Spin Locks and Contention Companion slides for The Art of Multiprocessor Programming by Maurice Herlihy & Nir Shavit Modified by Rajeev Alur for CIS 640, University of Pennsylvania

Art of Multiprocessor Programming 2 Muddy children’s puzzle (Common Knowledge) A group of kids are playing. A stranger walks by and announces “Some of you have mud on your forehead Each kid can see everyone else’s forehead, but not his/her own (and they don’t talk to one another) Stranger says “Raise your hand if you conclude that you have mud on your forehead”. Nobody does. Stranger keeps on repeating the statement. If k kids have muddy foreheads, then exactly these k kids raise their hands after the stranger repeats the statement exactly k times

Art of Multiprocessor Programming 3 Muddy children’s puzzle Why does this happen? For every k: –If >=k kids have muddy foreheads, then in the first k-1 rounds nobody raises hands –If k kids have muddy foreheads, then in the k-th round, exactly muddy kids raise their hands This claim can be proved by induction on k –Base case k=1 –Inductive case (assume for k, and prove for k+1)

Art of Multiprocessor Programming 4 What is the role of stranger’s statement? Let p stand for “> 0 kids have muddy foreheads” Assuming >1 kids are muddy, stranger announcing p does not add to anyone’s information However, without stranger’s announcement, nobody will ever raise their hands So what’s going on Well, the base case for our proof fails, but exactly what information do kids acquire from the stranger’s announcement?

Art of Multiprocessor Programming 5 Common Knowledge E p : Everybody knows p E E p: Everybody knows that everybody knows p E k p : defined similarly (k repetitions) C p : p is “common knowledge”: limit of Everybody knows that everybody knows …. For k =2, each kid knows p, but not Ep, and after stranger’s announcement, each kid knows E p If k kids are muddy, before announcement, each kid knows E k-1 p, but not E k p Stranger makes p the common knowledge

Art of Multiprocessor Programming6 Mutual Exclusion Focus so far: Correctness Models –Accurate –But idealized Protocols –Elegant –Important –But used in practice

Art of Multiprocessor Programming7 New Focus: Performance Models –More complicated –Still focus on principles Protocols –Elegant –Important –And realistic

Art of Multiprocessor Programming8 Kinds of Architectures SISD (Uniprocessor) –Single instruction stream –Single data stream SIMD (Vector) –Single instruction –Multiple data MIMD (Multiprocessors) –Multiple instruction –Multiple data.

Art of Multiprocessor Programming9 Kinds of Architectures SISD (Uniprocessor) –Single instruction stream –Single data stream SIMD (Vector) –Single instruction –Multiple data MIMD (Multiprocessors) –Multiple instruction –Multiple data. Our space (1)

Art of Multiprocessor Programming10 MIMD Architectures Memory Contention Communication Contention Communication Latency Shared Bus memory Distributed

Art of Multiprocessor Programming11 Today: Revisit Mutual Exclusion Think of performance, not just correctness and progress Begin to understand how performance depends on our software properly utilizing the multiprocessor machine’s hardware And get to know a collection of locking algorithms… (1)

Art of Multiprocessor Programming12 What Should you do if you can’t get a lock? Keep trying –“spin” or “busy-wait” –Good if delays are short Give up the processor –Good if delays are long –Always good on uniprocessor (1)

Art of Multiprocessor Programming13 What Should you do if you can’t get a lock? Keep trying –“spin” or “busy-wait” –Good if delays are short Give up the processor –Good if delays are long –Always good on uniprocessor our focus

Art of Multiprocessor Programming14 Basic Spin-Lock CS Resets lock upon exit spin lock critical section...

Art of Multiprocessor Programming15 Basic Spin-Lock CS Resets lock upon exit spin lock critical section... …lock introduces sequential bottleneck

Art of Multiprocessor Programming16 Basic Spin-Lock CS Resets lock upon exit spin lock critical section... …lock suffers from contention

Art of Multiprocessor Programming17 Basic Spin-Lock CS Resets lock upon exit spin lock critical section... Notice: these are distinct phenomena …lock suffers from contention

Art of Multiprocessor Programming18 Test-and-Set Primitive Boolean value Test-and-set (TAS) –Swap true with current value –Return value tells if prior value was true or false Can reset just by writing false TAS aka “getAndSet”

