Spin Locks and Contention
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1 Spin Locks and Contention Companion slides for The Art of Multiprocessor Programming by Maurice Herlihy & Nir Shavit Modified for Software1 students by Lior Wolf and Mati Shomrat
2 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 Programming 2
3 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 Programming 3
4 MIMD Architectures memory Shared Bus Memory Contention Communication Contention Communication Latency Distributed Art of Multiprocessor Programming 4
5 What Should you do if you can t Keep trying get a lock? spin or busy-wait Good if delays are short Give up the processor [ask another thread to run expensive since switching is pricey] Good if delays are long Always good on uniprocessor Art of Multiprocessor Programming 5 (1)
6 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 Programming 6
7 Basic Spin-Lock CS. spin lock critical section Resets lock upon exit Art of Multiprocessor Programming 7
8 Basic Spin-Lock lock introduces sequential bottleneck CS. spin lock critical section Resets lock upon exit Art of Multiprocessor Programming 8
9 Basic Spin-Lock lock suffers from contention CS. spin lock critical section Resets lock upon exit Art of Multiprocessor Programming 9
10 Basic Spin-Lock lock suffers from contention CS. spin lock critical section Resets lock upon exit Notice: these are distinct phenomena Art of Multiprocessor Programming 10
11 Basic Spin-Lock lock suffers from contention. spin lock CS critical section Resets lock upon exit Seq Bottleneck no parallelism Art of Multiprocessor Programming 11
12 Basic Spin-Lock lock suffers from contention CS. spin lock critical section Resets lock upon exit Contention??? Art of Multiprocessor Programming 12
13 Review: Test-and-Set 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 Programming 13
14 Review: Test-and-Set public class AtomicBoolean { boolean value; public synchronized boolean getandset(boolean newvalue) { boolean prior = value; value = newvalue; return prior; } } Art of Multiprocessor Programming 14(5)
15 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 Programming 15
16 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 Programming 16
17 Review: Test-and-Set AtomicBoolean lock = new AtomicBoolean(false) boolean prior = lock.getandset(true) Art of Multiprocessor Programming 17
18 Review: Test-and-Set AtomicBoolean lock = new AtomicBoolean(false) boolean prior = lock.getandset(true) Swapping in true is called test-and-set or TAS Art of Multiprocessor Programming 18(5)
19 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 Programming 19
20 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 Programming 20
21 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 Programming 21
22 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 Programming 22
23 Test-and-set Lock class TASlock { AtomicBoolean state = new AtomicBoolean(false); Release lock by resetting state to false void lock() { while (state.getandset(true)) {} } void unlock() { state.set(false); }} Art of Multiprocessor Programming 23
24 Space Complexity TAS spin-lock has small footprint N thread spin-lock uses O(1) space As opposed to O(n) in solutions that keep record of who else is interested (we ll see later) Art of Multiprocessor Programming 24
25 Performance Experiment n threads Increment shared counter 1 million times How long should it take? How long does it take? Art of Multiprocessor Programming 25
26 time Graph no speedup because of sequential bottleneck ideal threads Art of Multiprocessor Programming 26
27 time Mystery #1 TAS lock threads Ideal What is going on? Art of Multiprocessor Programming 27 (1)
28 Test-and-Test-and-Set Locks Main idea: Split the following lock line to two while (state.getandset(true)) {} Art of Multiprocessor Programming 28
29 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 Programming 29
30 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 Programming 30
31 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 Programming 31
32 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 Programming 32
33 time Mystery #2 TAS lock TTAS lock Ideal threads Art of Multiprocessor Programming 33
34 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 Programming 34
35 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 Programming 35
36 Bus-Based Architectures cache cache Bus cache memory Art of Multiprocessor Programming 36
37 Bus-Based Architectures Random access memory (10s of cycles) cache cache Bus cache memory Art of Multiprocessor Programming 37
38 Bus-Based Architectures Shared Bus Broadcast medium One broadcaster at a time Processors and memory all snoop cache cache Bus cache memory Art of Multiprocessor Programming 38
