/**
    *
    * 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 com.google.common.base.Preconditions;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.LinkedBlockingQueue;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.atomic.AtomicReference;
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.util.ByteRange;
  import org.apache.hadoop.hbase.util.SimpleMutableByteRange;
  
  /**
   * 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 HeapMemStoreLAB implements MemStoreLAB {
  
    static final String CHUNK_SIZE_KEY = "hbase.hregion.memstore.mslab.chunksize";
    static final int CHUNK_SIZE_DEFAULT = 2048 * 1024;
    static final String MAX_ALLOC_KEY = "hbase.hregion.memstore.mslab.max.allocation";
    static final int MAX_ALLOC_DEFAULT = 256 * 1024; // allocs bigger than this don't go through
                                                     // allocator
  
    static final Log LOG = LogFactory.getLog(HeapMemStoreLAB.class);
  
    private AtomicReference<Chunk> curChunk = new AtomicReference<Chunk>();
    // A queue of chunks contained by this memstore, used with chunk pool
    private BlockingQueue<Chunk> chunkQueue = null;
    final int chunkSize;
    final int maxAlloc;
    private final MemStoreChunkPool chunkPool;
  
    // This flag is for closing this instance, its set when clearing snapshot of
    // memstore
    private volatile boolean closed = false;
    // This flag is for reclaiming chunks. Its set when putting chunks back to
    // pool
    private AtomicBoolean reclaimed = new AtomicBoolean(false);
    // Current count of open scanners which reading data from this MemStoreLAB
    private final AtomicInteger openScannerCount = new AtomicInteger();
  
    // Used in testing
    public HeapMemStoreLAB() {
      this(new Configuration());
    }
  
    public HeapMemStoreLAB(Configuration conf) {
      chunkSize = conf.getInt(CHUNK_SIZE_KEY, CHUNK_SIZE_DEFAULT);
      maxAlloc = conf.getInt(MAX_ALLOC_KEY, MAX_ALLOC_DEFAULT);
      this.chunkPool = MemStoreChunkPool.getPool(conf);
      // currently chunkQueue is only used for chunkPool
      if (this.chunkPool != null) {
        // set queue length to chunk pool max count to avoid keeping reference of
        // too many non-reclaimable chunks
        chunkQueue = new LinkedBlockingQueue<Chunk>(chunkPool.getMaxCount());
      }
  
      // 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.
    */
   @Override
   public ByteRange 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 SimpleMutableByteRange(c.data, allocOffset, size);
       }
 
       // not enough space!
       // try to retire this chunk
       tryRetireChunk(c);
     }
   }
 
   /**
    * Close this instance since it won't be used any more, try to put the chunks
    * back to pool
    */
   @Override
   public void close() {
     this.closed = true;
     // We could put back the chunks to pool for reusing only when there is no
     // opening scanner which will read their data
     if (chunkPool != null && openScannerCount.get() == 0
         && reclaimed.compareAndSet(false, true)) {
       chunkPool.putbackChunks(this.chunkQueue);
     }
   }
 
   /**
    * Called when opening a scanner on the data of this MemStoreLAB
    */
   @Override
   public void incScannerCount() {
     this.openScannerCount.incrementAndGet();
   }
 
   /**
    * Called when closing a scanner on the data of this MemStoreLAB
    */
   @Override
   public void decScannerCount() {
     int count = this.openScannerCount.decrementAndGet();
     if (chunkPool != null && count == 0 && this.closed
         && reclaimed.compareAndSet(false, true)) {
       chunkPool.putbackChunks(this.chunkQueue);
     }
   }
 
   /**
    * Try to retire the current chunk if it is still
    * <code>c</code>. Postcondition is that curChunk.get()
    * != c
    * @param c the chunk to retire
    * @return true if we won the race to retire the chunk
    */
   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 = (chunkPool != null) ? chunkPool.getChunk() : new Chunk(chunkSize);
       if (curChunk.compareAndSet(null, c)) {
         // we won race - now we need to actually do the expensive
         // allocation step
         c.init();
         if (chunkQueue != null && !this.closed && !this.chunkQueue.offer(c)) {
           if (LOG.isTraceEnabled()) {
             LOG.trace("Chunk queue is full, won't reuse this new chunk. Current queue size: "
                 + chunkQueue.size());
           }
         }
         return c;
       } else if (chunkPool != null) {
         chunkPool.putbackChunk(c);
       }
       // someone else won race - that's fine, we'll try to grab theirs
       // in the next iteration of the loop.
     }
   }
 
   Chunk getCurrentChunk() {
     return this.curChunk.get();
   }
 
   BlockingQueue<Chunk> getChunkQueue() {
     return this.chunkQueue;
   }
 
   /**
    * A chunk of memory out of which allocations are sliced.
    */
   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
      */
     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 {
         if (data == null) {
           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");
     }
 
     /**
      * Reset the offset to UNINITIALIZED before before reusing an old chunk
      */
     void reset() {
       if (nextFreeOffset.get() != UNINITIALIZED) {
         nextFreeOffset.set(UNINITIALIZED);
         allocCount.set(0);
       }
     }
 
     /**
      * 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());
     }
 
     int getNextFreeOffset() {
       return this.nextFreeOffset.get();
     }
   }
 }