Art of Multiprocessor Programming19 Test-and-Set public class AtomicBoolean { boolean value; public synchronized boolean getAndSet(boolean newValue) { boolean prior = value; value = newValue; return prior; } (5)

Art of Multiprocessor Programming20 Review: Test-and-Set public class AtomicBoolean { boolean value; public synchronized boolean getAndSet(boolean newValue) { boolean prior = value; value = newValue; return prior; } Package java.util.concurrent.atomic

Art of Multiprocessor Programming21 Review: Test-and-Set public class AtomicBoolean { boolean value; public synchronized boolean getAndSet(boolean newValue) { boolean prior = value; value = newValue; return prior; } Swap old and new values

Art of Multiprocessor Programming22 Test-and-Set AtomicBoolean lock = new AtomicBoolean(false) … boolean prior = lock.getAndSet(true)

Art of Multiprocessor Programming23 Test-and-Set AtomicBoolean lock = new AtomicBoolean(false) … boolean prior = lock.getAndSet(true) (5) Swapping in true is called “test-and-set” or TAS

Art of Multiprocessor Programming24 Test-and-Set Locks Locking –Lock is free: value is false –Lock is taken: value is true Acquire lock by calling TAS –If result is false, you win –If result is true, you lose Release lock by writing false

Art of Multiprocessor Programming25 Test-and-set Lock class TASlock { AtomicBoolean state = new AtomicBoolean(false); void lock() { while (state.getAndSet(true)) {} } void unlock() { state.set(false); }}

Art of Multiprocessor Programming26 Test-and-set Lock class TASlock { AtomicBoolean state = new AtomicBoolean(false); void lock() { while (state.getAndSet(true)) {} } void unlock() { state.set(false); }} Lock state is AtomicBoolean

Art of Multiprocessor Programming27 Test-and-set Lock class TASlock { AtomicBoolean state = new AtomicBoolean(false); void lock() { while (state.getAndSet(true)) {} } void unlock() { state.set(false); }} Keep trying until lock acquired

Art of Multiprocessor Programming28 Test-and-set Lock class TASlock { AtomicBoolean state = new AtomicBoolean(false); void lock() { while (state.getAndSet(true)) {} } void unlock() { state.set(false); }} Release lock by resetting state to false

Art of Multiprocessor Programming29 Space Complexity TAS spin-lock has small “footprint” N thread spin-lock uses O(1) space As opposed to O(n) Peterson/Bakery How did we overcome the  (n) lower bound? We used a combined read-write operation…

Art of Multiprocessor Programming30 Performance Experiment –n threads –Increment shared counter 1 million times How long should it take? How long does it take?

Art of Multiprocessor Programming31 Mystery #1 time threads TAS lock Ideal (1) What is going on?

Art of Multiprocessor Programming32 Test-and-Test-and-Set Locks Lurking stage –Wait until lock “looks” free –Spin while read returns true (lock taken) Pouncing state –As soon as lock “looks” available –Read returns false (lock free) –Call TAS to acquire lock –If TAS loses, back to lurking

Art of Multiprocessor Programming33 Test-and-test-and-set Lock class TTASlock { AtomicBoolean state = new AtomicBoolean(false); void lock() { while (true) { while (state.get()) {} if (!state.getAndSet(true)) return; }

Art of Multiprocessor Programming34 Test-and-test-and-set Lock class TTASlock { AtomicBoolean state = new AtomicBoolean(false); void lock() { while (true) { while (state.get()) {} if (!state.getAndSet(true)) return; } Wait until lock looks free

Art of Multiprocessor Programming35 Test-and-test-and-set Lock class TTASlock { AtomicBoolean state = new AtomicBoolean(false); void lock() { while (true) { while (state.get()) {} if (!state.getAndSet(true)) return; } Then try to acquire it

Art of Multiprocessor Programming36 Mystery #2 TAS lock TTAS lock Ideal time threads

Art of Multiprocessor Programming37 Mystery Both –TAS and TTAS –Do the same thing (in our model) Except that –TTAS performs much better than TAS –Neither approaches ideal

Art of Multiprocessor Programming38 Opinion Our memory abstraction is broken TAS & TTAS methods –Are provably the same (in our model) –Except they aren’t (in field tests) Need a more detailed model …

Art of Multiprocessor Programming39 Bus-Based Architectures Bus cache memory cache

Art of Multiprocessor Programming40 Bus-Based Architectures Bus cache memory cache Random access memory (10s of cycles)

