SNZI: Scalable Non-Zero Indicator

Copyright 2007 Sun Microsystems, Inc

SNZI: ScalableNon-Zero Indicator

Yossi Lev (Brown University & Sun Microsystems Laboratories)Joint work with:

Faith Ellen (University of Toronto)Victor Luchangco and

Mark Moir (Sun Microsystems Laboratories)

Presence Indicator

• Threads Arrive and Depart a room

• Query: Is there anybody in there?

Room

Room01

Simple Solution: Counter

• Threads Arrive and Depart a room

• Query: Is there anybody in there?

• Problem: Not Scalable

210Counter

Simple Solution: Counter

• Problem: Not Scalable– Arrive/Depart nonscalable– Query nonscalable

• Observation: Counter Semantics too strong– Answers: how many threads in room– All we asked: are there any threads in room

• Task: Exploit weaker semantics to develop A Scalable Non-Zero Indicator (SNZI)

SNZI Specification

• State: – Surplus: a nonnegative integer

• Operations:– Arrive: increment surplus– Depart: decrement surplus– Query: return whether surplus ≠ 0

• Well Formedness: Surplus ≥ 0

Solution’s Requirements

• Linearizable• Lock Free• Query reads a 1-bit indicator in a given word

– Update using LL/SC• Captures modifications by the outside world

(spurious failures)

– Scalability: Minimize modifications to indicator bit

Agenda

• Two SNZI solutions– Base solution:

Separate indicator and surplus data (Query scalability)– Hierarchical solution:

Implement one SNZI using the other (Arrive/Depart scalability)

• Applications• Performance

Room

Separate Surplus and Indicator:Naïve Attempt

• A simple counter and an indicator bit• Set or UnSet bit after updating counter

– 0 1 transition: set the bit– 1 0 transition: unset the bit

01210Counter

Indicator

Room


• A simple counter and an indicator bit• Set or UnSet bit after updating counter• What can go wrong?

01210Counter

Indicator

Oh oh…I’m in the

dark!

Zzzz…

Zzzz…


• A simple counter and an indicator bit• Set or UnSet bit after updating counter• What can go wrong?

– Delay in setting bit causes unnoticed arrivals– Delay in unsetting bit causes obsolete writes

Separate Surplus and Indicator:Our solution

• 1. Add an “Announce” bit to the counter word:Says: “Indicator needed to be set”– Set announce on 0 1 transition– Clear announce after setting indicator– “Help” setting indicator if announce bit set

Room


01210

false

Counter &Announce bit Indicator


Room


01211true


• 1. Add an “Announce” bit to the counter word:Says: “Indicator needed to be set”– Set announce on 0 1 transition– Clear announce after setting indicator– “Help” setting indicator if announce bit setZzzz…

Room



01212true


Zzzz…

Room



01212

false


Zzzz…

Room


• 2. Prevent obsolete writes:Unset the indicator using LL/SC– Read counter in between, unset only if still 0

01211

false


Room

LL( )

Read Counter,If still 0

SC( , false)


• 2. Prevent obsolete writes:Unset the indicator using LL/SC– Read counter in between, unset only if still 0

01210

false


Indicatorwasn’t writtenin between

Wasn’t writtenbefore LL

Hierarchical SNZI

• Base SNZI took care of Query Scalability• Hierarchical SNZI: Arrive/Depart Scalability

– Implement SNZI using a parent SNZI– Parent surplus > 0 iff a child surplus > 0

• Arrange solution in treeArrive/Depart at leaves, Query the root

Base SNZI

Hierarchical SNZI Filter

Hierarchical SNZI:The Basics

• Invariant: Parent surplus > 0 iff child surplus > 0• Similar to base SNZI, use a counter

– 0 1 transition triggers Arrive at parent– 1 0 transition triggers Depart at parent– Help arriving at parent during 0 1 transition

• Unlike base SNZI– Use intermediate value ½ : 0 ½ 1– Parent is not a bit: it has a surplus

