com.aliyun.openservices.shade.io.prometheus.client.DoubleAdder Maven / Gradle / Ivy
/*
* 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/
*
* Source: http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/DoubleAdder.java?revision=1.12
*/
package com.aliyun.openservices.shade.io.prometheus.client;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
/**
* One or more variables that together maintain an initially zero
* {@code double} sum. When updates (method {@link #add}) are
* contended across threads, the set of variables may grow dynamically
* to reduce contention. Method {@link #sum} (or, equivalently {@link
* #doubleValue}) returns the current total combined across the
* variables maintaining the sum.
*
* This class extends {@link Number}, but does not define
* methods such as {@code equals}, {@code hashCode} and {@code
* compareTo} because instances are expected to be mutated, and so are
* not useful as collection keys.
*
*
jsr166e note: This class is targeted to be placed in
* java.util.concurrent.atomic.
*
* @since 1.8
* @author Doug Lea
*/
public class DoubleAdder extends Striped64 implements Serializable {
private static final long serialVersionUID = 7249069246863182397L;
/**
* Update function. Note that we must use "long" for underlying
* representations, because there is no compareAndSet for double,
* due to the fact that the bitwise equals used in any CAS
* implementation is not the same as double-precision equals.
* However, we use CAS only to detect and alleviate contention,
* for which bitwise equals works best anyway. In principle, the
* long/double conversions used here should be essentially free on
* most platforms since they just re-interpret bits.
*
* Similar conversions are used in other methods.
*/
final long fn(long v, long x) {
return Double.doubleToRawLongBits
(Double.longBitsToDouble(v) +
Double.longBitsToDouble(x));
}
/**
* Creates a new adder with initial sum of zero.
*/
public DoubleAdder() {
}
/**
* Adds the given value.
*
* @param x the value to add
*/
public void add(double x) {
Cell[] as; long b, v; int[] hc; Cell a; int n;
if ((as = cells) != null ||
!casBase(b = base,
Double.doubleToRawLongBits
(Double.longBitsToDouble(b) + x))) {
boolean uncontended = true;
if ((hc = threadHashCode.get()) == null ||
as == null || (n = as.length) < 1 ||
(a = as[(n - 1) & hc[0]]) == null ||
!(uncontended = a.cas(v = a.value,
Double.doubleToRawLongBits
(Double.longBitsToDouble(v) + x))))
retryUpdate(Double.doubleToRawLongBits(x), hc, uncontended);
}
}
/**
* Returns the current sum. The returned value is NOT an
* atomic snapshot; invocation in the absence of concurrent
* updates returns an accurate result, but concurrent updates that
* occur while the sum is being calculated might not be
* incorporated. Also, because floating-point arithmetic is not
* strictly associative, the returned result need not be identical
* to the value that would be obtained in a sequential series of
* updates to a single variable.
*
* @return the sum
*/
public double sum() {
// On concurrent `sum` and `set`, it is acceptable to `get` an outdated `value`.
// On concurrent `sum` and `add`, it is acceptable to `get` an outdated `value`.
// On concurrent `sum` and `set` and `add`, it is possible to `get` an outdated `value`.
// Correctness is guaranteed by `volatile` memory access ordering and visibility semantics.
// Program order:
// - writes in `set` - `busy` (CAS), `cells` (Wc), `base` (Wb), `busy`
// - reads in `sum` - `cells` (Rc), `base` (Rb), `busy`, `cells` (Cc), `base` (Cb)
// Note that:
// - `busy` is written after `cells` and `base`
// - `busy` is read after `cells` and `base`, then `cells` and `base` is re-read after `busy`
// In other words:
// - if we see the write to `busy`, then we must see the write to `cells` and `busy` on re-read
// - if we don't see the write to `busy`, then we must retry as we have no guarantees
// Execution order (in the former case):
// - serial
// - old result - Rc, Rb, Cc, Cb, Wc, Wb
// - new result - Wc, Wb, Rc, Rb, Cc, Cb
// - concurrent
// - old result - Rc, Wc, Rb, Wb, Cc, Cb - retry (superfluous)
// - new result - Wc, Rc, Wb, Rb, Cc, Cb
// - invalid result - Rc, Wc, Wb, Rb, Cc, Cb - retry
// - invalid result - Wc, Rc, Rb, Wb, Cc, Cb - retry
Cell[] as = cells; long b = base;
while (as != null && !(busy == 0 && cells == as && base == b)) {
// busy waiting, retry loop
Thread.yield();
as = cells; b = base;
}
double sum = Double.longBitsToDouble(b);
if (as != null) {
int n = as.length;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null)
sum += Double.longBitsToDouble(a.value);
}
}
return sum;
}
/**
* Resets variables maintaining the sum to zero. This method may
* be a useful alternative to creating a new adder, but is only
* effective if there are no concurrent updates. Because this
* method is intrinsically racy, it should only be used when it is
* known that no threads are concurrently updating.
*/
public void reset() {
internalReset(0L);
}
public void set(double x) {
// On concurrent `set` and `set`, it should be acceptable to lose one `set` measurement.
// On concurrent `set` and `add`, it should be acceptable to lose the `add` measurement.
// Correctness is ensured by different techniques:
// - `set` waits on contention (blocking)
// - `add` avoids contention (non-blocking)
// - `sum` retries on conflicts (non-blocking)
// Performance characteristics by use cases:
// - only `set` - `cells` is always `null` - no allocations
// - only `add` - `cells` allocated on contention
// - mixed `set` and `add` - `cells` allocated on contention, `cells` deallocated on `set`
for (;;) {
Cell[] as;
if ((as = cells) != null) { // have cells
if (busy == 0 && casBusy()) {
try {
if (cells == as) { // recheck under lock
// update cells and base (not atomic)
cells = null;
base = Double.doubleToLongBits(x);
break;
}
} finally {
busy = 0;
}
}
} else { // no cells
// update base (atomic)
base = Double.doubleToLongBits(x);
break;
}
}
}
/**
* Equivalent in effect to {@link #sum} followed by {@link
* #reset}. This method may apply for example during quiescent
* points between multithreaded computations. If there are
* updates concurrent with this method, the returned value is
* not guaranteed to be the final value occurring before
* the reset.
*
* @return the sum
*/
public double sumThenReset() {
Cell[] as = cells;
double sum = Double.longBitsToDouble(base);
base = 0L;
if (as != null) {
int n = as.length;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null) {
long v = a.value;
a.value = 0L;
sum += Double.longBitsToDouble(v);
}
}
}
return sum;
}
/**
* Returns the String representation of the {@link #sum}.
* @return the String representation of the {@link #sum}
*/
public String toString() {
return Double.toString(sum());
}
/**
* Equivalent to {@link #sum}.
*
* @return the sum
*/
public double doubleValue() {
return sum();
}
/**
* Returns the {@link #sum} as a {@code long} after a
* narrowing primitive conversion.
*/
public long longValue() {
return (long)sum();
}
/**
* Returns the {@link #sum} as an {@code int} after a
* narrowing primitive conversion.
*/
public int intValue() {
return (int)sum();
}
/**
* Returns the {@link #sum} as a {@code float}
* after a narrowing primitive conversion.
*/
public float floatValue() {
return (float)sum();
}
private void writeObject(ObjectOutputStream s) throws IOException {
s.defaultWriteObject();
s.writeDouble(sum());
}
private void readObject(ObjectInputStream s)
throws IOException, ClassNotFoundException {
s.defaultReadObject();
busy = 0;
cells = null;
base = Double.doubleToRawLongBits(s.readDouble());
}
}