Art of Multiprocessor Programming41 Bus-Based Architectures cache memory cache Shared Bus Broadcast medium One broadcaster at a time Processors and memory all “snoop” Bus

Art of Multiprocessor Programming42 Bus-Based Architectures Bus cache memory cache Per-Processor Caches Small Fast: 1 or 2 cycles Address & state information

Art of Multiprocessor Programming43 Jargon Watch Cache hit –“I found what I wanted in my cache” –Good Thing Cache miss –“I had to go all the way to memory for that data” –Bad Thing

Art of Multiprocessor Programming44 Caveat This model is still a simplification –But not in any essential way –Illustrates basic principles Will discuss complexities later

Art of Multiprocessor Programming45 Bus Processor Issues Load Request cache memory cache data

Art of Multiprocessor Programming46 Bus Processor Issues Load Request Bus cache memory cache data Gimme data

Art of Multiprocessor Programming47 cache Bus Memory Responds Bus memory cache data Got your data right here data

Art of Multiprocessor Programming48 Bus Processor Issues Load Request memory cache data Gimme data

Art of Multiprocessor Programming49 Bus Processor Issues Load Request Bus memory cache data Gimme data

Art of Multiprocessor Programming50 Bus Processor Issues Load Request Bus memory cache data I got data

Art of Multiprocessor Programming51 Bus Other Processor Responds memory cache data I got data data Bus

Art of Multiprocessor Programming52 Bus Other Processor Responds memory cache data Bus

Art of Multiprocessor Programming53 Modify Cached Data Bus data memory cachedata (1)

Art of Multiprocessor Programming54 Modify Cached Data Bus data memory cachedata (1)

Art of Multiprocessor Programming55 memory Bus data Modify Cached Data cachedata

Art of Multiprocessor Programming56 memory Bus data Modify Cached Data cache What’s up with the other copies? data

Art of Multiprocessor Programming57 Cache Coherence We have lots of copies of data –Original copy in memory –Cached copies at processors Some processor modifies its own copy –What do we do with the others? –How to avoid confusion?

Art of Multiprocessor Programming58 Write-Back Caches Accumulate changes in cache Write back when needed –Need the cache for something else –Another processor wants it On first modification –Invalidate other entries –Requires non-trivial protocol …

Art of Multiprocessor Programming59 Write-Back Caches Cache entry has three states –Invalid: contains raw seething bits –Valid: I can read but I can’t write –Dirty: Data has been modified Intercept other load requests Write back to memory before using cache

Art of Multiprocessor Programming60 Bus Invalidate memory cachedata

Art of Multiprocessor Programming61 Bus Invalidate Bus memory cachedata Mine, all mine!

Art of Multiprocessor Programming62 Bus Invalidate Bus memory cachedata cache Uh,oh

Art of Multiprocessor Programming63 cache Bus Invalidate memory cachedata Other caches lose read permission

Art of Multiprocessor Programming64 cache Bus Invalidate memory cachedata Other caches lose read permission This cache acquires write permission

Art of Multiprocessor Programming65 cache Bus Invalidate memory cachedata Memory provides data only if not present in any cache, so no need to change it now (expensive) (2)

Art of Multiprocessor Programming66 cache Bus Another Processor Asks for Data memory cachedata (2) Bus

Art of Multiprocessor Programming67 cache data Bus Owner Responds memory cachedata (2) Bus Here it is!

Art of Multiprocessor Programming68 Bus End of the Day … memory cachedata (1) Reading OK, no writing data

Art of Multiprocessor Programming69 Mutual Exclusion What do we want to optimize? –Bus bandwidth used by spinning threads –Release/Acquire latency –Acquire latency for idle lock

Art of Multiprocessor Programming70 Simple TASLock TAS invalidates cache lines Spinners –Miss in cache –Go to bus Thread wants to release lock –delayed behind spinners

Art of Multiprocessor Programming71 Test-and-test-and-set Wait until lock “looks” free –Spin on local cache –No bus use while lock busy Problem: when lock is released –Invalidation storm …

Art of Multiprocessor Programming72 Local Spinning while Lock is Busy Bus memory busy

Art of Multiprocessor Programming73 Bus On Release memory freeinvalid free

Art of Multiprocessor Programming74 On Release Bus memory freeinvalid free miss Everyone misses, rereads (1)

Art of Multiprocessor Programming75 On Release Bus memory freeinvalid free TAS(…) Everyone tries TAS (1)

Art of Multiprocessor Programming76 Problems Everyone misses –Reads satisfied sequentially Everyone does TAS –Invalidates others’ caches Eventually quiesces after lock acquired –How long does this take?

