/*
* Licensed to ElasticSearch and Shay Banon under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. ElasticSearch licenses this
* file to you 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.elasticsearch.common.lab;
import org.elasticsearch.common.Preconditions;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
public class LongsLAB {
private AtomicReference<Chunk> curChunk = new AtomicReference<Chunk>();
final int chunkSize;
final int maxAlloc;
public LongsLAB(int chunkSize, int maxAlloc) {
this.chunkSize = chunkSize;
this.maxAlloc = maxAlloc;
// if we don't exclude allocations >CHUNK_SIZE, we'd infiniteloop on one!
Preconditions.checkArgument(maxAlloc <= chunkSize);
}
/**
* Allocate a slice of the given length.
* <p/>
* If the size is larger than the maximum size specified for this
* allocator, returns null.
*/
public Allocation allocateLongs(int size) {
Preconditions.checkArgument(size >= 0, "negative size");
// Callers should satisfy large allocations directly from JVM since they
// don't cause fragmentation as badly.
if (size > maxAlloc) {
return null;
}
while (true) {
Chunk c = getOrMakeChunk();
// Try to allocate from this chunk
int allocOffset = c.alloc(size);
if (allocOffset != -1) {
// We succeeded - this is the common case - small alloc
// from a big buffer
return new Allocation(c.data, allocOffset);
}
// not enough space!
// try to retire this chunk
tryRetireChunk(c);
}
}
/**
* Try to retire the current chunk if it is still
* <code>c</code>. Postcondition is that curChunk.get()
* != c
*/
private void tryRetireChunk(Chunk c) {
@SuppressWarnings("unused")
boolean weRetiredIt = curChunk.compareAndSet(c, null);
// If the CAS succeeds, that means that we won the race
// to retire the chunk. We could use this opportunity to
// update metrics on external fragmentation.
//
// If the CAS fails, that means that someone else already
// retired the chunk for us.
}
/**
* Get the current chunk, or, if there is no current chunk,
* allocate a new one from the JVM.
*/
private Chunk getOrMakeChunk() {
while (true) {
// Try to get the chunk
Chunk c = curChunk.get();
if (c != null) {
return c;
}
// No current chunk, so we want to allocate one. We race
// against other allocators to CAS in an uninitialized chunk
// (which is cheap to allocate)
c = new Chunk(chunkSize);
if (curChunk.compareAndSet(null, c)) {
// we won race - now we need to actually do the expensive
// allocation step
c.init();
return c;
}
// someone else won race - that's fine, we'll try to grab theirs
// in the next iteration of the loop.
}
}
/**
* A chunk of memory out of which allocations are sliced.
*/
private static class Chunk {
/**
* Actual underlying data
*/
private long[] data;
private static final int UNINITIALIZED = -1;
private static final int OOM = -2;
/**
* Offset for the next allocation, or the sentinel value -1
* which implies that the chunk is still uninitialized.
*/
private AtomicInteger nextFreeOffset = new AtomicInteger(UNINITIALIZED);
/**
* Total number of allocations satisfied from this buffer
*/
private AtomicInteger allocCount = new AtomicInteger();
/**
* Size of chunk in longs
*/
private final int size;
/**
* Create an uninitialized chunk. Note that memory is not allocated yet, so
* this is cheap.
*
* @param size in longs
*/
private Chunk(int size) {
this.size = size;
}
/**
* Actually claim the memory for this chunk. This should only be called from
* the thread that constructed the chunk. It is thread-safe against other
* threads calling alloc(), who will block until the allocation is complete.
*/
public void init() {
assert nextFreeOffset.get() == UNINITIALIZED;
try {
data = new long[size];
} catch (OutOfMemoryError e) {
boolean failInit = nextFreeOffset.compareAndSet(UNINITIALIZED, OOM);
assert failInit; // should be true.
throw e;
}
// Mark that it's ready for use
boolean initted = nextFreeOffset.compareAndSet(
UNINITIALIZED, 0);
// We should always succeed the above CAS since only one thread
// calls init()!
Preconditions.checkState(initted,
"Multiple threads tried to init same chunk");
}
/**
* Try to allocate <code>size</code> longs from the chunk.
*
* @return the offset of the successful allocation, or -1 to indicate not-enough-space
*/
public int alloc(int size) {
while (true) {
int oldOffset = nextFreeOffset.get();
if (oldOffset == UNINITIALIZED) {
// The chunk doesn't have its data allocated yet.
// Since we found this in curChunk, we know that whoever
// CAS-ed it there is allocating it right now. So spin-loop
// shouldn't spin long!
Thread.yield();
continue;
}
if (oldOffset == OOM) {
// doh we ran out of ram. return -1 to chuck this away.
return -1;
}
if (oldOffset + size > data.length) {
return -1; // alloc doesn't fit
}
// Try to atomically claim this chunk
if (nextFreeOffset.compareAndSet(oldOffset, oldOffset + size)) {
// we got the alloc
allocCount.incrementAndGet();
return oldOffset;
}
// we raced and lost alloc, try again
}
}
@Override
public String toString() {
return "Chunk@" + System.identityHashCode(this) +
" allocs=" + allocCount.get() + "waste=" +
(data.length - nextFreeOffset.get());
}
}
/**
* The result of a single allocation. Contains the chunk that the
* allocation points into, and the offset in this array where the
* slice begins.
*/
public static class Allocation {
private final long[] data;
private final int offset;
private Allocation(long[] data, int off) {
this.data = data;
this.offset = off;
}
@Override
public String toString() {
return "Allocation(data=" + data +
" with capacity=" + data.length +
", off=" + offset + ")";
}
public long[] getData() {
return data;
}
public int getOffset() {
return offset;
}
}
}