JDK 17 java.base.jmod - Base Module
JDK 17 java.base.jmod is the JMOD file for JDK 17 Base module.
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JDK 17 Base module source code files are stored in \fyicenter\jdk-17.0.5\lib\src.zip\java.base.
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⏎ java/util/concurrent/atomic/Striped64.java
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
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* 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 java.util.concurrent.atomic;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.Arrays;
import java.util.concurrent.ThreadLocalRandom;
import java.util.function.DoubleBinaryOperator;
import java.util.function.LongBinaryOperator;
/**
* A package-local class holding common representation and mechanics
* for classes supporting dynamic striping on 64bit values. The class
* extends Number so that concrete subclasses must publicly do so.
*/
@SuppressWarnings({"removal","serial"})
abstract class Striped64 extends Number {
/*
* This class maintains a lazily-initialized table of atomically
* updated variables, plus an extra "base" field. The table size
* is a power of two. Indexing uses masked per-thread hash codes.
* Nearly all declarations in this class are package-private,
* accessed directly by subclasses.
*
* Table entries are of class Cell; a variant of AtomicLong padded
* (via @Contended) to reduce cache contention. Padding is
* overkill for most Atomics because they are usually 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 (with a huge negative performance impact) without
* this precaution.
*
* In part because Cells are relatively large, we avoid creating
* them until they are needed. When there is no contention, all
* updates are made to the base field. Upon first contention (a
* failed CAS on base update), the table is initialized to size 2.
* The table size is doubled upon further contention until
* reaching the nearest power of two greater than or equal to the
* number of CPUS. Table slots remain empty (null) until they are
* needed.
*
* A single spinlock ("cellsBusy") is used for initializing and
* resizing the table, as well as populating slots with new Cells.
* There is no need for a blocking lock; when the lock is not
* available, threads try other slots (or the base). During these
* retries, there is increased contention and reduced locality,
* which is still better than alternatives.
*
* The Thread probe fields maintained via ThreadLocalRandom serve
* as per-thread hash codes. We let them remain uninitialized as
* zero (if they come in this way) until they contend at slot
* 0. They are then initialized to values that typically do not
* often conflict with others. Contention and/or table collisions
* are indicated by failed CASes when performing an update
* operation. Upon a collision, if the table size is less than
* the capacity, it is doubled in size unless some other thread
* holds the lock. If a hashed slot is empty, and lock is
* available, a new Cell is created. Otherwise, if the slot
* exists, a CAS is tried. Retries proceed by "double hashing",
* using a secondary hash (Marsaglia XorShift) to try to find a
* free slot.
*
* The table size is capped because, when there are more threads
* than CPUs, supposing that 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 collisions
* only become known via CAS failures, convergence can be slow,
* and because threads are typically not bound to CPUS forever,
* may not occur at all. However, despite these limitations,
* observed contention rates are typically low in these cases.
*
* It is possible for a Cell to become unused when threads that
* once hashed to it terminate, as well as in the case where
* doubling the table causes no thread to hash to it under
* expanded mask. We do not try to detect or remove such cells,
* under the assumption that for long-running instances, observed
* contention levels will recur, so the cells will eventually be
* needed again; and for short-lived ones, it does not matter.
*/
/**
* Padded variant of AtomicLong supporting only raw accesses plus CAS.
*
* JVM intrinsics note: It would be possible to use a release-only
* form of CAS here, if it were provided.
*/
@jdk.internal.vm.annotation.Contended static final class Cell {
volatile long value;
Cell(long x) { value = x; }
final boolean cas(long cmp, long val) {
return VALUE.weakCompareAndSetRelease(this, cmp, val);
}
final void reset() {
VALUE.setVolatile(this, 0L);
}
final void reset(long identity) {
VALUE.setVolatile(this, identity);
}
final long getAndSet(long val) {
return (long)VALUE.getAndSet(this, val);
}
// VarHandle mechanics
private static final VarHandle VALUE;
static {
try {
MethodHandles.Lookup l = MethodHandles.lookup();
VALUE = l.findVarHandle(Cell.class, "value", long.class);
} catch (ReflectiveOperationException e) {
throw new ExceptionInInitializerError(e);
}
}
}
/** Number of CPUS, to place bound on table size */
static final int NCPU = Runtime.getRuntime().availableProcessors();
/**
* Table of cells. When non-null, size is a power of 2.
