/*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.jctools.maps;
import static org.jctools.util.UnsafeAccess.UNSAFE;
import java.io.Serializable;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
/**
* An auto-resizing table of {@code longs}, supporting low-contention CAS
* operations. Updates are done with CAS's to no particular table element.
* The intent is to support highly scalable counters, r/w locks, and other
* structures where the updates are associative, loss-free (no-brainer), and
* otherwise happen at such a high volume that the cache contention for
* CAS'ing a single word is unacceptable.
*
* @since 1.5
* @author Cliff Click
*/
public class ConcurrentAutoTable implements Serializable {
// --- public interface ---
/**
* Add the given value to current counter value. Concurrent updates will
* not be lost, but addAndGet or getAndAdd are not implemented because the
* total counter value (i.e., {@link #get}) is not atomically updated.
* Updates are striped across an array of counters to avoid cache contention
* and has been tested with performance scaling linearly up to 768 CPUs.
*/
public void add( long x ) { add_if( x); }
/** {@link #add} with -1 */
public void decrement() { add_if(-1L); }
/** {@link #add} with +1 */
public void increment() { add_if( 1L); }
/** Atomically set the sum of the striped counters to specified value.
* Rather more expensive than a simple store, in order to remain atomic.
*/
public void set( long x ) {
CAT newcat = new CAT(null,4,x);
// Spin until CAS works
while( !CAS_cat(_cat,newcat) ) {/*empty*/}
}
/**
* Current value of the counter. Since other threads are updating furiously
* the value is only approximate, but it includes all counts made by the
* current thread. Requires a pass over the internally striped counters.
*/
public long get() { return _cat.sum(); }
/** Same as {@link #get}, included for completeness. */
public int intValue() { return (int)_cat.sum(); }
/** Same as {@link #get}, included for completeness. */
public long longValue() { return _cat.sum(); }
/**
* A cheaper {@link #get}. Updated only once/millisecond, but as fast as a
* simple load instruction when not updating.
*/
public long estimate_get( ) { return _cat.estimate_sum(); }
/**
* Return the counter's {@code long} value converted to a string.
*/
public String toString() { return _cat.toString(); }
/**
* A more verbose print than {@link #toString}, showing internal structure.
* Useful for debugging.
*/
public void print() { _cat.print(); }
/**
* Return the internal counter striping factor. Useful for diagnosing
* performance problems.
*/
public int internal_size() { return _cat._t.length; }
// Only add 'x' to some slot in table, hinted at by 'hash'. The sum can
// overflow. Value is CAS'd so no counts are lost. The CAS is retried until
// it succeeds. Returned value is the old value.
private long add_if( long x ) { return _cat.add_if(x,hash(),this); }
// The underlying array of concurrently updated long counters
private volatile CAT _cat = new CAT(null,16/*Start Small, Think Big!*/,0L);
private static AtomicReferenceFieldUpdater<ConcurrentAutoTable,CAT> _catUpdater =
AtomicReferenceFieldUpdater.newUpdater(ConcurrentAutoTable.class,CAT.class, "_cat");
private boolean CAS_cat( CAT oldcat, CAT newcat ) { return _catUpdater.compareAndSet(this,oldcat,newcat); }
// Hash spreader
private static int hash() {
//int h = (int)Thread.currentThread().getId();
int h = System.identityHashCode(Thread.currentThread());
return h<<3; // Pad out cache lines. The goal is to avoid cache-line contention
}
// --- CAT -----------------------------------------------------------------
private static class CAT implements Serializable {
// Unsafe crud: get a function which will CAS arrays
private static final int _Lbase = UNSAFE.arrayBaseOffset(long[].class);
private static final int _Lscale = UNSAFE.arrayIndexScale(long[].class);
private static long rawIndex(long[] ary, int i) {
assert i >= 0 && i < ary.length;
return _Lbase + i * _Lscale;
}
private static boolean CAS( long[] A, int idx, long old, long nnn ) {
return UNSAFE.compareAndSwapLong( A, rawIndex(A,idx), old, nnn );
}
//volatile long _resizers; // count of threads attempting a resize
//static private final AtomicLongFieldUpdater<CAT> _resizerUpdater =
// AtomicLongFieldUpdater.newUpdater(CAT.class, "_resizers");
private final CAT _next;
private volatile long _fuzzy_sum_cache;
private volatile long _fuzzy_time;
private static final int MAX_SPIN=1;
private final long[] _t; // Power-of-2 array of longs
CAT( CAT next, int sz, long init ) {
_next = next;
_t = new long[sz];
_t[0] = init;
}
// Only add 'x' to some slot in table, hinted at by 'hash'. The sum can
// overflow. Value is CAS'd so no counts are lost. The CAS is attempted
// ONCE.
public long add_if( long x, int hash, ConcurrentAutoTable master ) {
final long[] t = _t;
final int idx = hash & (t.length-1);
// Peel loop; try once fast
long old = t[idx];
final boolean ok = CAS( t, idx, old, old+x );
if( ok ) return old; // Got it
// Try harder
int cnt=0;
while( true ) {
old = t[idx];
if( CAS( t, idx, old, old+x ) ) break; // Got it!
cnt++;
}
if( cnt < MAX_SPIN ) return old; // Allowable spin loop count
if( t.length >= 1024*1024 ) return old; // too big already
// Too much contention; double array size in an effort to reduce contention
//long r = _resizers;
//final int newbytes = (t.length<<1)<<3/*word to bytes*/;
//while( !_resizerUpdater.compareAndSet(this,r,r+newbytes) )
// r = _resizers;
//r += newbytes;
if( master._cat != this ) return old; // Already doubled, don't bother
//if( (r>>17) != 0 ) { // Already too much allocation attempts?
// // We could use a wait with timeout, so we'll wakeup as soon as the new
// // table is ready, or after the timeout in any case. Annoyingly, this
// // breaks the non-blocking property - so for now we just briefly sleep.
// //synchronized( this ) { wait(8*megs); } // Timeout - we always wakeup
// try { Thread.sleep(r>>17); } catch( InterruptedException e ) { }
// if( master._cat != this ) return old;
//}
CAT newcat = new CAT(this,t.length*2,0);
// Take 1 stab at updating the CAT with the new larger size. If this
// fails, we assume some other thread already expanded the CAT - so we
// do not need to retry until it succeeds.
while( master._cat == this && !master.CAS_cat(this,newcat) ) {/*empty*/}
return old;
}
// Return the current sum of all things in the table. Writers can be
// updating the table furiously, so the sum is only locally accurate.
public long sum( ) {
long sum = _next == null ? 0 : _next.sum(); // Recursively get cached sum
final long[] t = _t;
for( long cnt : t ) sum += cnt;
return sum;
}
// Fast fuzzy version. Used a cached value until it gets old, then re-up
// the cache.
public long estimate_sum( ) {
// For short tables, just do the work
if( _t.length <= 64 ) return sum();
// For bigger tables, periodically freshen a cached value
long millis = System.currentTimeMillis();
if( _fuzzy_time != millis ) { // Time marches on?
_fuzzy_sum_cache = sum(); // Get sum the hard way
_fuzzy_time = millis; // Indicate freshness of cached value
}
return _fuzzy_sum_cache; // Return cached sum
}
public String toString( ) { return Long.toString(sum()); }
public void print() {
long[] t = _t;
System.out.print("["+t[0]);
for( int i=1; i<t.length; i++ )
System.out.print(","+t[i]);
System.out.print("]");
if( _next != null ) _next.print();
}
}
}