Art of Multiprocessor Programming77 Mystery Explained TAS lock TTAS lock Ideal time threads Better than TAS but still not as good as ideal

Art of Multiprocessor Programming78 Solution: Introduce Delay spin lock time d r1dr1d r2dr2d If the lock looks free But I fail to get it There must be contention Better to back off than to collide again

Art of Multiprocessor Programming79 Dynamic Example: Exponential Backoff time d 2d4d spin lock If I fail to get lock –wait random duration before retry –Each subsequent failure doubles expected wait

Art of Multiprocessor Programming80 Exponential Backoff Lock public class Backoff implements lock { public void lock() { int delay = MIN_DELAY; while (true) { while (state.get()) {} if (!lock.getAndSet(true)) return; sleep(random() % delay); if (delay < MAX_DELAY) delay = 2 * delay; }}}

Art of Multiprocessor Programming81 Exponential Backoff Lock public class Backoff implements lock { public void lock() { int delay = MIN_DELAY; while (true) { while (state.get()) {} if (!lock.getAndSet(true)) return; sleep(random() % delay); if (delay < MAX_DELAY) delay = 2 * delay; }}} Fix minimum delay

Art of Multiprocessor Programming82 Exponential Backoff Lock public class Backoff implements lock { public void lock() { int delay = MIN_DELAY; while (true) { while (state.get()) {} if (!lock.getAndSet(true)) return; sleep(random() % delay); if (delay < MAX_DELAY) delay = 2 * delay; }}} Wait until lock looks free

Art of Multiprocessor Programming83 Exponential Backoff Lock public class Backoff implements lock { public void lock() { int delay = MIN_DELAY; while (true) { while (state.get()) {} if (!lock.getAndSet(true)) return; sleep(random() % delay); if (delay < MAX_DELAY) delay = 2 * delay; }}} If we win, return

Art of Multiprocessor Programming84 Exponential Backoff Lock public class Backoff implements lock { public void lock() { int delay = MIN_DELAY; while (true) { while (state.get()) {} if (!lock.getAndSet(true)) return; sleep(random() % delay); if (delay < MAX_DELAY) delay = 2 * delay; }}} Back off for random duration

Art of Multiprocessor Programming85 Exponential Backoff Lock public class Backoff implements lock { public void lock() { int delay = MIN_DELAY; while (true) { while (state.get()) {} if (!lock.getAndSet(true)) return; sleep(random() % delay); if (delay < MAX_DELAY) delay = 2 * delay; }}} Double max delay, within reason

Art of Multiprocessor Programming86 Spin-Waiting Overhead TTAS Lock Backoff lock time threads

Art of Multiprocessor Programming87 Backoff: Other Issues Good –Easy to implement –Beats TTAS lock Bad –Must choose parameters carefully –Not portable across platforms

Art of Multiprocessor Programming88 Idea Avoid useless invalidations –By keeping a queue of threads Each thread –Notifies next in line –Without bothering the others

Art of Multiprocessor Programming89 Anderson Queue Lock flags next TFFFFFFF idle

Art of Multiprocessor Programming90 Anderson Queue Lock flags next TFFFFFFF acquiring getAndIncrement

Art of Multiprocessor Programming91 Anderson Queue Lock flags next TFFFFFFF acquiring getAndIncrement

Art of Multiprocessor Programming92 Anderson Queue Lock flags next TFFFFFFF acquired Mine!

Art of Multiprocessor Programming93 Anderson Queue Lock flags next TFFFFFFF acquired acquiring

Art of Multiprocessor Programming94 Anderson Queue Lock flags next TFFFFFFF acquired acquiring getAndIncrement

Art of Multiprocessor Programming95 Anderson Queue Lock flags next TFFFFFFF acquired acquiring getAndIncrement

Art of Multiprocessor Programming96 acquired Anderson Queue Lock flags next TFFFFFFF acquiring

Art of Multiprocessor Programming97 released Anderson Queue Lock flags next TTFFFFFF acquired

Art of Multiprocessor Programming98 released Anderson Queue Lock flags next TTFFFFFF acquired Yow!