• Undo extra arrivals

Room

Hierarchical SNZI

0Counter Parent SNZI

Surplus

0

• Invariant: Parent surplus > 0 iff child surplus > 0

Increment Counter

Room

Hierarchical SNZI

½Counter Parent SNZI

Surplus

0


Increment Counter: 0 ½

Room

Hierarchical SNZI


Surplus

1



Arrive at Parent

Try ½ 1 transition

Room

Hierarchical SNZI


Surplus

1



Arrive at Parent

Try ½ 1 transition

Room

Hierarchical SNZI


Surplus

1



Arrive at Parent

Try ½ 1 transition

Room

Hierarchical SNZI


Surplus

1



Arrive at Parent

Read Counter as ½:

Help Arrive at Parent

Room

Hierarchical SNZI


Surplus

2



Arrive at Parent

Read Counter as ½:


Try ½ 1 transition

Room

Hierarchical SNZI


Surplus

2



Arrive at Parent

Try ½ 1 transition

Undo: Depart at Parent

Read Counter as ½:


Try ½ 1 transition

Update counter for myself

Only one succeeds: single contribution to parent per 01

1

Caveats

• Counter must have a version number– Avoid the ABA problem– Also in Base algorithm

• Undoing arrives at parent must happen after the executing Arrive incremented the counter– Helper defers undoing an Arrive operation until after

incrementing the counter for itself– Otherwise indicator might “flicker”

Detailed scenarios in the paper

Applications

• Hybrid Transactional Memory (ASPLOS 06)– HW and SW transactions run concurrently– HW transactions pay overhead for conflict detection

with SW transactions– Avoid overhead if no SW transactions are running:

Are there any SW transactions out there?

– Query performance is importantExecuted by HW transactions

Read Indicators

• STM: Read Ownership Is any transaction reading this location?

• In addition:– Reset operation:

• All readers logically “disappear”• New readers can arrive and depart

before old readers departed

• Indicator should work as if old readers are not there

– Needed because writer invalidate old readers

Room

SNZI-R• Added an Epoch to the indicator

– Indicator set iff someone in room that arrived in the current epoch• Operations:

– Reset starts a new epoch– Arrive at current epoch– Depart at the epoch we arrived at– Query returns bit and current epoch

Indicator

1975

Epoch1975Reset2007

Room

SNZI-R• Added an Epoch to the indicator

– Indicator set iff someone in room that arrived in the current epoch• Operations:

– Reset starts a new epoch– Arrive at current epoch– Depart at the epoch we arrived at– Query returns bit and current epoch

Indicator

2007

Epoch

1975

2007

Evaluation

• System: 48-processor Sun FireTM 6800• Experiment:

– Visiting threads: keep arriving and departing.– Query thread: keeps querying the indicator.

• Various tree depths• Measured:

– Visit (Arrive+Depart) and Query throughput

when varying #visiting threads

• Compared with a simple counter implementation

Performance: Query Scalability

SNZI: Query Throughput

0.1

1

10

100

1000

0 8 16 24 32 40 48

Number of Visiting Threads

Que

ries/

us

Simple

Depth 0

Depth 1

Depth 2

Depth 3

Depth 4

Depth 5

g

Performance: Visiting Scalability

SNZI: Visit Throughput (per thread)

1

10

100

1000

10000

0 8 16 24 32 40 48


Vis

its/m

s

Simple

Depth 0

Depth 1

Depth 2

Depth 3

Depth 4

Depth 5

Performance: Visiting Scalability

SNZI: Visit Throughput (per thread)

100

1000

10000

1 2 3 4 5 6


Vis

its/m

s

Simple

Depth 0

Depth 1

Depth 2

Depth 3

Depth 4

Depth 5

• SuperSNZI: Simple counter with a SNZI indicator

• Arrive by modifying counter if not contended• Use SNZI otherwise• Depart accordingly

Performance: SuperSNZI

SuperSNZI: Visits Throughput (per thread)

100

1000

10000

1 2 3 4 5 6

Numbe of Visiting Threads

Vis

its/m

s

Simple

Depth 0

Depth 1

Depth 2

Depth 3

Depth 4

Depth 5

• SuperSNZI: Simple counter with a SNZI indicator

• Arrive by modifying counter if not contended• Use SNZI otherwise• Depart accordingly

Conclusion

• Presence indicator– Can be implemented using a simple counter– Counter semantics too strong Doesn’t scale

– Exploits the weaker semantics we needto provide SNZI: A Scalable Non-Zero Indicator

Perform much better than a simple counter

– Useful in practice

Thank You!

Documents

SNZI: Scalable Non-Zero Indicator