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/*
* Quasar: lightweight strands and actors for the JVM.
* Copyright (c) 2013-2015, Parallel Universe Software Co. All rights reserved.
*
* This program and the accompanying materials are dual-licensed under
* either the terms of the Eclipse Public License v1.0 as published by
* the Eclipse Foundation
*
* or (per the licensee's choosing)
*
* under the terms of the GNU Lesser General Public License version 3.0
* as published by the Free Software Foundation.
*/
/*
* Based on j.u.c.LinkedTransferQueue:
*/
/*
* 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 co.paralleluniverse.strands.channels;
import co.paralleluniverse.common.util.DelegatingEquals;
import co.paralleluniverse.common.util.UtilUnsafe;
import co.paralleluniverse.fibers.SuspendExecution;
import co.paralleluniverse.strands.Strand;
import co.paralleluniverse.strands.Synchronization;
import co.paralleluniverse.strands.Timeout;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
/**
*
* @author Doug Lea
* @author pron
*/
public class TransferChannel implements StandardChannel, Selectable, Synchronization {
private Throwable closeException;
private volatile boolean sendClosed;
private boolean receiveClosed;
private static final Object CHANNEL_CLOSED = new Object();
private static final Object NO_MATCH = new Object();
private static final Object LOST = new Object();
public TransferChannel() {
}
@Override
public final int capacity() {
return 0;
}
@Override
public boolean isSingleProducer() {
return false;
}
@Override
public boolean isSingleConsumer() {
return false;
}
@Override
public boolean equals(Object other) {
if (other instanceof DelegatingEquals)
return other.equals(this);
return super.equals(other);
}
@Override
public void send(Message message) throws SuspendExecution, InterruptedException {
if (message == null)
throw new IllegalArgumentException("message is null");
if (isSendClosed())
return;
if (xfer1(message, true, SYNC, 0) != null)
throw new InterruptedException(); // failure possible only due to interrupt
}
@Override
public boolean send(Message message, long timeout, TimeUnit unit) throws SuspendExecution, InterruptedException {
if (message == null)
throw new IllegalArgumentException("message is null");
if (isSendClosed())
return true;
if (xfer1(message, true, TIMED, unit.toNanos(timeout)) == null)
return true;
if (!Strand.interrupted())
return false;
throw new InterruptedException();
}
@Override
public boolean send(Message message, Timeout timeout) throws SuspendExecution, InterruptedException {
return send(message, timeout.nanosLeft(), TimeUnit.NANOSECONDS);
}
@Override
public boolean trySend(Message message) {
if (message == null)
throw new IllegalArgumentException("message is null");
if (isSendClosed())
return true;
boolean res = (trySendOrReceive(message, true) == null);
return res;
}
@Override
public void close() {
if (!sendClosed) {
sendClosed = true;
signalWaitersOnClose();
}
}
@Override
public void close(Throwable t) {
if (!sendClosed) {
closeException = t;
sendClosed = true;
signalWaitersOnClose();
}
}
private void setReceiveClosed() {
if (!receiveClosed)
this.receiveClosed = true;
}
private Message closeValue() {
if (closeException != null)
throw new ProducerException(closeException);
return null;
}
@Override
public Message tryReceive() {
if (receiveClosed)
return null;
final Object m = trySendOrReceive(null, false);
if (m == CHANNEL_CLOSED)
return closeValue();
return (Message) m;
}
@Override
public Object register() {
// for queues, a simple registration is always a receive
return receive0();
}
@Override
public Object register(SelectAction action) {
return xfer0((SelectActionImpl) action);
}
@Override
public boolean tryNow(Object token) {
Token t = (Token) token;
return t.n.isMatched();
}
@Override
public void unregister(Object token) {
Token t = (Token) token;
if (token == null)
return;
Node p = t.n;
Node pred = t.pred;
Object x = p.item;
if (!((x == p) || ((x == null) == p.isData))) {
if (p.casItem(x, p)) // cancel
unsplice(pred, p);
}
}
@Override
public Message receive() throws SuspendExecution, InterruptedException {
if (receiveClosed)
return closeValue();
