The Future of Concurrency Theory

Renaissance or Reformation?

(Met dank aan Maurice Herlihy)

Frits Vaandrager

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Le Quatorze Juillet

SAN FRANCISCO, May 7. 2004 - Intel said on Friday that it was scrapping its development of two microprocessors, a move that is a shift in the company's business strategy….

New York Times

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Moore’s Law

Clock speed

flattening sharply

Transistor count still

rising

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Still on some of your desktops: The Uniprocesor

memory

cpu

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In the Enterprise: The Shared Memory

Multiprocessor(SMP)

cache

BusBus

shared memory

cachecache

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Your New Desktop: The Multicore Processor

(CMP)

cache

BusBus

shared memory

cachecacheAll on the same chip

Sun T2000Niagara

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Multicores are Here• “Learn how the multi-core processor

architecture plays a central role in Intel's platform approach. ….”

• “AMD is leading the industry to multi-core technology for the x86 based computing market …”

• “Sun's multicore strategy centers around multi-threaded software. ... “

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Renaissance?• World (re)discovers

Concurrency Theory achievements

• This has already happened (sort-of)

World learns of concurrency results

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Reformation?• Can we respond to

the Real World’s challenges?

• Are we working on problems that matter?

• Can we recognize what’s going to be important?

Bonfire of the Vanities

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In Classic Antiquity• Time cured software

bloat• Double your path

length?– Wait 6 months, until– Processor speed

catches up

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• Multiprocessor companies failed in 80s

• Outstripped by sequential processors

• Field respected, but not taken seriously

Parallelism Didn’t Matter

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The Old Order Lies in Ruins

• Six months means more cores, same clock speed

• Must exploit more parallelism

• No one really knows how to do this

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What Keeps Microsoft and Intel awake at Night?

• If more cores does not deliver more value …

• Then why upgrade?

?

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Washing Machine Science?• Computers could

become like washing machines

• You don’t trade it in every 2 years for a cooler model

• You keep it until it breaks

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No Cores Please, we’re Theorists!

• Computer Science is driven by Moore’s law

• Each year we can do things we couldn’t do last year

• Means funding, students, excitement

!

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With Sudden Relevance Comes Great Responsibility

• Many challenges involve– Concurrent algorithms– Data structures– Formal models– Complexity & lower bounds,– …

• Stuff we’re good at.

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Concurrent Programming Today

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Coarse-Grained Locking

Easily made correct …But not scalable.

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Fine-Grained Locking

Here comes trouble …

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Locks are not Robust

If a thread holdinga lock is delayed

…

No one else can make progress

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Locking Relies on Conventions

• Relation between– Lock bit and object bits– Exists only in programmer’s mind

/* * When a locked buffer is visible to the I/O layer * BH_Launder is set. This means before unlocking * we must clear BH_Launder,mb() on alpha and then * clear BH_Lock, so no reader can see BH_Launder set * on an unlocked buffer and then risk to deadlock. */

Actual comment from Linux

Kernel(hat tip: Bradley Kuszmaul)

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Sadistic Homework

enq(x) deq(y)FIFO queue

No interference if ends “far

enough” apart

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Sadistic Homework

enq(x) deq(y)FIFO queue

Interference OK if ends “close

enough” together

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You Try It …

• One lock?– Too Conservative

• Locks at each end?– Deadlock, too

complicated, etc

• Publishable result?– Once, maybe still?

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Downey’s Room Party Problem

“A dean of students should keep order in the students' house. In order to do this, he can enter a room with too many students (in order to break up a too large party) or he can enter an empty room to conduct a search. Otherwise, the dean may not enter a room. If the dean is in a room, no additional students may enter, but students may leave. In that case, the dean has to stay until all students have left.There is only one dean, and no limitation on the number of students in one room.”

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Don’t be ShyMarc Schoolderman (first year CS student from Nijmegen) discovered bugs in published “solution” of Downey using a model checker

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The Transactional Manifesto

• What we do now is inadequate to meet the multicore challenge

• Research Agenda– Replace locking with a transactional API – Design languages to support this model– Implement the run-time to be fast

enough

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Public void enq(item x) { Qnode q = new Qnode(x); q.next = this.tail; this.tail.next = q;}

Sadistic Homework Revisited

(1)

Write sequential Code

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Public void LeftEnq(item x) { atomic { Qnode q = new Qnode(x); q.next = this.tail; this.tail.next = q; }}

Sadistic Homework Revisited

(1)

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Public void LeftEnq(item x) { atomic { Qnode q = new Qnode(x); q.next = this.tail; this.tail.next = q; }}

Sadistic Homework Revisited

(1)

Enclose in atomic block

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Warning• Not always this

simple– Conditional waits– Enhanced concurrency– Complex patterns

• But often it is– Works for sadistic

homework

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Public void Transfer(Queue<T> q1, q2){ atomic { T x = q1.deq(); q2.enq(x); }}

Composition

(1)

Trivial or what?

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Not All Skittles and Beer• Algorithmic choices

– Lower bounds– Better algorithms

• Language design• Semantic issues

– Like memory models

– Atomicity checking

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Contention Management & Scheduling

• How to resolve conflicts?

• Who moves forward and who rolls back?

• Lots of empirical work but formal work in infancy Judgment of Solomon

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Strong vs Weak Isolation

• How do transactional & non-transactional threads synchronize?

• Similar to memory-model theory?

• Efficient algorithms?

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Single Global Lock Semantics?

• Transactions act as if it acquires SGL

• Good:– Intuitively appealing

• Bad:– What about aborted

transactions?– Expensive?

• Need better models

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Formal Models of Performance

• Asynchrony

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Formal Models of Performance

• Asynchrony• Multi-level

Memory

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Formal Models of Performance

• Asynchrony• Multi-level

Memory• Contention

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Formal Verification• Concurrent

algorithms are hard

• Need routine verification of real algorithms

• Model checking?• Theorem proving?• Probably both

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An Insurmountable Opportunity!

• Multicore forces us to rethink almost everything

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An Insurmountable Opportunity!

• Multicore forces us to rethink almost everything

• The fate of CS as a vibrant field depends on our success

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An Insurmountable Opportunity!

• Multicore forces us to rethink almost everything

• The fate of CS as a vibrant field depends on our success

• Concurrency theory has unique insights & advantages

Intreedag 2008 45

An Insurmountable Opportunity!

• Multicore forces us to rethink almost everything

• The fate of CS as a vibrant field depends on our success

• Concurrency theory has unique insights & advantages

• Are we equal to the task?