Art of Multiprocessor Programming99 Anderson Queue Lock class ALock implements Lock { boolean[] flags={true,false,…,false}; AtomicInteger next = new AtomicInteger(0); ThreadLocal mySlot;

Art of Multiprocessor Programming100 Anderson Queue Lock class ALock implements Lock { boolean[] flags={true,false,…,false}; AtomicInteger next = new AtomicInteger(0); ThreadLocal mySlot; One flag per thread

Art of Multiprocessor Programming101 Anderson Queue Lock class ALock implements Lock { boolean[] flags={true,false,…,false}; AtomicInteger next = new AtomicInteger(0); ThreadLocal mySlot; Next flag to use

Art of Multiprocessor Programming102 Anderson Queue Lock class ALock implements Lock { boolean[] flags={true,false,…,false}; AtomicInteger next = new AtomicInteger(0); ThreadLocal mySlot; Thread-local variable

Art of Multiprocessor Programming103 Anderson Queue Lock public lock() { mySlot = next.getAndIncrement(); while (!flags[mySlot % n]) {}; flags[mySlot % n] = false; } public unlock() { flags[(mySlot+1) % n] = true; }

Art of Multiprocessor Programming104 Anderson Queue Lock public lock() { mySlot = next.getAndIncrement(); while (!flags[mySlot % n]) {}; flags[mySlot % n] = false; } public unlock() { flags[(mySlot+1) % n] = true; } Take next slot

Art of Multiprocessor Programming105 Anderson Queue Lock public lock() { mySlot = next.getAndIncrement(); while (!flags[mySlot % n]) {}; flags[mySlot % n] = false; } public unlock() { flags[(mySlot+1) % n] = true; } Spin until told to go

Art of Multiprocessor Programming106 Anderson Queue Lock public lock() { myslot = next.getAndIncrement(); while (!flags[myslot % n]) {}; flags[myslot % n] = false; } public unlock() { flags[(myslot+1) % n] = true; } Prepare slot for re-use

Art of Multiprocessor Programming107 Anderson Queue Lock public lock() { mySlot = next.getAndIncrement(); while (!flags[mySlot % n]) {}; flags[mySlot % n] = false; } public unlock() { flags[(mySlot+1) % n] = true; } Tell next thread to go

Art of Multiprocessor Programming108 Performance Shorter handover than backoff Curve is practically flat Scalable performance FIFO fairness queue TTAS

Art of Multiprocessor Programming109 Anderson Queue Lock Good –First truly scalable lock –Simple, easy to implement Bad –Space hog –One bit per thread Unknown number of threads? Small number of actual contenders?

Art of Multiprocessor Programming110 CLH Lock FIFO order Small, constant-size overhead per thread

Art of Multiprocessor Programming111 Initially false tail idle

Art of Multiprocessor Programming112 Initially false tail idle Queue tail

Art of Multiprocessor Programming113 Initially false tail idle Lock is free

Art of Multiprocessor Programming114 Initially false tail idle

Art of Multiprocessor Programming115 Purple Wants the Lock false tail acquiring

Art of Multiprocessor Programming116 Purple Wants the Lock false tail acquiring true

Art of Multiprocessor Programming117 Purple Wants the Lock false tail acquiring true Swap

Art of Multiprocessor Programming118 Purple Has the Lock false tail acquired true

Art of Multiprocessor Programming119 Red Wants the Lock false tail acquired acquiring true

Art of Multiprocessor Programming120 Red Wants the Lock false tail acquired acquiring true Swap true

Art of Multiprocessor Programming121 Red Wants the Lock false tail acquired acquiring true

Art of Multiprocessor Programming122 Red Wants the Lock false tail acquired acquiring true

Art of Multiprocessor Programming123 Red Wants the Lock false tail acquired acquiring true Implicit Linked list

Art of Multiprocessor Programming124 Red Wants the Lock false tail acquired acquiring true

Art of Multiprocessor Programming125 Red Wants the Lock false tail acquired acquiring true Actually, it spins on cached copy

Art of Multiprocessor Programming126 Purple Releases false tail release acquiring false true false Bingo!

