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/*
* 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 io.trino.array;
import io.airlift.slice.SizeOf;
import io.airlift.slice.Slice;
import io.trino.spi.block.Block;
import io.trino.spi.block.MapHashTables;
import it.unimi.dsi.fastutil.longs.Long2IntOpenHashMap;
import static io.airlift.slice.SizeOf.instanceSize;
import static java.lang.String.format;
import static java.lang.reflect.Array.getLength;
// This class tracks how many times an objects have been referenced
// in order not to over count memory in complex objects such as SliceBigArray or BlockBigArray.
// Ideally the key set should be the object references themselves.
// But it turns out such implementation can lead to long concurrent marking time with G1GC.
// With several attempts including breaking down the object arrays into object big arrays,
// the problem can be solved but tracking overhead is still high.
// Benchmark results show using hash maps with primitive arrays perform way better than object arrays.
// Therefore, we use concatenation of identity hash code and size of the object to identify an object.
// This may lead to hash collision, in which case we will under count memory usage.
// But we believe the performance boost is worth the cost.
public final class ReferenceCountMap
extends Long2IntOpenHashMap
{
private static final int INSTANCE_SIZE = instanceSize(ReferenceCountMap.class);
/**
* Increments the reference count of an object by 1 and returns the updated reference count
*/
public int incrementAndGet(Object key)
{
return addTo(getHashCode(key), 1) + 1;
}
/**
* Decrements the reference count of an object by 1 and returns the updated reference count
*/
public int decrementAndGet(Object key)
{
long hashCode = getHashCode(key);
int previousCount = addTo(hashCode, -1);
if (previousCount == 1) {
remove(hashCode);
}
return previousCount - 1;
}
/**
* Returns the size of this map in bytes.
*/
public long sizeOf()
{
return INSTANCE_SIZE + SizeOf.sizeOf(key) + SizeOf.sizeOf(value);
}
/**
* Get the 64-bit hash code for an object
*/
private static long getHashCode(Object key)
{
// identityHashCode of two objects are not guaranteed to be different.
// Any additional identity information can reduce collisions.
// In the most cases below, we use size of an object to be the extra identity.
// Experiments show that with 100 million objects created, using identityHashCode has a collision rate around 2.5%.
// However, if these 100 million objects are combined with 10 different sizes, the collision rate is around 0.1%.
// The collision rate can be lower than 0.001% if there are 1000 different sizes.
int extraIdentity;
if (key == null) {
extraIdentity = 0;
}
else if (key instanceof Block) {
extraIdentity = (int) ((Block) key).getRetainedSizeInBytes();
}
else if (key instanceof Slice) {
extraIdentity = (int) ((Slice) key).getRetainedSize();
}
else if (key.getClass().isArray()) {
extraIdentity = getLength(key);
}
else if (key instanceof MapHashTables) {
extraIdentity = (int) ((MapHashTables) key).getRetainedSizeInBytes();
}
else {
throw new IllegalArgumentException(format("Unsupported type for %s", key));
}
return (((long) System.identityHashCode(key)) << Integer.SIZE) + extraIdentity;
}
}