Object m = xfer1(null, false, SYNC, 0);
if (m != null) {
if (m == CHANNEL_CLOSED)
return closeValue();
return (Message) m;
}
Thread.interrupted();
throw new InterruptedException();
}
protected Message receiveInternal(long timeout, TimeUnit unit) throws SuspendExecution, InterruptedException {
if (receiveClosed)
return closeValue();
Object m = xfer1(null, false, TIMED, unit.toNanos(timeout));
if (m != null || !Strand.interrupted()) {
if (m == CHANNEL_CLOSED)
return closeValue();
return (Message) m;
}
throw new InterruptedException();
}
@Override
public Message receive(long timeout, TimeUnit unit) throws SuspendExecution, InterruptedException {
return receiveInternal(timeout, unit);
}
@Override
public Message receive(Timeout timeout) throws SuspendExecution, InterruptedException {
return receiveInternal(timeout.nanosLeft(), TimeUnit.NANOSECONDS);
}
boolean isSendClosed() {
return sendClosed;
}
@Override
public boolean isClosed() {
if (receiveClosed)
return true;
// racy, but that's OK because we don't guarantee anything if we return false
if (sendClosed && size() == 0) {
setReceiveClosed();
return true;
}
return false;
}
private void signalWaitersOnClose() {
for (Node p = head; p != null;) {
if (!p.isMatched()) {
if (!p.isData) {
if (p.casItem(null, CHANNEL_CLOSED)) // match waiting requesters with CHANNEL_CLOSED
Strand.unpark(p.waiter, this); // ... and wake 'em up
} else
p.tryMatchData();
}
Node n = p.next;
if (n != p)
p = n;
else
p = head;
}
}
/////////////////////////////////////////
private static final long serialVersionUID = -3223113410248163686L;
/**
* True if on multiprocessor
*/
private static final boolean MP = Runtime.getRuntime().availableProcessors() > 1;
/**
* The number of times to spin (with randomly interspersed calls
* to Strand.yield) on multiprocessor before blocking when a node
* is apparently the first waiter in the queue. See above for
* explanation. Must be a power of two. The value is empirically
* derived -- it works pretty well across a variety of processors,
* numbers of CPUs, and OSes.
*/
private static final int FRONT_SPINS = 1 << 7;
/**
* The number of times to spin before blocking when a node is
* preceded by another node that is apparently spinning. Also
* serves as an increment to FRONT_SPINS on phase changes, and as
* base average frequency for yielding during spins. Must be a
* power of two.
*/
private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
/**
* The maximum number of estimated removal failures (sweepVotes)
* to tolerate before sweeping through the queue unlinking
* cancelled nodes that were not unlinked upon initial
* removal. See above for explanation. The value must be at least
* two to avoid useless sweeps when removing trailing nodes.
*/
static final int SWEEP_THRESHOLD = 32;
/**
* Queue nodes. Uses Object, not E, for items to allow forgetting
* them after use. Relies heavily on Unsafe mechanics to minimize
* unnecessary ordering constraints: Writes that are intrinsically
* ordered wrt other accesses or CASes use simple relaxed forms.
*/
static final class Node {
final boolean isData; // false if this is a request node
volatile SelectActionImpl sa;
volatile Object item; // initially non-null if isData; CASed to match
volatile Node next;
volatile Strand waiter; // null until waiting
// CAS methods for fields
final boolean casNext(Node cmp, Node val) {
return NEXT.compareAndSet(this, cmp, val);
}
final boolean casItem(Object cmp, Object val) {
// assert cmp == null || cmp.getClass() != Node.class;
return ITEM.compareAndSet(this, cmp, val);
}
/**
* Constructs a new node. Uses relaxed write because item can
* only be seen after publication via casNext.
*/
Node(SelectActionImpl sa) {
ITEM.set(this, sa.message()); // UNSAFE.putObject(this, itemOffset, sa.message()); // relaxed write
SA.set(this, sa); // UNSAFE.putObject(this, saOffset, sa); // relaxed write
this.isData = sa.isData();
}
Node(Object item, boolean isData) {
ITEM.set(this, item); // UNSAFE.putObject(this, itemOffset, item); // relaxed write
this.isData = isData;
}
/**
* Links node to itself to avoid garbage retention. Called
* only after CASing head field, so uses relaxed write.
