| 16 |
|
|
| 17 |
|
/** |
| 18 |
|
* A set of variables that together maintain a sum. When updates |
| 19 |
< |
* (method {@link #add}) are contended across threads, the set of |
| 20 |
< |
* adders may grow to reduce contention. Method {@link #sum} returns |
| 21 |
< |
* the current combined total across these adders. This value is |
| 22 |
< |
* <em>NOT</em> an atomic snapshot (concurrent updates may occur while |
| 23 |
< |
* the sum is being calculated), and so cannot be used alone for |
| 24 |
< |
* fine-grained synchronization control. |
| 25 |
< |
* |
| 19 |
> |
* (method {@link #add}) are contended across threads, this set of |
| 20 |
> |
* adder variables may grow dynamically to reduce contention. Method |
| 21 |
> |
* {@link #sum} returns the current combined total across these |
| 22 |
> |
* adders. This value is <em>NOT</em> an atomic snapshot (concurrent |
| 23 |
> |
* updates may occur while the sum is being calculated), and so cannot |
| 24 |
> |
* be used alone for fine-grained synchronization control. |
| 25 |
> |
* |
| 26 |
|
* <p> This class may be applicable when many threads frequently |
| 27 |
|
* update a common sum that is used for purposes such as collecting |
| 28 |
|
* statistics. In this case, performance may be significantly faster |
| 31 |
|
* only one thread can ever update the sum, performance may be |
| 32 |
|
* significantly slower than just updating a local variable. |
| 33 |
|
* |
| 34 |
< |
* @author Doug Lea |
| 34 |
> |
* <p>A StripedAdder may optionally be constructed with a given |
| 35 |
> |
* expected contention level; i.e., the number of threads that are |
| 36 |
> |
* expected to concurrently update the sum. Supplying an accurate |
| 37 |
> |
* value may improve performance by reducing the need for dynamic |
| 38 |
> |
* adjustment. |
| 39 |
> |
* |
| 40 |
> |
* @author Doug Lea |
| 41 |
|
*/ |
| 42 |
|
public class StripedAdder implements Serializable { |
| 43 |
|
private static final long serialVersionUID = 7249069246863182397L; |
| 44 |
|
|
| 45 |
|
/* |
| 46 |
< |
* Overview: We maintain a table of AtomicLongs (padded to reduce |
| 47 |
< |
* false sharing). The table is indexed by per-thread hash codes |
| 48 |
< |
* that are initialized as random values. The table doubles in |
| 49 |
< |
* size upon contention (as indicated by failed CASes when |
| 50 |
< |
* performing add()), but is capped at the nearest power of two >= |
| 51 |
< |
* #cpus: At that point, contention should be infrequent if each |
| 52 |
< |
* thread has a unique index; so we instead adjust hash codes to |
| 53 |
< |
* new random values upon contention rather than expanding. A |
| 54 |
< |
* single spinlock is used for resizing the table as well as |
| 46 |
> |
* Overview: We maintain a table of Atomic long variables. The |
| 47 |
> |
* table is indexed by per-thread hash codes that are initialized |
| 48 |
> |
* to random values. |
| 49 |
> |
* |
| 50 |
> |
* The table doubles in size upon contention (as indicated by |
| 51 |
> |
* failed CASes when performing add()), but is capped at the |
| 52 |
> |
* nearest power of two >= #CPUS. This reflects the idea that, |
| 53 |
> |
* when there are more threads than CPUs, then if each thread were |
| 54 |
> |
* bound to a CPU, there would exist a perfect hash function |
| 55 |
> |
* mapping threads to slots that eliminates collisions. When we |
| 56 |
> |
* reach capacity, we search for this mapping by randomly varying |
| 57 |
> |
* the hash codes of colliding threads. Because search is random, |
| 58 |
> |
* and failures only become known via CAS failures, convergence |
| 59 |
> |
* will be slow, and because threads are typically not bound to |
| 60 |
> |
* CPUS forever, may not occur at all. However, despite these |
| 61 |
> |
* limitations, observed contention is typically very low in these |
| 62 |
> |
* cases. |
| 63 |
> |
* |
| 64 |
> |
* Table entries are of class Adder; a form of AtomicLong padded |
| 65 |
> |
* to reduce cache contention on most processors. Padding is |
| 66 |
> |
* overkill for most Atomics because they are most often |
| 67 |
> |
* irregularly scattered in memory and thus don't interfere much |
| 68 |
> |
* with each other. But Atomic objects residing in arrays will |
| 69 |
> |
* tend to be placed adjacent to each other, and so will most |
| 70 |
> |
* often share cache lines without this precaution. Except for |
| 71 |
> |
* slot adders[0], Adders are constructed upon first use, which |
| 72 |
> |
