Scheduling Memory Transactions

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)

Transactional Memory Implementations

Hardware Transactional Memory Transactional Memory [Herlihy & Moss, '93] Transactional Memory Coherence and Consistency [Hammond et al., '04] Unbounded transactional memory [Ananian, Asanovic, Kuszmaul,

Leiserson, Lie, '05]…

Software Transactional Memory Software Transactional Memory [Shavit &Touitou, '97] DSTM [Herlihy, Luchangco, Moir, Scherer, '03] RSTM [Marathe et al., '06] WSTM [Harris & Fraser, '03], OSTM [Fraser, '04], ASTM [Marathe,

Scherer, Scott, '05], SXM [Herlihy]…

“Conventional” STM system high-level structure

TM system

OS-scheduler-controlledthreads

Contention

Manager

ContentionDetection

arbitrate

proceed

Abort/retry, waitgreedy

Aggressive

Karma

Polka

Passive

Polite

Talk outline

Preliminaries Memory Transactions Scheduling: Rationale CAR-STM Adaptive TM Schedulers TM-scheduling OS support

Loser resumes execution after pre-determined waiting periodo May resume execution too earlyo May resume execution too late

Repeated collisions occur under high contentiono Livelockso Performance may become worse than single lock

Scheduling-based CM to the rescue.

Conventional conflict resolution policies are often insufficient

TM schedulers: rationale

Transactional threads controlled by TM-aware schedulero Kernel-level, user-level

Richer “tool-box“ for reducing and/or preventing transaction conflicts

Improve performance under high-contention

“Adaptive Transaction Scheduling for transactional memory systems”, Yoo & Lee, SPAA'08

“CAR-STM: Scheduling-based collision avoidance and resolution for software transactional memory”, Dolev, Hendler & Suissa, PODC '08

“Steal-on-abort: dynamic transaction reordering to reduce conflicts in transactional memory”, Ansari , Jarvis, Kirkham, Kotsedilis, Lujan and Watson, HiPEAC'09

The first TM schedulers

Our work

“CAR-STM: Scheduling-based collision avoidance and resolution for software transactional memory” [Dolev, Hendler & Suissa, PODC '08]

“On the impact of Serializing Contention Management on STM performance” [Heber, Hendler & Suissa, OPODIS '09]

“Scheduling support for transactional memory contention management” [Fedorova, Felber, Hendler, Lawall, Maldonado, Marlier Muller & Suissa, PPoPP'10]

CAR-STM (Collision Avoidance and Reduction for STM) Design Goals

Limit Parallelism to a single transaction per core (or hardware thread)

Serialize conflicting transactions

Contention avoidance

CAR-STM high-level architecture

Transaction queue #1

TQ thread

TQ thread

Transaction thread

T-Info

Core #1

Serializing contention

mgr.

Dispatcher

Collision

Avoider

Core #k

Transaction queue #k

TQ-Entry Structure

Transaction queue #1

TQ thread

TQ thread

Transaction thread

T-Info

Core #1

Serializing contention

mgr.

Dispatcher

Collision

Avoider

Core #k

Transaction queue #k

wrapper method

Transaction data

T-Info

Trans. thread

Lock, condition var

Transaction dispatching processEnque transaction in most-conflicting queue. Put thread to sleep, notify TQ thread.

4

4

Transaction execution

TQ thread

Core #i

Transaction queue #i

wrapper method

Transaction data

T-Info

Trans. threadLock, condition var

Serializing Contention Managers

When two transactions collide, fail the newer transaction and move it to the TQ of the older transaction

Fast elimination of live-lock scenarios Two SCMs implemented

o Basic (BSCM) – move failed transaction to end of the other transactions' TQ

o Permanent (PSCM) – Make the failed transaction a subordinate-transaction of the other transaction

PSCM

Ta

Transaction

queue #1

TQ thread

Core #1

PSCM

Tb

Transaction

queue #k

TQ thread

Core #k

Tc

Td Te

Transactions a and b collide, b is older

PSCM

Transaction queue #1

TQ thread

Core #1

PSCM

Tb

Transaction queue #k

TQ thread

Core #k

Ta

Tc

Td Te

Losing transaction and its subordinates are made subordinates of winning transaction

Ta Tc

Execution time: STMBench7R/W dominated workloads

Throughput: STMBench7R/W dominated workloads

CAR-STM Shortcomings

May restrict parallelism too much At most a single transactional thread per

core/hardware-thread Transitive serialization

High overhead

Non-adaptive

Talk outline

Preliminaries Memory Transactions Scheduling: Rationale CAR-STM Adaptive TM Scheduling TM-scheduling OS support

“On the impact of Serializing Contention Management on STM performance”

CBench – synthetic benchmark generating workloads with pre-determined length and abort probability.