*/
final void forgetNext() {
NEXT.set(this, this); // UNSAFE.putObject(this, nextOffset, this);
}
/**
* Sets item to self and waiter to null, to avoid garbage
* retention after matching or cancelling. Uses relaxed writes
* because order is already constrained in the only calling
* contexts: item is forgotten only after volatile/atomic
* mechanics that extract items. Similarly, clearing waiter
* follows either CAS or return from park (if ever parked;
* else we don't care).
*/
final void forgetContents() {
ITEM.set(this, this); // UNSAFE.putObject(this, itemOffset, this);
SA.set(this, null); // UNSAFE.putObject(this, saOffset, null);
WAITER.set(this, null); // UNSAFE.putObject(this, waiterOffset, null);
}
/**
* Returns true if this node has been matched, including the
* case of artificial matches due to cancellation.
*/
final boolean isMatched() {
Object x = item;
return (x == this) || ((x == null) == isData);
}
/**
* Returns true if this is an unmatched request node.
*/
final boolean isUnmatchedRequest() {
return !isData && item == null;
}
/**
* Returns true if a node with the given mode cannot be
* appended to this node because this node is unmatched and
* has opposite data mode.
*/
final boolean cannotPrecede(boolean haveData) {
boolean d = isData;
Object x;
return d != haveData && (x = item) != this && (x != null) == d;
}
/**
* Tries to artificially match a data node -- used by remove.
*/
final boolean tryMatchData() {
// assert isData;
Object x = item;
if (x != null && x != this && casItem(x, null)) {
Strand.unpark(waiter, this);
return true;
}
return false;
}
boolean lease() {
final SelectActionImpl sa = this.sa;
if (sa == null)
return true;
return sa.lease();
}
void returnLease() {
final SelectActionImpl sa = this.sa;
if (sa != null)
sa.returnLease();
}
void won() {
final SelectActionImpl sa = this.sa;
if (sa != null) {
Object x = item;
sa.setItem(x == CHANNEL_CLOSED ? null : x);
sa.won();
}
}
private static final long serialVersionUID = -3375979862319811754L;
private static final VarHandle ITEM;
private static final VarHandle SA;
private static final VarHandle NEXT;
private static final VarHandle WAITER;
static {
try {
MethodHandles.Lookup l = MethodHandles.lookup();
Class k = Node.class;
ITEM = l.findVarHandle(k, "item", Object.class);
SA = l.findVarHandle(k, "sa", SelectActionImpl.class);
NEXT = l.findVarHandle(k, "next", k);
WAITER = l.findVarHandle(k, "waiter", Strand.class);
} catch (ReflectiveOperationException e) {
throw new ExceptionInInitializerError(e);
}
}
// // Unsafe mechanics
// private static final sun.misc.Unsafe UNSAFE;
// private static final long itemOffset;
// private static final long saOffset;
// private static final long nextOffset;
// private static final long waiterOffset;
//
// static {
// try {
// UNSAFE = UtilUnsafe.getUnsafe();
// Class k = Node.class;
// itemOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("item"));
// saOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("sa"));
// nextOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("next"));
// waiterOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("waiter"));
// } catch (Exception e) {
// throw new Error(e);
// }
// }
}
static class Token {
final Node n;
final Node pred;
public Token(Node n, Node pred) {
this.n = n;
this.pred = pred;
}
}
/**
* head of the queue; null until first enqueue
*/
transient volatile Node head;
/**
* tail of the queue; null until first append
*/
private transient volatile Node tail;
/**
* The number of apparent failures to unsplice removed nodes
*/
private transient volatile int sweepVotes;
// CAS methods for fields
private boolean casTail(Node cmp, Node val) {
return TAIL.compareAndSet(this, cmp, val); // UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
}
private boolean casHead(Node cmp, Node val) {
return HEAD.compareAndSet(this, cmp, val); // UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
}
private boolean casSweepVotes(int cmp, int val) {
return SWEEP_VOTES.compareAndSet(this, cmp, val); // UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val);
}
/*
* Possible values for "how" argument in xfer method.
