/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF 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.apache.cassandra.utils;
import java.nio.ByteBuffer;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicReference;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* The SlabAllocator is a bump-the-pointer allocator that allocates
* large (2MB by default) regions and then doles them out to threads that request
* slices into the array.
* <p/>
* The purpose of this class is to combat heap fragmentation in long lived
* objects: by ensuring that all allocations with similar lifetimes
* only to large regions of contiguous memory, we ensure that large blocks
* get freed up at the same time.
* <p/>
* Otherwise, variable length byte arrays allocated end up
* interleaved throughout the heap, and the old generation gets progressively
* more fragmented until a stop-the-world compacting collection occurs.
*/
public class SlabAllocator extends Allocator
{
private static final Logger logger = LoggerFactory.getLogger(SlabAllocator.class);
private final static int REGION_SIZE = 1024 * 1024;
private final static int MAX_CLONED_SIZE = 128 * 1024; // bigger than this don't go in the region
// globally stash any Regions we allocate but are beaten to using, and use these up before allocating any more
private static final ConcurrentLinkedQueue<Region> RACE_ALLOCATED = new ConcurrentLinkedQueue<>();
private final AtomicReference<Region> currentRegion = new AtomicReference<Region>();
private final AtomicInteger regionCount = new AtomicInteger(0);
private AtomicLong unslabbed = new AtomicLong(0);
public ByteBuffer allocate(int size)
{
assert size >= 0;
if (size == 0)
return ByteBufferUtil.EMPTY_BYTE_BUFFER;
// satisfy large allocations directly from JVM since they don't cause fragmentation
// as badly, and fill up our regions quickly
if (size > MAX_CLONED_SIZE)
{
unslabbed.addAndGet(size);
return ByteBuffer.allocate(size);
}
while (true)
{
Region region = getRegion();
// Try to allocate from this region
ByteBuffer cloned = region.allocate(size);
if (cloned != null)
return cloned;
// not enough space!
currentRegion.compareAndSet(region, null);
}
}
/**
* Get the current region, or, if there is no current region, allocate a new one
*/
private Region getRegion()
{
while (true)
{
// Try to get the region
Region region = currentRegion.get();
if (region != null)
return region;
// No current region, so we want to allocate one. We race
// against other allocators to CAS in a Region, and if we fail we stash the region for re-use
region = RACE_ALLOCATED.poll();
if (region == null)
region = new Region(REGION_SIZE);
if (currentRegion.compareAndSet(null, region))
{
regionCount.incrementAndGet();
logger.trace("{} regions now allocated in {}", regionCount, this);
return region;
}
// someone else won race - that's fine, we'll try to grab theirs
// in the next iteration of the loop.
RACE_ALLOCATED.add(region);
}
}
/**
* @return a lower bound on how much space has been allocated
*/
public long getMinimumSize()
{
return unslabbed.get() + (regionCount.get() - 1) * (long)REGION_SIZE;
}
/**
* A region of memory out of which allocations are sliced.
*
* This serves two purposes:
* - to provide a step between initialization and allocation, so that racing to CAS a
* new region in is harmless
* - encapsulates the allocation offset
*/
private static class Region
{
/**
* Actual underlying data
*/
private final ByteBuffer data;
/**
* Offset for the next allocation, or the sentinel value -1
* which implies that the region is still uninitialized.
*/
private final AtomicInteger nextFreeOffset = new AtomicInteger(0);
/**
* Total number of allocations satisfied from this buffer
*/
private final AtomicInteger allocCount = new AtomicInteger();
/**
* Create an uninitialized region. Note that memory is not allocated yet, so
* this is cheap.
*
* @param size in bytes
*/
private Region(int size)
{
data = ByteBuffer.allocate(size);
}
/**
* Try to allocate <code>size</code> bytes from the region.
*
* @return the successful allocation, or null to indicate not-enough-space
*/
public ByteBuffer allocate(int size)
{
while (true)
{
int oldOffset = nextFreeOffset.get();
if (oldOffset + size > data.capacity()) // capacity == remaining
return null;
// Try to atomically claim this region
if (nextFreeOffset.compareAndSet(oldOffset, oldOffset + size))
{
// we got the alloc
allocCount.incrementAndGet();
return (ByteBuffer) data.duplicate().position(oldOffset).limit(oldOffset + size);
}
// we raced and lost alloc, try again
}
}
@Override
public String toString()
{
return "Region@" + System.identityHashCode(this) +
" allocs=" + allocCount.get() + "waste=" +
(data.capacity() - nextFreeOffset.get());
}
}
}