ARROW-7538: [Java] Clarify actual and desired size in AllocationManager

java/memory/src/main/java/org/apache/arrow/memory/AllocationManager.java

@@ -53,19 +53,17 @@ public abstract class AllocationManager {
   // see JIRA for details
   private final LowCostIdentityHashMap<BaseAllocator, BufferLedger> map = new LowCostIdentityHashMap<>();
   private final long amCreationTime = System.nanoTime();
-  private final long size;
 
   // The ReferenceManager created at the time of creation of this AllocationManager
   // is treated as the owning reference manager for the underlying chunk of memory
   // managed by this allocation manager
   private volatile BufferLedger owningLedger;
   private volatile long amDestructionTime = 0;
 
-  protected AllocationManager(BaseAllocator accountingAllocator, long size) {
+  protected AllocationManager(BaseAllocator accountingAllocator) {
     Preconditions.checkNotNull(accountingAllocator);
     accountingAllocator.assertOpen();
 
-    this.size = size;
     this.root = accountingAllocator.root;
 
     // we do a no retain association since our creator will want to retrieve the newly created
@@ -181,11 +179,11 @@ void release(final BufferLedger ledger) {
   /**
    * Return the size of underlying chunk of memory managed by this Allocation Manager.
    *
+   * <p>The underlying memory chunk managed can be different from the original requested size.
+   *
    * @return size of underlying memory chunk
    */
-  public long getSize() {
-    return size;
-  }
+  public abstract long getSize();
 
   /**
    * Return the absolute memory address pointing to the fist byte of underling memory chunk.

java/memory/src/main/java/org/apache/arrow/memory/NettyAllocationManager.java

@@ -34,13 +34,13 @@ public class NettyAllocationManager extends AllocationManager {
   static final UnsafeDirectLittleEndian EMPTY = INNER_ALLOCATOR.empty;
   static final long CHUNK_SIZE = INNER_ALLOCATOR.getChunkSize();
 
-  private final int size;
+  private final int allocatedSize;
   private final UnsafeDirectLittleEndian memoryChunk;
 
-  NettyAllocationManager(BaseAllocator accountingAllocator, int size) {
-    super(accountingAllocator, size);
-    this.memoryChunk = INNER_ALLOCATOR.allocate(size);
-    this.size = memoryChunk.capacity();
+  NettyAllocationManager(BaseAllocator accountingAllocator, int requestedSize) {
+    super(accountingAllocator);
+    this.memoryChunk = INNER_ALLOCATOR.allocate(requestedSize);
+    this.allocatedSize = memoryChunk.capacity();
   }
 
   /**
@@ -61,9 +61,14 @@ protected void release0() {
     memoryChunk.release();
   }
 
+  /**
+   * Returns the underlying memory chunk size managed.
+   *
+   * <p>NettyAllocationManager rounds requested size up to the next power of two.
+   */
   @Override
   public long getSize() {
-    return size;
+    return allocatedSize;
   }
 
   /**

java/memory/src/test/java/org/apache/arrow/memory/TestBaseAllocator.java

@@ -391,9 +391,9 @@ public void testCustomizedAllocationManager() {
   private BaseAllocator createAllocatorWithCustomizedAllocationManager() {
     return new RootAllocator(BaseAllocator.configBuilder()
         .maxAllocation(MAX_ALLOCATION)
-        .allocationManagerFactory((accountingAllocator, size) -> new AllocationManager(accountingAllocator, size) {
+        .allocationManagerFactory((accountingAllocator, requestedSize) -> new AllocationManager(accountingAllocator) {
           private final Unsafe unsafe = getUnsafe();
-          private final long address = unsafe.allocateMemory(size);
+          private final long address = unsafe.allocateMemory(requestedSize);
 
           @Override
           protected long memoryAddress() {
@@ -402,10 +402,15 @@ protected long memoryAddress() {
 
           @Override
           protected void release0() {
-            unsafe.setMemory(address, size, (byte) 0);
+            unsafe.setMemory(address, requestedSize, (byte) 0);
             unsafe.freeMemory(address);
           }
 
+          @Override
+          public long getSize() {
+            return requestedSize;
+          }
+
           private Unsafe getUnsafe() {
             Field f = null;
             try {