39 Per-Processor Caches Small Fast: 1 or 2 cycles Address & state information Bus-Based Architectures cache cache Bus cache memory Art of Multiprocessor Programming 39
40 Jargon Watch Cache hit I found what I wanted in my cache Good Thing Art of Multiprocessor Programming 40
41 Jargon Watch Cache hit I found what I wanted in my cache Good Thing Cache miss I had to shlep all the way to memory for that data Bad Thing Art of Multiprocessor Programming 41
42 Cave Canem This model is still a simplification But not in any essential way Illustrates basic principles Art of Multiprocessor Programming 42
43 Processor Issues Load Request cache cache Bus cache memory data Art of Multiprocessor Programming 43
44 Processor Issues Load Request Gimme data cache cache Bus cache memory data Art of Multiprocessor Programming 44
45 Memory Responds cache cache cache Got your data right here Bus memory data Bus Art of Multiprocessor Programming 45
46 Processor Issues Load Request Gimme data data cache Bus cache memory data Art of Multiprocessor Programming 46
47 Processor Issues Load Request Gimme data data cache Bus cache memory data Art of Multiprocessor Programming 47
48 Processor Issues Load Request I got data data cache Bus cache memory data Art of Multiprocessor Programming 48
49 Other Processor Responds I got data data cache cache Bus Bus memory data Art of Multiprocessor Programming 49
50 Other Processor Responds data cache cache Bus Bus memory data Art of Multiprocessor Programming 50
51 Modify Cached Data data data Bus cache memory data Art of Multiprocessor Programming 51 (1)
52 Modify Cached Data data data Bus cache memory data Art of Multiprocessor Programming 52 (1)
53 Modify Cached Data data data Bus cache memory data Art of Multiprocessor Programming 53
54 Modify Cached Data data data Bus cache What s up with the other copies? memory data Art of Multiprocessor Programming 54
55 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 Programming 55
56 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 Programming 56
57 Write-Back Caches Cache entry has three states Invalid: meaningless content Valid: I can read but I can t write (may be cached elsewhere) Dirty: Data has been modified Intercept other load requests Write back to memory before using cache Art of Multiprocessor Programming 57
58 Invalidate data data Bus cache memory data Art of Multiprocessor Programming 58
59 Invalidate Mine, all mine! data data Bus cache memory data Art of Multiprocessor Programming 59
60 Invalidate Uh,oh cache data data Bus cache memory data Art of Multiprocessor Programming 60
61 Invalidate Other caches lose read permission cache data Bus cache memory data Art of Multiprocessor Programming 61
62 Invalidate Other caches lose read permission cache data Bus cache This cache acquires write permission memory data Art of Multiprocessor Programming 62
63 Invalidate Memory provides data only if not present in any cache, so no need to change it now (expensive) cache data Bus cache memory data Art of Multiprocessor Programming 63(2)
64 Another Processor Asks for Data cache data Bus cache memory data Art of Multiprocessor Programming 64(2)
65 Owner Responds Here it is! cache data Bus cache memory data Art of Multiprocessor Programming 65(2)
66 End of the Day data data Bus cache memory data Reading OK, no writing Art of Multiprocessor Programming 66 (1)
67 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 Programming 67
68 Simple TASLock TAS invalidates cache lines Spinners Miss in cache Go to bus Thread wants to release lock delayed behind spinners Art of Multiprocessor Programming 68
69 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 Programming 69
70 Local Spinning while Lock is Busy busy busy Bus busy memory busy Art of Multiprocessor Programming 70
71 On Release invalid invalid Bus free memory free Art of Multiprocessor Programming 71
72 Everyone misses, rereads On Release miss invalid miss invalid Bus free memory free Art of Multiprocessor Programming 72 (1)
73 Everyone tries TAS On Release TAS( ) invalid TAS( ) invalid Bus free memory free Art of Multiprocessor Programming 73 (1)
74 Problems Everyone misses Reads satisfied sequentially Everyone does TAS Invalidates others caches Eventually quiesces after lock acquired How long does this take? Linearly with the number of processors Art of Multiprocessor Programming 74
75 time Mystery Explained TAS lock TTAS lock threads Ideal Better than TAS but still not as good as ideal Art of Multiprocessor Programming 77
76 Solution: Introduce Delay If the lock looks free But I fail to get it There must be lots of contention Better to back off than to collide again time r 2 d r 1 d d spin lock Art of Multiprocessor Programming 78