*/
transient volatile Cell[] cells;
/**
* Base value, used mainly when there is no contention, but also as
* a fallback during table initialization races. Updated via CAS.
*/
transient volatile long base;
/**
* Spinlock (locked via CAS) used when resizing and/or creating Cells.
*/
transient volatile int cellsBusy;
/**
* Package-private default constructor.
*/
Striped64() {
}
/**
* CASes the base field.
*/
final boolean casBase(long cmp, long val) {
return BASE.weakCompareAndSetRelease(this, cmp, val);
}
final long getAndSetBase(long val) {
return (long)BASE.getAndSet(this, val);
}
/**
* CASes the cellsBusy field from 0 to 1 to acquire lock.
*/
final boolean casCellsBusy() {
return CELLSBUSY.compareAndSet(this, 0, 1);
}
/**
* Returns the probe value for the current thread.
* Duplicated from ThreadLocalRandom because of packaging restrictions.
*/
static final int getProbe() {
return (int) THREAD_PROBE.get(Thread.currentThread());
}
/**
* Pseudo-randomly advances and records the given probe value for the
* given thread.
* Duplicated from ThreadLocalRandom because of packaging restrictions.
*/
static final int advanceProbe(int probe) {
probe ^= probe << 13; // xorshift
probe ^= probe >>> 17;
probe ^= probe << 5;
THREAD_PROBE.set(Thread.currentThread(), probe);
return probe;
}
/**
* Handles cases of updates involving initialization, resizing,
* creating new Cells, and/or contention. See above for
* explanation. This method suffers the usual non-modularity
* problems of optimistic retry code, relying on rechecked sets of
* reads.
*
* @param x the value
* @param fn the update function, or null for add (this convention
* avoids the need for an extra field or function in LongAdder).
* @param wasUncontended false if CAS failed before call
* @param index thread index from getProbe
*/
final void longAccumulate(long x, LongBinaryOperator fn,
boolean wasUncontended, int index) {
if (index == 0) {
ThreadLocalRandom.current(); // force initialization
index = getProbe();
wasUncontended = true;
}
for (boolean collide = false;;) { // True if last slot nonempty
Cell[] cs; Cell c; int n; long v;
if ((cs = cells) != null && (n = cs.length) > 0) {
if ((c = cs[(n - 1) & index]) == null) {
if (cellsBusy == 0) { // Try to attach new Cell
Cell r = new Cell(x); // Optimistically create
if (cellsBusy == 0 && casCellsBusy()) {
try { // Recheck under lock
Cell[] rs; int m, j;
if ((rs = cells) != null &&
(m = rs.length) > 0 &&
rs[j = (m - 1) & index] == null) {
rs[j] = r;
break;
}
} finally {
cellsBusy = 0;
}
continue; // Slot is now non-empty
}
}
collide = false;
}
else if (!wasUncontended) // CAS already known to fail
wasUncontended = true; // Continue after rehash
else if (c.cas(v = c.value,
(fn == null) ? v + x : fn.applyAsLong(v, x)))
break;
else if (n >= NCPU || cells != cs)
collide = false; // At max size or stale
else if (!collide)
collide = true;
else if (cellsBusy == 0 && casCellsBusy()) {
try {
if (cells == cs) // Expand table unless stale
cells = Arrays.copyOf(cs, n << 1);
} finally {
cellsBusy = 0;
}
collide = false;
continue; // Retry with expanded table
}
index = advanceProbe(index);
}
else if (cellsBusy == 0 && cells == cs && casCellsBusy()) {
try { // Initialize table
if (cells == cs) {
Cell[] rs = new Cell[2];
rs[index & 1] = new Cell(x);
cells = rs;
break;
}
} finally {
cellsBusy = 0;
}
}
// Fall back on using base
else if (casBase(v = base,
(fn == null) ? v + x : fn.applyAsLong(v, x)))
break;
}
}
private static long apply(DoubleBinaryOperator fn, long v, double x) {
double d = Double.longBitsToDouble(v);
d = (fn == null) ? d + x : fn.applyAsDouble(d, x);
return Double.doubleToRawLongBits(d);
}
/**
* Same as longAccumulate, but injecting long/double conversions
* in too many places to sensibly merge with long version, given
* the low-overhead requirements of this class. So must instead be
* maintained by copy/paste/adapt.