Art of Multiprocessor Programming127 Purple Releases tail released acquired true

Art of Multiprocessor Programming128 Space Usage Let –L = number of locks –N = number of threads ALock –O(LN) CLH lock –O(L+N)

Art of Multiprocessor Programming129 CLH Queue Lock class Qnode { AtomicBoolean locked = new AtomicBoolean(true); }

Art of Multiprocessor Programming130 CLH Queue Lock class CLHLock implements Lock { AtomicReference tail; ThreadLocal myNode = new Qnode(); public void lock() { myNode.locked.set(true); Qnode pred = tail.getAndSet(myNode); while (pred.locked) {} }} (3)

Art of Multiprocessor Programming131 CLH Queue Lock class CLHLock implements Lock { AtomicReference tail; ThreadLocal myNode = new Qnode(); public void lock() { mynode.locked.set(true); Qnode pred = tail.getAndSet(myNode); while (pred.locked) {} }} (3) Queue tail

Art of Multiprocessor Programming132 CLH Queue Lock class CLHLock implements Lock { AtomicReference tail; ThreadLocal myNode = new Qnode(); public void lock() { Qnode pred = tail.getAndSet(myNode); while (pred.locked) {} }} (3) Thread-local Qnode

Art of Multiprocessor Programming133 CLH Queue Lock class CLHLock implements Lock { AtomicReference tail; ThreadLocal myNode = new Qnode(); public void lock() { mynode.locked.set(true); Qnode pred = tail.getAndSet(myNode); while (pred.locked) {} }} (3) Swap in my node

Art of Multiprocessor Programming134 CLH Queue Lock class CLHLock implements Lock { AtomicReference tail; ThreadLocal myNode = new Qnode(); public void lock() { mynode.locked.set(true); Qnode pred = tail.getAndSet(myNode); while (pred.locked) {} }} (3) Spin until predecessor releases lock

Art of Multiprocessor Programming135 CLH Queue Lock Class CLHLock implements Lock { … public void unlock() { myNode.locked.set(false); myNode = pred; } (3)

Art of Multiprocessor Programming136 CLH Queue Lock Class CLHLock implements Lock { … public void unlock() { myNode.locked.set(false); myNode = pred; } (3) Notify successor

Art of Multiprocessor Programming137 CLH Queue Lock Class CLHLock implements Lock { … public void unlock() { myNode.locked.set(false); myNode = pred; } (3) Recycle predecessor’s node

Art of Multiprocessor Programming138 CLH Queue Lock Class CLHLock implements Lock { … public void unlock() { myNode.locked.set(false); myNode = pred; } (3) (Code in book shows how it’s done using myPred reference.)

Art of Multiprocessor Programming139 CLH Lock Good –Lock release affects predecessor only –Small, constant-sized space Bad –Doesn’t work for uncached NUMA architectures

Art of Multiprocessor Programming140 NUMA Architecturs Acronym: –Non-Uniform Memory Architecture Illusion: –Flat shared memory Truth: –No caches (sometimes) –Some memory regions faster than others

Art of Multiprocessor Programming141 NUMA Machines Spinning on local memory is fast

Art of Multiprocessor Programming142 NUMA Machines Spinning on remote memory is slow

Art of Multiprocessor Programming143 CLH Lock Each thread spins on predecessor’s memory Could be far away …

Art of Multiprocessor Programming144 MCS Lock FIFO order Spin on local memory only Small, Constant-size overhead

Art of Multiprocessor Programming145 Initially false idle tail

Art of Multiprocessor Programming146 Acquiring false true acquiring (allocate Qnode) tail

Art of Multiprocessor Programming147 Acquiring false tail false true acquired swap

Art of Multiprocessor Programming148 Acquiring false tail false true acquired

Art of Multiprocessor Programming149 Acquired false tail false true acquired

Art of Multiprocessor Programming150 Acquiring tail false acquired acquiring true swap

Art of Multiprocessor Programming151 Acquiring tail acquired acquiring true false

Art of Multiprocessor Programming152 Acquiring tail acquired acquiring true false

Art of Multiprocessor Programming153 Acquiring tail acquired acquiring true false

Art of Multiprocessor Programming154 Acquiring tail acquired acquiring true false

Art of Multiprocessor Programming155 Acquiring tail acquired acquiring true Yes! false

Art of Multiprocessor Programming156 MCS Queue Lock class Qnode { boolean locked = false; qnode next = null; }

Art of Multiprocessor Programming157 MCS Queue Lock class MCSLock implements Lock { AtomicReference tail; public void lock() { Qnode qnode = new Qnode(); Qnode pred = tail.getAndSet(qnode); if (pred != null) { qnode.locked = true; pred.next = qnode; while (qnode.locked) {} }}} (3)

