THE UNIVERSAL TRANSACTIONAL MEMORY CONSTRUCTION Jons-Tobias - - PowerPoint PPT Presentation

THE UNIVERSAL TRANSACTIONAL MEMORY CONSTRUCTION

Jons-Tobias Wamhoff and Christof Fetzer Dresden University of Technology, Germany

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The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

MOTIVATION

• Universal construction
• Shows how to convert sequential algorithm into concurrent

wait-free algorithm

• Can one base such a construction on TM?
• Wait-free progress for all correct operations
• Tolerate crashes and non-terminating operations

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The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

ASYNCHRONOUS MULTICORE SYSTEM MODEL (AMSM)

• Asynchronous model with features of

current multi-core systems

• Performance counters for executed

cycles per thread

• Size of memory is bound
• Operations (transactions) can be

executed by any thread

• Compare-and-Swap (CAS),

Fetch-and-Increment (FAI)

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Thread n Operating System Thread 1 Perf Ctr 1 Perf Ctr n

Code Code

Transactional Memory Shared Memory

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

Thread n Operating System Thread 1 Perf Ctr 1 Perf Ctr n

Code Code

Transactional Memory Shared Memory

AMSM CRASH FAILURES

• Threads can crash (stop taking steps)

caused by:

• Operating system, hardware, signal:

not detectable in AMSM

• Program code (bug): detected by

runtime and converted in exception

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The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

AMSM PROGRAMS

• Log defines total order on a sequence of

invocations (operations)

• FIFO sequential execution
• Multiple logs executed in parallel
• Invocations (transactions) applied to shared

memory

• Sequential object to transform into wait-free

linerizable object

• Transactions can be non-terminating
• Terminate correct transactions within finite

number of steps

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Log 1 Log 0 Invocation 0

txBegin txRead txWrite txCommit

S1

txBegin txRead txWrite txCommit

S2

txBegin txRead txWrite txCommit

S3

txBegin txRead txWrite txCommit

S4

Invocation 1

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

INVOCATION PROCESSING

How can one process invocations from k logs using n threads in a finite number of steps?

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Thread n Operating System Thread 1 Perf Ctr 1 Perf Ctr n

Code Code

Transactional Memory Shared Memory

Log 1 Log 0 Invocation 0

txBegin txRead txWrite txCommit

S1

txBegin txRead txWrite txCommit

S2

txBegin txRead txWrite txCommit

S3

txBegin txRead txWrite txCommit

S4

Invocation 1

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

UNIVERSAL TRANSACTIONAL MEMORY CONSTRUCTION

• Universal construction transforms program from one valid

state to another valid state

• Use TM to isolate modifications until commit
• Schedule transactions on threads for wait-free progress

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The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

STATES

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Memory ... ...

1 2 3 4

ct key

S0

chunks

1 1 2 3 1 4 S1 1 1 2 2 2 3 1 4 S2 3 1 2 2 2 3 3 4 3 S3

current

NULL

State History

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

TRANSACTION SCHEDULING ONTO THREADS

• A thread‘s commit time is FAI(CT)
• SCT = ftx(SCT-1)
• CT-1 = i * k + l
• Log l = (CT-1) mod k
• Invocation i = (CT-1) div k

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Log k Log 1 Invocation 0

txBegin txRead txWrite txCommit

S1

txBegin txRead txWrite txCommit

Sk

txBegin txRead txWrite txCommit

Sk+1

txBegin txRead txWrite txCommit

S2k

Invocation 1 ...

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

TRANSACTION SCHEDULING ONTO THREADS

• ct4 = FAI(CT4)
• S4 = ftx(S3)
• ct4-1 = i * k + l
• Log l1 = 3 mod 2
• Invocation i1 = 3 div 2

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Log 1 Log 0 Invocation 0

txBegin txRead txWrite txCommit

S1

txBegin txRead txWrite txCommit

S2

txBegin txRead txWrite txCommit

S3

txBegin txRead txWrite txCommit

S4

Invocation 1

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

txBegin(ct)

• Clone current head of state history
• Chunks are contained only by

reference

• Set new commit time
• Keep base version

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1 1 2 2 2 3 1 4 S2

current

NULL 1 1 2 2 2 3 1 4 S4 B2

ct key

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

txWrite(addr, val)

• Identify chunk using hash function
• Clone chunk if still reference
• Update clone with value

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1 1 2 2 2 3 1 4 S2

current

NULL 1 1 4 2 2 3 1 4 S4 B2

ct key

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

txRead(addr)

• Identify chunk using hash function
• Keep commit time of chunk in read-

set if written by predecessor

• Return value of address

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1 1 2 2 2 3 1 4 S2

current

NULL 1 1 4 2 2 3 1 4 S4 B2

ct key

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

txCommit

• Wait until SCT-1 is available
• Validate against SCT-1 that read-

set versions unchanged

• Failure: abort and retry
• Success: update chunk

references and append state to history using CAS

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1 1 2 2 2 3 1 4 S2 3 1 2 2 2 3 3 4 3 S3

current

1 1 4 2 2 3 1 4 S4 B2

ct key

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

txCommitProceeding(ct)

• Wait until SCT-1 is available
• Clone SCT-1 and append as SCT

using CAS

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ct key

1 1 2 2 2 3 1 4 S2 3 1 2 2 2 3 3 4 3 S3

current

3 1 2 2 2 3 3 4 3 S4

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

UNIVERSAL CONSTRUCTION FOR THE GOOD CASE

16 Thread 2 Thread 3 Thread 4 Operating System Thread 1 Log 1 Log 0 Log 2

CT8 s1=ftx(S0) s3=ftx(S2) s2=ftx(S1) ct4 ct5 ct6 ct7 s4=ftx(S3) s5=ftx(S4) s6=ftx(S5) NULL

Invocation 0 Invocation 1 Invocation 2

S3

Global State next CT

NULL NULL

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

DEALING WITH THREAD CRASHES

• Thread crashes not detectable in

AMSM: need helping

• Waiting in txCommit for SCT-1 used

for helping

• Process SCT-1 = ftx(SCT-2)
• Progress as long as one thread survives

17 Log 1 Log 0

txBegin txRead txWrite txCommit

S3

txBegin txRead txWrite txCommit

S4

Invocation 1 Thread 2 Thread 1

ct3 ct4 S2

current

NULL S4

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

DEALING WITH TRANSACTION CRASHES

• Transaction crashes detectable in

AMSM: throw exception

• Considered as persistent failures
• Commit proceeding state
• No further invocations can be added

to the log

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Log 1 Log 0

txBegin txRead txWrite txCommit

S3

txBegin txRead txWrite txCommit

S4

Invocation 1

S2

current

S3=S2 NULL S5=S4

Empty

NULL

Invocation 2

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

TOLERATING NON- TERMINATING TRANSACTIONS

• Non-terminating transaction: ftx(SCT-1) never returns
• Use performance counter to assign quota of steps

for invocation

• When exceeded commit proceeding and retry

with larger quota

• Quota is unknown and usually large
• Bounded number of states (bound memory)

means bound number of steps for transaction to complete

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Log 1 Log 0

txBegin txRead txWrite txCommit

S3

txBegin txRead txWrite txCommit

S4

Invocation 1

S2

current

S3=S2 NULL NULL

Invocation 2

txBegin txRead txWrite txCommit

S5 t=12 t=13 t=23

The Universal Transactional Memory Construction TRANSACT‘11, San Jose, CA

CONCLUSION

• Wait-free progress under AMSM by:
• Decoupling of work from threads for helping
• Isolating changes in transactions to mask failures

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