Reactive Spin-locks: A Self-tuning Approach Phuong Hoai Ha Marina Papatriantafilou Philippas Tsigas I-SPAN ’05, Las Vegas, Dec. 7 th – 9 th, 2005

I-SPAN '052 Outline Mutual exclusion –Overhead –Available reactive spin-locks New reactive spin-lock –Model –Algorithm –Evaluation Conclusions

I-SPAN '053 Mutual exclusion Performance goals: –Low latency –Low contention –…–… Entry sectionCritical sectionExit sectionNoncritical sec. Lock released Requests issued Arbitration Lock sent to winner

I-SPAN '054 Spin-lock categories Arbitrating locks: –Determine who is the next lock-holder in advance, e.g. ticket-locks, queue-locks. –Advantages: Prevent processors from causing bursts in network traffic and high contention on the lock. Non-arbitrating locks: –E.g. Test-and-set locks –Advantages: Exploit locality/cache Tolerate failures in the Entry section.

I-SPAN '055 Arbitrating vs. non-arbitrating locks Interconnection Network Interconnection Network Interconnection Network Interconnection Network

I-SPAN '056 Available reactive spin-lock algorithms Drawbacks: –Their reactive schemes rely on Fixed experimental thresholds –The thresholds frequently become inappropriate in variable and unpredictable environments like multiprogramming systems –E.g. ticket locks with proportional backoff, test-and-test-and- set locks with exponential backoff Known probability distributions of some inputs –The assumption is not usually feasible.

I-SPAN '057 New reactive spin-lock algorithm Ideas –A non-arbitrating lock with adaptive sensible backoff delay. Advantages –Its reactive scheme is self-tuning Neither experimentally tuned thresholds nor probability distributions of inputs are needed –It combines advantages of both arbitrating and non- arbitrating spin-lock categories. It can exploit locality as well as reduce contention on the lock.

I-SPAN '058 Find sensible backoff delay Need to optimize trade-off between: –Latency The interval between a pair of lock-release and lock-acquisition –Contention on the lock This is an online problem. Load on the lock  delay=?

I-SPAN '059 Reactive scheme – Increase delay only when the load on lock is the highest so far, – When increasing delay, increase just enough to keep the competitive ratio c = P - (P-1)/P 1/(P-1) Bounds for loads on the lock: 1  l t  P During a load-rising phase: Similar for load-dropping phase In each load-rising/load-dropping phase, the reactive scheme is competitive with competitive ration c=  (ln(P))

I-SPAN '0510 Interconnection Network Interconnection Network Algorithm The algorithm guarantees mutual exclusion and non- livelock. Its space complexity is log(P).

I-SPAN '0511 Evaluation Benchmarks –Spark98 kernel: lmv –SPLASH-2 suite: Volrend and Radiosity Representatives: –Arbitrating: ticket lock with (tuned) proportional backoff –Non-arbitrating: test-and-test-and-set lock with (tuned) exponential backoff System –A ccNUMA SGI Origin2000 with MHz MIPS R1000 processors.

I-SPAN '0512 Experimental results

I-SPAN '0513 Experimental results (2)

I-SPAN '0514 Experimetal results (3)

I-SPAN '0515 Conclusions We have designed and implemented a new reactive spin-lock: –It is self-tuning. –It combines advantages of both arbitrating and non- arbitrating locks –Its reactive scheme is competitive with c=  (ln(P))  The lock automatically adjusts its backoff delay reasonably according to loads on the lock as well as applications

Thanks for your attention!

I-SPAN '0517 Estimate delay bases Fairness –A fair lock helps parallel application gain performance since the application threads can execute their non- critical section in parallel. –Definition: Heuristic to estimate base l, where a, b are system documented constants and DoCS is the delay outside CS, where n i is #lock-acquisitions of a processor in  t and N is #processors

I-SPAN '0518 NUMA Another parameter that makes the problem harder is NUMA –Latency is much different –E.g. ccNUMA SGI Origin2000

I-SPAN '0519 Model: An online problem A sequence of loads on the lock are unfolded on-the-fly. When observing a load, the algorithm must decide how much its current backoff delay should be lengthened. –If increasing delay too soon, it will waste time on a long delay when the lock becomes available –If not increasing delay in time, it will cause high contention on the lock  it must increase delay at high loads reasonably  Goal is to maximize  t  delay t.load t,where  t  delay t  P

I-SPAN '0520 Algorithm LockType: Initial delay = L.counter x base l The algorithm guarantees mutual exclusion and non-livelock. Its space complexity is log(P). Acquire( Lock pL) L = FAA(pL.L, ) if L.lock then delay = ComputeDelay(L) cond = do sleep(delay) L = pL.L if L.lock then delay = ComputeDelay(L) continue; cond = FAA(pL.L, ) while cond.lock Release( Lock pL) do L = pL.L while not CAS(pL.L,L, )