Art of Multiprocessor Programming158 MCS Queue Lock class MCSLock implements Lock { AtomicReference tail; public void lock() { Qnode qnode = new Qnode(); Qnode pred = tail.getAndSet(qnode); if (pred != null) { qnode.locked = true; pred.next = qnode; while (qnode.locked) {} }}} (3) Make a QNode

Art of Multiprocessor Programming159 MCS Queue Lock class MCSLock implements Lock { AtomicReference tail; public void lock() { Qnode qnode = new Qnode(); Qnode pred = tail.getAndSet(qnode); if (pred != null) { qnode.locked = true; pred.next = qnode; while (qnode.locked) {} }}} (3) add my Node to the tail of queue

Art of Multiprocessor Programming160 MCS Queue Lock class MCSLock implements Lock { AtomicReference tail; public void lock() { Qnode qnode = new Qnode(); Qnode pred = tail.getAndSet(qnode); if (pred != null) { qnode.locked = true; pred.next = qnode; while (qnode.locked) {} }}} (3) Fix if queue was non-empty

Art of Multiprocessor Programming161 MCS Queue Lock class MCSLock implements Lock { AtomicReference tail; public void lock() { Qnode qnode = new Qnode(); Qnode pred = tail.getAndSet(qnode); if (pred != null) { qnode.locked = true; pred.next = qnode; while (qnode.locked) {} }}} (3) Wait until unlocked

Art of Multiprocessor Programming162 MCS Queue Unlock class MCSLock implements Lock { AtomicReference tail; public void unlock() { if (qnode.next == null) { if (tail.CAS(qnode, null) return; while (qnode.next == null) {} } qnode.next.locked = false; }} (3)

Art of Multiprocessor Programming163 MCS Queue Lock class MCSLock implements Lock { AtomicReference tail; public void unlock() { if (qnode.next == null) { if (tail.CAS(qnode, null) return; while (qnode.next == null) {} } qnode.next.locked = false; }} (3) Missing successor?

Art of Multiprocessor Programming164 MCS Queue Lock class MCSLock implements Lock { AtomicReference tail; public void unlock() { if (qnode.next == null) { if (tail.CAS(qnode, null) return; while (qnode.next == null) {} } qnode.next.locked = false; }} (3) If really no successor, return

Art of Multiprocessor Programming165 MCS Queue Lock class MCSLock implements Lock { AtomicReference tail; public void unlock() { if (qnode.next == null) { if (tail.CAS(qnode, null) return; while (qnode.next == null) {} } qnode.next.locked = false; }} (3) Otherwise wait for successor to catch up

Art of Multiprocessor Programming166 MCS Queue Lock class MCSLock implements Lock { AtomicReference queue; public void unlock() { if (qnode.next == null) { if (tail.CAS(qnode, null) return; while (qnode.next == null) {} } qnode.next.locked = false; }} (3) Pass lock to successor

Art of Multiprocessor Programming167 Purple Release false releasing swap false (2)

Art of Multiprocessor Programming168 Purple Release false releasing swap false By looking at the queue, I see another thread is active (2)

Art of Multiprocessor Programming169 Purple Release false releasing swap false By looking at the queue, I see another thread is active I have to wait for that thread to finish (2)

Art of Multiprocessor Programming170 Purple Release false releasing prepare to spin true

Art of Multiprocessor Programming171 Purple Release false releasing spinning true

Art of Multiprocessor Programming172 Purple Release false releasing spinning true false

Art of Multiprocessor Programming173 Purple Release false releasing true Acquired lock false

Art of Multiprocessor Programming174 Abortable Locks What if you want to give up waiting for a lock? For example –Timeout –Database transaction aborted by user

Art of Multiprocessor Programming175 Back-off Lock Aborting is trivial –Just return from lock() call Extra benefit: –No cleaning up –Wait-free –Immediate return

Art of Multiprocessor Programming176 Queue Locks Can’t just quit –Thread in line behind will starve Need a graceful way out Timeout Queue Lock

Art of Multiprocessor Programming177 One Lock To Rule Them All? TTAS+Backoff, CLH, MCS, ToLock… Each better than others in some way There is no one solution Lock we pick really depends on: – the application – the hardware – which properties are important