*/
private static final int NOW = 0; // for untimed poll, tryTransfer
private static final int ASYNC = 1; // for offer, put, add
private static final int SYNC = 2; // for transfer, take
private static final int TIMED = 3; // for timed poll, tryTransfer
@SuppressWarnings("unchecked")
static E cast(Object item) {
// assert item == null || item.getClass() != Node.class;
return (E) item;
}
private Object trySendOrReceive(Message e, boolean haveData) {
if (haveData && (e == null))
throw new NullPointerException();
Object item = tryMatch(null, e, haveData);
if (item != NO_MATCH)
return item;
return e;
}
/**
* Implements all queuing methods. See above for explanation.
*
* @param e the item or null for take
* @param haveData true if this is a put, else a take
* @param how NOW, ASYNC, SYNC, or TIMED
* @param nanos timeout in nanosecs, used only if mode is TIMED
* @return an item if matched, else e
* @throws NullPointerException if haveData mode but e is null
*/
private Object xfer1(Message e, boolean haveData, int how, long nanos) throws SuspendExecution {
assert how == SYNC || how == TIMED;
if (haveData && (e == null))
throw new NullPointerException();
Node s = null; // the node to append, if needed
retry:
for (;;) { // restart on append race
Object item = tryMatch(null, e, haveData);
if (item != NO_MATCH)
return item;
if (s == null)
s = new Node(e, haveData);
Node pred = tryAppend(s, haveData);
if (pred == null)
continue retry; // lost race vs opposite mode
if (!haveData && sendClosed) {
s.item = CHANNEL_CLOSED;
unsplice(pred, s);
setReceiveClosed();
return CHANNEL_CLOSED;
}
return awaitMatch(s, pred, e, (how == TIMED), nanos);
}
}
private Token receive0() {
Node s = new Node(null, false); // the node to append
retry:
for (;;) { // restart on append race
Object item = tryMatch(null, null, false);
if (item != NO_MATCH) {
s.item = item;
return new Token(s, null);
}
Node pred = tryAppend(s, false);
if (pred == null)
continue retry; // lost race vs opposite mode
if (sendClosed) {
s.item = CHANNEL_CLOSED;
unsplice(pred, s);
setReceiveClosed();
return new Token(s, null);
}
requestUnpark(s, Strand.currentStrand());
return new Token(s, pred);
}
}
private Token xfer0(SelectActionImpl e) {
final boolean haveData = e.isData();
Node s = null; // the node to append, if needed
retry:
for (;;) { // restart on append race
if (!e.lease())
return null;
if (isClosed() || (isSendClosed() && e.isData())) {
e.setItem(null);
e.won();
return null;
}
Object item = tryMatch(e, e.message(), haveData);
if (item == LOST)
return null;
if (item != NO_MATCH) {
e.setItem(item == CHANNEL_CLOSED ? null : (Message) item);
e.won();
return null;
}
e.returnLease();
if (s == null) {
s = new Node(e);
requestUnpark(s, e.selector().getWaiter());
}
Node pred = tryAppend(s, haveData);
if (pred == null)
continue retry; // lost race vs opposite mode
return new Token(s, pred);
}
}
private Object tryMatch(SelectActionImpl sa, Message e, boolean haveData) {
boolean closed = isSendClosed(); // must be read before trying to match so as not to miss puts
for (Node h = head, p = h; p != null;) { // find & match first node
boolean isData = p.isData;
Object item = p.item;
if (item != p && (item != null) == isData) { // unmatched
if (isData == haveData) // can't match
break;
// avoid deadlock by ordering lease acquisition:
// if p requires a lease and is of lower hashCode than sa, we return sa's lease, acquire p's, and then reacquire sa's.
SelectActionImpl sa2 = p.sa;
boolean leasedp;
if (sa != null && sa2 != null
&& sa2.selector().id < sa.selector().id) {
sa.returnLease();
leasedp = sa2.lease();
if (!sa.lease()) {
if (leasedp)
sa2.returnLease();
return LOST;
}
} else
leasedp = p.lease();
if (leasedp) {
if (p.casItem(item, e)) { // match
p.won();
for (Node q = p; q != h;) {
Node n = q.next; // update by 2 unless singleton
if (head == h && casHead(h, n == null ? q : n)) {
h.forgetNext();
break;
} // advance and retry
if ((h = head) == null
|| (q = h.next) == null || !q.isMatched())
break; // unless slack < 2
}
Strand.unpark(p.waiter, this);
return item;
} else
p.returnLease();
}
}
Node n = p.next;
p = (p != n) ? n : (h = head); // Use head if p offlist
}
if (closed) {
assert !haveData;
setReceiveClosed();
return CHANNEL_CLOSED;
}
return NO_MATCH;
}
/**
* Tries to append node s as tail.