* further improves per-thread locality and helps reduce (an |
| 73 |
> |
* already large) footprint. |
| 74 |
> |
* |
| 75 |
> |
* A single spinlock is used for resizing the table as well as |
| 76 |
|
* populating slots with new Adders. Upon lock contention, threads |
| 77 |
< |
* just try other slots rather than blocking. We guarantee that at |
| 78 |
< |
* least one slot exists, so retries will eventually find a |
| 79 |
< |
* candidate Adder. |
| 77 |
> |
* try other slots rather than blocking. We guarantee that at |
| 78 |
> |
* least one slot (0) exists, so retries will eventually find a |
| 79 |
> |
* candidate Adder. During these retries, there is increased |
| 80 |
> |
* contention and reduced locality, which is still better than |
| 81 |
> |
* alternatives. |
| 82 |
|
*/ |
| 83 |
|
|
| 84 |
< |
/** |
| 84 |
> |
/** |
| 85 |
|
* Number of processors, to place a cap on table growth. |
| 86 |
|
*/ |
| 87 |
|
static final int NCPU = Runtime.getRuntime().availableProcessors(); |
| 88 |
|
|
| 89 |
|
/** |
| 90 |
< |
* Version of AtomicLong padded to avoid sharing cache |
| 62 |
< |
* lines on most processors |
| 90 |
> |
* Padded version of AtomicLong |
| 91 |
|
*/ |
| 92 |
|
static final class Adder extends AtomicLong { |
| 93 |
|
long p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd; |
| 94 |
|
Adder(long x) { super(x); } |
| 95 |
|
} |
| 96 |
|
|
| 97 |
< |
/** |
| 98 |
< |
* Holder for the thread-local hash code. |
| 97 |
> |
/** |
| 98 |
> |
* Holder for the thread-local hash code. The code starts off with |
| 99 |
> |
* a given random value, but may be set to a different |
| 100 |
> |
* pseudo-random value (using a cheaper but adequate xorshift |
| 101 |
> |
* generator) upon collisions. |
| 102 |
|
*/ |
| 103 |
|
static final class HashCode { |
| 104 |
|
int code; |
| 110 |
|
*/ |
| 111 |
|
static final class ThreadHashCode extends ThreadLocal<HashCode> { |
| 112 |
|
static final Random rng = new Random(); |
| 113 |
< |
public HashCode initialValue() { |
| 113 |
> |
public HashCode initialValue() { |
| 114 |
|
int h = rng.nextInt(); |
| 115 |
|
return new HashCode((h == 0) ? 1 : h); // ensure nonzero |
| 116 |
|
} |
| 124 |
|
static final ThreadHashCode threadHashCode = new ThreadHashCode(); |
| 125 |
|
|
| 126 |
|
/** |
| 127 |
< |
* Table of adders. Initially of size 2; grows to be at most NCPU. |
| 127 |
> |
* Table of adders. Minimum size 2. Size grows to be at most NCPU. |
| 128 |
|
*/ |
| 129 |
|
private transient volatile Adder[] adders; |
| 130 |
|
|
| 136 |
|
private final AtomicInteger mutex; |
| 137 |
|
|
| 138 |
|
/** |
| 139 |
< |
* Marsaglia XorShift for rehashing on collisions |
| 139 |
> |
* Marsaglia XorShift random generator for rehashing on collisions |
| 140 |
|
*/ |
| 141 |
< |
private static int xorShift(int r) { |
| 141 |
> |
private static int xorShift(int r) { |
| 142 |
|
r ^= r << 13; |
| 143 |
|
r ^= r >>> 17; |
| 144 |
|
return r ^ (r << 5); |
| 145 |
|
} |
| 146 |
|
|
| 147 |
|
/** |
| 148 |
< |
* Creates a new adder with initially zero sum. |
| 148 |
> |
* Creates a new adder with zero sum. |
| 149 |
|
*/ |
| 150 |
|
public StripedAdder() { |
| 151 |
< |
Adder[] as = new Adder[2]; |
| 151 |
> |
this(2); |
| 152 |
> |
} |
| 153 |
> |
|
| 154 |
> |
/** |
| 155 |
> |
* Creates a new adder with zero sum, and with stripes presized |
| 156 |
> |
* for the given expected contention level. |
| 157 |
> |
* |
| 158 |
> |
* @param expectedContention the expected number of threads that |
| 159 |
> |
* will concurrently update the sum. |
| 160 |
> |
*/ |
| 161 |
> |
public StripedAdder(int expectedContention) { |
| 162 |
> |
int cap = (expectedContention < NCPU) ? expectedContention : NCPU; |
| 163 |
> |
int size = 2; |
| 164 |
> |
while (size < cap) |
| 165 |
> |
size <<= 1; |
| 166 |
> |
Adder[] as = new Adder[size]; |
| 167 |
|
as[0] = new Adder(0); // ensure at least one available adder |
| 168 |
|
this.adders = as; |
| 169 |
|
this.mutex = new AtomicInteger(); |
| 207 |
|
mutex.set(0); |
| 208 |
|
} |
| 209 |
|
if (created) { |
| 210 |
< |
hc.code = h; // Use this adder next time |
| 210 |
> |
hc.code = h; // Use this adder next time |
| 211 |
|
break; |
| 212 |
|
} |
| 213 |
|
} |
| 218 |
|
* Returns an estimate of the current sum. The result is |
| 219 |
|
* calculated by summing multiple variables, so may not be |
| 220 |
|
* accurate if updates occur concurrently with this method. |
| 221 |
< |
* |
| 221 |
> |
* |
| 222 |
|
* @return the estimated sum |
| 223 |
|
*/ |
| 224 |
|
public long sum() { |