*/
final void doubleAccumulate(double x, DoubleBinaryOperator fn,
boolean wasUncontended, int index) {
if (index == 0) {
ThreadLocalRandom.current(); // force initialization
index = getProbe();
wasUncontended = true;
}
for (boolean collide = false;;) { // True if last slot nonempty
Cell[] cs; Cell c; int n; long v;
if ((cs = cells) != null && (n = cs.length) > 0) {
if ((c = cs[(n - 1) & index]) == null) {
if (cellsBusy == 0) { // Try to attach new Cell
Cell r = new Cell(Double.doubleToRawLongBits(x));
if (cellsBusy == 0 && casCellsBusy()) {
try { // Recheck under lock
Cell[] rs; int m, j;
if ((rs = cells) != null &&
(m = rs.length) > 0 &&
rs[j = (m - 1) & index] == null) {
rs[j] = r;
break;
}
} finally {
cellsBusy = 0;
}
continue; // Slot is now non-empty
}
}
collide = false;
}
else if (!wasUncontended) // CAS already known to fail
wasUncontended = true; // Continue after rehash
else if (c.cas(v = c.value, apply(fn, v, x)))
break;
else if (n >= NCPU || cells != cs)
collide = false; // At max size or stale
else if (!collide)
collide = true;
else if (cellsBusy == 0 && casCellsBusy()) {
try {
if (cells == cs) // Expand table unless stale
cells = Arrays.copyOf(cs, n << 1);
} finally {
cellsBusy = 0;
}
collide = false;
continue; // Retry with expanded table
}
index = advanceProbe(index);
}
else if (cellsBusy == 0 && cells == cs && casCellsBusy()) {
try { // Initialize table
if (cells == cs) {
Cell[] rs = new Cell[2];
rs[index & 1] = new Cell(Double.doubleToRawLongBits(x));
cells = rs;
break;
}
} finally {
cellsBusy = 0;
}
}
// Fall back on using base
else if (casBase(v = base, apply(fn, v, x)))
break;
}
}
// VarHandle mechanics
private static final VarHandle BASE;
private static final VarHandle CELLSBUSY;
private static final VarHandle THREAD_PROBE;
static {
try {
MethodHandles.Lookup l = MethodHandles.lookup();
BASE = l.findVarHandle(Striped64.class,
"base", long.class);
CELLSBUSY = l.findVarHandle(Striped64.class,
"cellsBusy", int.class);
l = java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction<>() {
public MethodHandles.Lookup run() {
try {
return MethodHandles.privateLookupIn(Thread.class, MethodHandles.lookup());
} catch (ReflectiveOperationException e) {
throw new ExceptionInInitializerError(e);
}
}});
THREAD_PROBE = l.findVarHandle(Thread.class,
"threadLocalRandomProbe", int.class);
} catch (ReflectiveOperationException e) {
throw new ExceptionInInitializerError(e);
}
}
}
⏎ java/util/concurrent/atomic/Striped64.java
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File name: java.base-17.0.5-src.zip File size: 8883851 bytes Release date: 2022-09-13 Download
2023-09-26, ≈924🔥, 1💬
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