/**
*
* 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.hadoop.hbase.regionserver;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.conf.Configuration;
import com.google.common.base.Preconditions;
/**
* A memstore-local allocation buffer.
* <p>
* The MemStoreLAB is basically a bump-the-pointer allocator that allocates
* big (2MB) byte[] chunks from and then doles it out to threads that request
* slices into the array.
* <p>
* The purpose of this class is to combat heap fragmentation in the
* regionserver. By ensuring that all KeyValues in a given memstore refer
* only to large chunks of contiguous memory, we ensure that large blocks
* get freed up when the memstore is flushed.
* <p>
* Without the MSLAB, the byte array allocated during insertion end up
* interleaved throughout the heap, and the old generation gets progressively
* more fragmented until a stop-the-world compacting collection occurs.
* <p>
* TODO: we should probably benchmark whether word-aligning the allocations
* would provide a performance improvement - probably would speed up the
* Bytes.toLong/Bytes.toInt calls in KeyValue, but some of those are cached
* anyway
*/
@InterfaceAudience.Private
public class MemStoreLAB {
private AtomicReference<Chunk> curChunk = new AtomicReference<Chunk>();
final static String CHUNK_SIZE_KEY = "hbase.hregion.memstore.mslab.chunksize";
final static int CHUNK_SIZE_DEFAULT = 2048 * 1024;
final int chunkSize;
final static String MAX_ALLOC_KEY = "hbase.hregion.memstore.mslab.max.allocation";
final static int MAX_ALLOC_DEFAULT = 256 * 1024; // allocs bigger than this don't go through allocator
final int maxAlloc;
public MemStoreLAB() {
this(new Configuration());
}
public MemStoreLAB(Configuration conf) {
chunkSize = conf.getInt(CHUNK_SIZE_KEY, CHUNK_SIZE_DEFAULT);
maxAlloc = conf.getInt(MAX_ALLOC_KEY, MAX_ALLOC_DEFAULT);
// if we don't exclude allocations >CHUNK_SIZE, we'd infiniteloop on one!
Preconditions.checkArgument(
maxAlloc <= chunkSize,
MAX_ALLOC_KEY + " must be less than " + CHUNK_SIZE_KEY);
}
/**
* Allocate a slice of the given length.
*
* If the size is larger than the maximum size specified for this
* allocator, returns null.
*/
public Allocation allocateBytes(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) {
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 byte[] 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 bytes */
private final int size;
/**
* Create an uninitialized chunk. Note that memory is not allocated yet, so
* this is cheap.
* @param size in bytes
*/
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 byte[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> bytes 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 byte[] data;
private final int offset;
private Allocation(byte[] data, int off) {
this.data = data;
this.offset = off;
}
@Override
public String toString() {
return "Allocation(" + "capacity=" + data.length + ", off=" + offset
+ ")";
}
byte[] getData() {
return data;
}
int getOffset() {
return offset;
}
}
}