*
* @param s the node to append
* @param haveData true if appending in data mode
* @return null on failure due to losing race with append in
* different mode, else s's predecessor, or s itself if no
* predecessor
*/
private Node tryAppend(Node s, boolean haveData) {
for (Node t = tail, p = t;;) { // move p to last node and append
Node n, u; // temps for reads of next & tail
if (p == null && (p = head) == null) {
if (casHead(null, s))
return s; // initialize
} else if (p.cannotPrecede(haveData))
return null; // lost race vs opposite mode
else if ((n = p.next) != null) // not last; keep traversing
p = p != t && t != (u = tail) ? (t = u) : // stale tail
(p != n) ? n : null; // restart if off list
else if (!p.casNext(null, s))
p = p.next; // re-read on CAS failure
else {
if (p != t) { // update if slack now >= 2
while ((tail != t || !casTail(t, s))
&& (t = tail) != null
&& (s = t.next) != null && // advance and retry
(s = s.next) != null && s != t);
}
return p;
}
}
}
/**
* Spins/yields/blocks until node s is matched or caller gives up.
*
* @param s the waiting node
* @param pred the predecessor of s, or s itself if it has no
* predecessor, or null if unknown (the null case does not occur
* in any current calls but may in possible future extensions)
* @param e the comparison value for checking match
* @param timed if true, wait only until timeout elapses
* @param nanos timeout in nanosecs, used only if timed is true
* @return matched item, or e if unmatched on interrupt or timeout
*/
private Message awaitMatch(Node s, Node pred, Message e, boolean timed, long nanos) throws SuspendExecution {
long lastTime = timed ? System.nanoTime() : 0L;
Strand w = Strand.currentStrand();
int spins = (w.isFiber() ? 0 : -1); // no spins in fiber; otherwise, initialized after first item and cancel checks
ThreadLocalRandom randomYields = null; // bound if needed
if (spins == 0)
requestUnpark(s, w);
for (;;) {
Object item = s.item;
if (item == CHANNEL_CLOSED)
setReceiveClosed();
if (item != e) { // matched
// assert item != s;
s.forgetContents(); // avoid garbage
return this.cast(item);
}
if ((w.isInterrupted() || (timed && nanos <= 0))
&& s.casItem(e, s)) { // cancel
unsplice(pred, s);
return e;
}
if (spins < 0) { // establish spins at/near front
if ((spins = spinsFor(pred, s.isData)) > 0)
randomYields = ThreadLocalRandom.current();
} else if (spins > 0) { // spin
--spins;
if (randomYields.nextInt(CHAINED_SPINS) == 0)
Strand.yield(); // occasionally yield
} else if (s.waiter == null) {
requestUnpark(s, w); // request unpark then recheck
} else if (timed) {
long now = System.nanoTime();
if ((nanos -= now - lastTime) > 0)
Strand.parkNanos(this, nanos);
lastTime = now;
} else {
Strand.park(this);
}
}
}
private void requestUnpark(Node s, Strand waiter) {
s.waiter = waiter;
}
/**
* Returns spin/yield value for a node with given predecessor and
* data mode. See above for explanation.
*/
private static int spinsFor(Node pred, boolean haveData) {
if (MP && pred != null) {
if (pred.isData != haveData) // phase change
return FRONT_SPINS + CHAINED_SPINS;
if (pred.isMatched()) // probably at front
return FRONT_SPINS;
if (pred.waiter == null) // pred apparently spinning
return CHAINED_SPINS;
}
return 0;
}
/* -------------- Traversal methods -------------- */
/**
* Returns the successor of p, or the head node if p.next has been
* linked to self, which will only be true if traversing with a
* stale pointer that is now off the list.
*/
final Node succ(Node p) {
Node next = p.next;
return (p == next) ? head : next;
}
/**
* Returns the first unmatched node of the given mode, or null if
* none. Used by methods isEmpty, hasWaitingConsumer.
