challenges in concurrent computing

Challenges in Concurrent Computing

Danny Hendler

From the New York Times …

May 7th, 2004 – “Intel said on Friday that it was scrapping its development of a faster Pentium 4 to focus on “dual core” microprocessors, a move that is a shift in the company's business strategy….”

May 8th,2004 – “Intel … [has] decided to focus its development efforts on “dual core” processors …. with two engines instead of one, allowing for greater efficiency because the processor workload is essentially shared.”

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Moore’s lawExponential growth in computing power

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The Future of Computing Speeding up uni-processors is harder and harder Intel, Sun (RIP), AMD, IBM now focusing on “multi-core”

architectures Already, multiprocessors are everywhere:

o Desktops

o Laptop

o Smart phones

o Smart TVs

o ...?

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How can programmers write correct, efficient and scalable algorithms for multiprocessors?

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Synchronization alternatives: coarse-grained locks

Pros Easy to

program

Cons Sequential

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Synchronization alternatives: fine-grained locks

Pros Potentially scalable

Cons Difficult to program

correctlyo Deadlocko Priority inversiono Convoying

Locks do not compose

compare-and-swapLoad-link/store-

conditionalSwap, fetch-and-add

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Synchronization alternatives: nonblocking

synchronization Various progress guarantees

o Wait-freedom, o Lock-freedomo Obstruction-freedom

Generally requires strong synchronization

Pros Potentially scalable Avoids lock-based hazards

Cons Extremely difficult to program

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Synchronization alternatives: Transactional Memory

A (memory) transaction is a sequence of memory reads and writes executed by a single thread that either commits or aborts

If a transaction commits, all the reads and writes appear to have executed atomically

If a transaction aborts, none of its operations take effect

Transaction operations aren't visible until they commit (if they do)

To be supported soon in hardware by Intel and IBM processors

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, מצטיינים 2012למאי 20סמינר

Talk outline

IntroductionNon-blocking algorithms (separate presentation)Randomized mutual exclusion (separate presentation)

Research directions

, מצטיינים 2012למאי 20סמינר

Talk outline

IntroductionNon-blocking algorithms (separate presentation)Randomized mutual exclusion (separate presentation)

Research directions

, מצטיינים 2012למאי 20סמינר

Talk outline

IntroductionNon-blocking algorithms (separate presentation)Randomized mutual exclusion (separate presentation)

Research directions

Devising efficient implementations of transactional memory Includes incorporating OS kernel support

Devising efficient and scalable lock-based and non-blocking data structures Some find their way to Java libraries

Devising algorithms that use few barrier instructions ... , מצטיינים 2012למאי 20סמינר

Research directions: practical

Research directions: theoretical

Lower bounds and impossibility results on deterministic and randomized mutual exclusion

Lower bounds and impossibility results on concurrent data-structures' (counters, stacks, queues, ...) complexity according to various metrics.

Formulating novel realistic metrics for concurrent algorithms and corresponding upper/lower bounds

, מצטיינים 2012למאי 20סמינר

, מצטיינים 2012למאי 20סמינר