A low-overhead serialization mechanism

Better understanding of adaptive serialization algorithms

A Low Overhead Serialization Mechanism(LO-SER)

Transactional threads

Conditionvariables

A Low Overhead Serialization Mechanism (cont'd)

1) t Identifies a collision

2) t calls contention manager: ABORT_OTHER

3) t change status of t' to ABORT (writes that t is winner)

tt'

4) t' identifies it was aborted

A Low Overhead Serialization Mechanism (cont'd)

t

t'

5) t' rolls back transaction and goes to sleep on the condition variable of t

6) Eventually t commits and broadcasts on its condition variable…

A Low Overhead Serialization Mechanism (cont'd)

tt'

Requirements for serialization mechanism

Commit broadcasts only if transaction won a collision since last broadcast (or start of execution)

No waiting cycles (deadlock-freedom)

Avoid race conditions

LO-SER algorithm: data structures

LO-SER algorithm: pseudo-code

LO-SER algorithm: pseudo-code (cont'd)

LO-SER algorithm: pseudo-code (cont'd)

Adaptive algorithms

Collect (local or global) statistics on contention level.

Apply serialization only when contention is high. Otherwise, apply a “conventional” contention-management algorithm.

We find that Stabilized adaptive algorithms perform better.

First adaptive TM scheduler:“Adaptive transaction scheduling for transactional memory systems” [Yoo & Lee, SPAA'08]

CBench Evaluation

CAR-STM incurs high overhead as compared with

other algorithms

Always serializing is bad in medium

contention

Always serializing is best in high contention

Always serializing incurs no overhead in the lack of contention

CBench EvaluationAdaptive

serialization fares well for all

contention levels

CBench Evaluation

Conventional CM performance

degrades for high contention

CBench Evaluation (cont'd)

CAR-STM has best efficiency but worst

throughput

RandomGraph Evaluation

Stabilized algorithm improves

throughput by up to 30%

Throughput and efficiency of conventional algorithms are

bad

Preliminaries Memory Transactions Scheduling: Rationale CAR-STM Adaptive TM Schedulers TM-scheduling OS support

Talk outline

“Scheduling Support for Transactional Memory Contention Management”

Implement CM scheduling support in the kernel scheduler (Linux & OpenSolaris) (Strict) serialization Soft serialization Time-slice extension

Different mechanisms for communication between user-level STM library and kernel scheduler

TM Library / Kernel Communication via Shared Memory Segment (Ser-k)

User code notifies kernel on events such as: transaction start, commit and abort (in which case thread yields)

Kernel code handles moving thread between ready and blocked queues

Soft Serialization

Instead of blocking, reduce loser thread priority and yield

Efficient in scenarios where loser transactions may take a different execution path when retrying (non-determinism)

Priority should be restored upon commit or when conflicting transactions terminate

Time-slice extention

Preemption in the midst of a transaction increases conflict “window of vulnerability”

Defer preemption of transactional threads avoid CPU monopolization by bounding number of

extensions and yielding after commit

May be combined with serialization/soft serialization

Evaluation (STMBench7, 16 core machine)

Conventional CM deteriorates when

threads>cores

Serializing by local spinning is efficient as long as threads ≤

cores

Evaluation - STMBench7 throughput

Serializing by sleeping on condition var is best when threads>cores, since system call

overhead is negligible (long transactions)

Evaluation - STMBench7 aborts data

Conclusions

Scheduling-based CM results in Improved throughput in high contention Improved efficiency in all contention levels LO-SER-based serialization incurs no visible overhead

Lightweight kernel support can improve performance and efficiency

Dynamically selecting best CM algorithm for workload at hand is a challenging research direction

Thank you. Any questions?