*/
private Node firstOfMode(boolean isData) {
for (Node p = head; p != null; p = succ(p)) {
if (!p.isMatched())
return (p.isData == isData) ? p : null;
}
return null;
}
/**
* Traverses and counts unmatched nodes of the given mode.
* Used by methods size and getWaitingConsumerCount.
*/
private int countOfMode(boolean data) {
int count = 0;
for (Node p = head; p != null;) {
if (!p.isMatched()) {
if (p.isData != data)
return 0;
if (++count == Integer.MAX_VALUE) // saturated
break;
}
Node n = p.next;
if (n != p)
p = n;
else {
count = 0;
p = head;
}
}
return count;
}
/* -------------- Removal methods -------------- */
/**
* Unsplices (now or later) the given deleted/cancelled node with
* the given predecessor.
*
* @param pred a node that was at one time known to be the
* predecessor of s, or null or s itself if s is/was at head
* @param s the node to be unspliced
*/
final void unsplice(Node pred, Node s) {
s.forgetContents(); // forget unneeded fields
/*
* See above for rationale. Briefly: if pred still points to
* s, try to unlink s. If s cannot be unlinked, because it is
* trailing node or pred might be unlinked, and neither pred
* nor s are head or offlist, add to sweepVotes, and if enough
* votes have accumulated, sweep.
*/
if (pred != null && pred != s && pred.next == s) {
Node n = s.next;
if (n == null
|| (n != s && pred.casNext(s, n) && pred.isMatched())) {
for (;;) { // check if at, or could be, head
Node h = head;
if (h == pred || h == s || h == null)
return; // at head or list empty
if (!h.isMatched())
break;
Node hn = h.next;
if (hn == null)
return; // now empty
if (hn != h && casHead(h, hn))
h.forgetNext(); // advance head
}
if (pred.next != pred && s.next != s) { // recheck if offlist
for (;;) { // sweep now if enough votes
int v = sweepVotes;
if (v < SWEEP_THRESHOLD) {
if (casSweepVotes(v, v + 1))
break;
} else if (casSweepVotes(v, 0)) {
sweep();
break;
}
}
}
}
}
}
/**
* Unlinks matched (typically cancelled) nodes encountered in a
* traversal from head.
*/
private void sweep() {
for (Node p = head, s, n; p != null && (s = p.next) != null;) {
if (!s.isMatched())
// Unmatched nodes are never self-linked
p = s;
else if ((n = s.next) == null) // trailing node is pinned
break;
else if (s == n) // stale
// No need to also check for p == s, since that implies s == n
p = head;
else
p.casNext(s, n);
}
}
/**
* Returns {@code true} if this queue contains no elements.
*
* @return {@code true} if this queue contains no elements
*/
boolean isEmpty() {
for (Node p = head; p != null; p = succ(p)) {
if (!p.isMatched())
return !p.isData;
}
return true;
}
boolean hasWaitingConsumer() {
return firstOfMode(false) != null;
}
int size() {
return countOfMode(true);
}
public int getWaitingConsumerCount() {
return countOfMode(false);
}
private static final VarHandle HEAD;
private static final VarHandle TAIL;
private static final VarHandle SWEEP_VOTES;
static {
try {
MethodHandles.Lookup l = MethodHandles.lookup();
Class k = TransferChannel.class;
HEAD = l.findVarHandle(k, "head", Node.class);
TAIL = l.findVarHandle(k, "tail", Node.class);
SWEEP_VOTES = l.findVarHandle(k, "sweepVotes", int.class);
} catch (ReflectiveOperationException e) {
throw new ExceptionInInitializerError(e);
}
}
// // Unsafe mechanics
// private static final sun.misc.Unsafe UNSAFE;
// private static final long headOffset;
// private static final long tailOffset;
// private static final long sweepVotesOffset;
//
// static {
// try {
// UNSAFE = UtilUnsafe.getUnsafe();
// Class k = TransferChannel.class;
// headOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("head"));
// tailOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("tail"));
// sweepVotesOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("sweepVotes"));
// } catch (Exception e) {
// throw new Error(e);
// }
// }
}
