, Nimes, France

Decision (, false);

TFAS_Consensus( proposal) { (first, ) TFAS(Decision, proposal); if first = then return proposal; else return first;}

Scalability Key idea: Combinability

eliminates contention & reduce load Ex: TFAS

OPODIS '09, Nimes, France

x=

TFAS(x,1)TFAS(x,2)

TFAS(x,3)TFAS(x,4)

TFAS(x,1) TFAS(x,3)

TFAS(x,1)

x=1

1 1 1

1

TFAS( var x, value v) atomically { (o, flago) (x, flagx); if flagx = false then (x, flagx) (v, true); end if return (o, flago);}

Note: CAS or LL/SC is not combinable

NB-FEB combining logic

OPODIS '09, Nimes, France

(x, [v1]) Successive primitive with parameter (x, [v2])

Load SAC SAS TFAS

Load Load SAC(v2) SAS(v2) TFAS(v2)

SAC SAC(v1) SAC(v2) SAS(v2) SAS(v2)

SAS SAS(v1) SAC(v2) SAS(v2) SAS(v1)

TFAS TFAS(v1) SAC(v2) SAS(v2) TFAS(v1)

Easy-to-use

Key idea: abstractions for productivity-layer programmers Non-blocking software transactional memory NBFEB-STM

OPODIS '09, Nimes, France

Road-map

NB-FEB Feasible Universal Scalable Easy-to-use

NBFEB-STM: a non-blocking STM

OPODIS '09, Nimes, France

NBFEB-STM

Models Objects are accessed within transactions No nested transactions Garbage collected programming languages (e.g.

Java)

Features Obstruction-free STM Eliminate conventional synch. hot spots in STMs Optimal space complexity (N)

OPODIS '09, Nimes, France

Challenge 1: TFAS-SAC interleavingCAS-based STMs NBFEB-STM

Need SAC to clear pointer’s flag

Overlapping TFAS1 & TFAS2 both may succeed due to SAC’s interference.

violate TMObj’s semantics

OPODIS '09, Nimes, France

TMObj

Old NewTM0

Old NewTM1

Copy

CAS1

Old NewTM2

CopyCAS2

locator

Key idea 1 Keep a linked list of locators

write-once pointer next

OPODIS '09, Nimes, France

locator

Old NewTM0

Old NewTM1

Old NewTM2

eliminate SAC interference

TFAS

TFAS

Challenge 2: Space complexity

OPODIS '09, Nimes, France

CAS-based STMs NBFEB-STM

TMObj

Old NewTM0

Old NewTM1

Old NewTM2

CAS2

locator locator

Old NewTM0

Old NewTM1

Old NewTM2

Key idea 2 Only the head is needed for further accesses break the list of obsolete locators

OPODIS '09, Nimes, France

locator

Old NewTM0

Old NewTM1

Old NewTM2

pi

pi

SAC

Optimal space complexity (N)

Challenge 3: Find the head

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locator

Old NewTM0

Old NewTM1

Old NewTM2

pi

pi

HeadX

Key idea 3 No nested transactions one active locator / thread

OPODIS '09, Nimes, France

i

0

1

……

N

TMObj locator

Old NewTM0

Old NewTM1

Old NewTM2

pi

pi

SAC

Correctness

NBFEB-STM fulfills the essential aspects of TM [Guerraoui, PPoPP ’08] Instantaneous commit Precluding inconsistent views Preserving real-time order

OPODIS '09, Nimes, France

Conclusions

Introduce a novel non-blocking full/empty bit primitive (NB-FEB) Scalable, universal, feasible and easy-to-use

Provide an abstraction, NBFEB-STM, built on top of the primitive.

OPODIS '09, Nimes, France

OPODIS '09, Nimes, France

Thanks for your attention!