/* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package jsr166e; import java.util.Arrays; import java.util.Random; import java.util.concurrent.atomic.AtomicInteger; import java.util.concurrent.atomic.AtomicLong; import java.io.IOException; import java.io.Serializable; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; /** * A set of variables that together maintain a sum. When updates * (method {@link #add}) are contended across threads, this set of * adder variables may grow dynamically to reduce contention. Method * {@link #sum} returns the current combined total across these * adders. This value is NOT an atomic snapshot (concurrent * updates may occur while the sum is being calculated), and so cannot * be used alone for fine-grained synchronization control. * *
This class may be applicable when many threads frequently * update a common sum that is used for purposes such as collecting * statistics. In this case, performance may be significantly faster * than using a shared {@link AtomicLong}, at the expense of using * significantly more space. On the other hand, if it is known that * only one thread can ever update the sum, performance may be * significantly slower than just updating a local variable. * *
A StripedAdder may optionally be constructed with a given
* expected contention level; i.e., the number of threads that are
* expected to concurrently update the sum. Supplying an accurate
* value may improve performance by reducing the need for dynamic
* adjustment.
*
* @author Doug Lea
*/
public class StripedAdder implements Serializable {
private static final long serialVersionUID = 7249069246863182397L;
/*
* Overview: We maintain a table of Atomic long variables. The
* table is indexed by per-thread hash codes that are initialized
* to random values.
*
* The table doubles in size upon contention (as indicated by
* failed CASes when performing add()), but is capped at the
* nearest power of two >= #CPUS. This reflects the idea that,
* when there are more threads than CPUs, then if each thread were
* bound to a CPU, there would exist a perfect hash function
* mapping threads to slots that eliminates collisions. When we
* reach capacity, we search for this mapping by randomly varying
* the hash codes of colliding threads. Because search is random,
* and failures only become known via CAS failures, convergence
* will be slow, and because threads are typically not bound to
* CPUS forever, may not occur at all. However, despite these
* limitations, observed contention is typically very low in these
* cases.
*
* Table entries are of class Adder; a form of AtomicLong padded
* to reduce cache contention on most processors. Padding is
* overkill for most Atomics because they are most often
* irregularly scattered in memory and thus don't interfere much
* with each other. But Atomic objects residing in arrays will
* tend to be placed adjacent to each other, and so will most
* often share cache lines without this precaution. Except for
* slot adders[0], Adders are constructed upon first use, which
* further improves per-thread locality and helps reduce (an
* already large) footprint.
*
* A single spinlock is used for resizing the table as well as
* populating slots with new Adders. Upon lock contention, threads
* try other slots rather than blocking. We guarantee that at
* least one slot (0) exists, so retries will eventually find a
* candidate Adder. During these retries, there is increased
* contention and reduced locality, which is still better than
* alternatives.
*/
/**
* Number of processors, to place a cap on table growth.
*/
static final int NCPU = Runtime.getRuntime().availableProcessors();
/**
* Padded version of AtomicLong
*/
static final class Adder extends AtomicLong {
long p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd;
Adder(long x) { super(x); }
}
/**
* Holder for the thread-local hash code. The code starts off with
* a given random value, but may be set to a different
* pseudo-random value (using a cheaper but adequate xorshift
* generator) upon collisions.
*/
static final class HashCode {
int code;
HashCode(int h) { code = h; }
}
/**
* The corresponding ThreadLocal class
*/
static final class ThreadHashCode extends ThreadLocal