77 Dynamic Example: Exponential Backoff time 4d 2d d spin lock If I fail to get lock wait random duration before retry Each subsequent failure doubles expected wait Art of Multiprocessor Programming 79
78 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 Programming 80
79 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 Programming 81
80 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 Programming 82
81 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 Programming 83
82 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 Programming 84
83 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 Programming 85
84 time Spin-Waiting Overhead TTAS Lock Backoff lock threads Art of Multiprocessor Programming 86
85 Backoff: Other Issues Good Easy to implement Beats TTAS lock Bad Must choose parameters carefully Not portable across platforms Art of Multiprocessor Programming 87
86 Idea Avoid useless invalidations By keeping a queue of threads Each thread Notifies next in line Without bothering the others Art of Multiprocessor Programming 88
87 Anderson Queue Lock next idle flags T F F F F F F F Art of Multiprocessor Programming 89
88 Anderson Queue Lock next acquiring getandincrement flags T F F F F F F F Art of Multiprocessor Programming 90
89 Anderson Queue Lock next acquiring getandincrement flags T F F F F F F F Art of Multiprocessor Programming 91
90 Anderson Queue Lock next acquired Mine! flags T F F F F F F F Art of Multiprocessor Programming 92
91 Anderson Queue Lock next acquired acquiring flags T F F F F F F F Art of Multiprocessor Programming 93
92 Anderson Queue Lock next acquired acquiring flags getandincrement T F F F F F F F Art of Multiprocessor Programming 94
93 Anderson Queue Lock next acquired acquiring flags getandincrement T F F F F F F F Art of Multiprocessor Programming 95
94 Anderson Queue Lock next acquired acquiring flags T F F F F F F F Art of Multiprocessor Programming 96
95 Anderson Queue Lock next released acquired flags T T F F F F F F Art of Multiprocessor Programming 97
96 Anderson Queue Lock next released acquired flags Yow! T T F F F F F F Art of Multiprocessor Programming 98
97 Anderson Queue Lock class ALock implements Lock { boolean[] flags={true,false,,false}; AtomicInteger next = new AtomicInteger(0); int[] slot = new int[n]; Art of Multiprocessor Programming 99
98 Anderson Queue Lock class ALock implements Lock { boolean[] flags={true,false,,false}; AtomicInteger next = new AtomicInteger(0); int[] slot = new int[n]; One flag per thread Art of Multiprocessor Programming 100
99 Anderson Queue Lock class ALock implements Lock { boolean[] flags={true,false,,false}; AtomicInteger next = new AtomicInteger(0); int[] slot = new int[n]; Next flag to use Art of Multiprocessor Programming 101
100 Anderson Queue Lock class ALock implements Lock { boolean[] flags={true,false,,false}; AtomicInteger next = new AtomicInteger(0); ThreadLocal<Integer> myslot; Thread-local variable Art of Multiprocessor Programming 102
101 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 Programming 103
102 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 Programming 104
103 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 Programming 105
104 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 Programming 106
105 Anderson Queue Lock public lock() { Tell next thread to go myslot = next.getandincrement(); while (!flags[myslot % n]) {}; flags[myslot % n] = false; } public unlock() { flags[(myslot+1) % n] = true; } Art of Multiprocessor Programming 107
106 Performance TTAS queue Shorter handover than backoff Curve is practically flat Scalable performance FIFO fairness Art of Multiprocessor Programming 108
107 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 Programming 109
108 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 Art of Multiprocessor Programming 110
109 This work is licensed under a Creative Commons Attribution- ShareAlike 2.5 License. You are free: to Share to copy, distribute and transmit the work to Remix to adapt the work Under the following conditions: Attribution. You must attribute the work to The Art of Multiprocessor Programming (but not in any way that suggests that the authors endorse you or your use of the work). Share Alike. If you alter, transform, or build upon this work, you may distribute the resulting work only under the same, similar or a compatible license. For any reuse or distribution, you must make clear to others the license terms of this work. The best way to do this is with a link to Any of the above conditions can be waived if you get permission from the copyright holder. Nothing in this license impairs or restricts the author's moral rights. Art of Multiprocessor Programming 111
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