[GitHub] yzhliu commented on a change in pull request #12647: NativeResource Management in Scala

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yzhliu commented on a change in pull request #12647: NativeResource Management in Scala
URL: https://github.com/apache/incubator-mxnet/pull/12647#discussion_r225739773
 
 

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 File path: scala-package/core/src/main/scala/org/apache/mxnet/NativeResource.scala
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 @@ -0,0 +1,186 @@
+/*
+ * 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.mxnet
+
+import org.apache.mxnet.Base.CPtrAddress
+import java.lang.ref.{PhantomReference, ReferenceQueue, WeakReference}
+import java.util.concurrent._
+
+import org.apache.mxnet.Base.checkCall
+import java.util.concurrent.atomic.AtomicLong
+
+
+/**
+  * NativeResource trait is used to manage MXNet Objects
+  * such as NDArray, Symbol, Executor, etc.,
+  * The MXNet Object calls NativeResource.register
+  * and assign the returned NativeResourceRef to PhantomReference
+  * NativeResource also implements AutoCloseable so MXNetObjects
+  * can be used like Resources in try-with-resources paradigm
+  */
+private[mxnet] trait NativeResource
+  extends AutoCloseable with WarnIfNotDisposed {
+
+  /**
+    * native Address associated with this object
+    */
+  def nativeAddress: CPtrAddress
+
+  /**
+    * Function Pointer to the NativeDeAllocator of nativeAddress
+    */
+  def nativeDeAllocator: (CPtrAddress => Int)
+
+  /** Call NativeResource.register to get the reference
+    */
+  val ref: NativeResourceRef
+
+  /**
+    * Off-Heap Bytes Allocated for this object
+    */
+  // intentionally making it a val, so it gets evaluated when defined
+  val bytesAllocated: Long
+
+  private[mxnet] var scope: ResourceScope = null
+
+  @volatile var disposed = false
+
+  override def isDisposed: Boolean = disposed || isDeAllocated
+
+  /**
+    * Register this object for PhantomReference tracking and in
+    * ResourceScope if used inside ResourceScope.
+    * @return NativeResourceRef that tracks reachability of this object
+    *         using PhantomReference
+    */
+  def register(): NativeResourceRef = {
+    scope = ResourceScope.getCurrentScope()
+    if (scope != null) scope.add(this)
+
+    NativeResource.totalBytesAllocated.getAndAdd(bytesAllocated)
+    // register with PhantomRef tracking to release incase the objects go
+    // out of reference within scope but are held for long time
+    NativeResourceRef.register(this, nativeDeAllocator)
+ }
+
+  // Removes this object from PhantomRef tracking and from ResourceScope
+  private def deRegister(removeFromScope: Boolean): Unit = {
+    NativeResourceRef.deRegister(ref)
+    if (scope != null && removeFromScope) scope.remove(this)
+  }
+
+  // Implements [[@link AutoCloseable.close]]
+  override def close(): Unit = {
+    dispose()
+  }
+
+  // Implements [[@link WarnIfNotDisposed.dispose]]
+  def dispose(): Unit = dispose(true)
+
+  private[mxnet] def dispose(removeFromScope: Boolean = true): Unit = {
+    if (!disposed) {
+      checkCall(nativeDeAllocator(this.nativeAddress))
+      deRegister(removeFromScope)
+      NativeResource.totalBytesAllocated.getAndAdd(-1*bytesAllocated)
+      disposed = true
+    }
+  }
+
+  /*
+  this is used by the WarnIfNotDisposed finalizer,
+  the object could be disposed by the GC without the need for explicit disposal
+  but the finalizer might not have run, then the WarnIfNotDisposed throws a warning
+   */
+  private[mxnet] def isDeAllocated(): Boolean = NativeResourceRef.isDeAllocated(ref)
+
+}
+
+private[mxnet] object NativeResource {
+  var totalBytesAllocated : AtomicLong = new AtomicLong(0)
+}
+
+// Do not make [[NativeResource.resource]] a member of the class,
+// this will hold reference and GC will not clear the object.
+private[mxnet] class NativeResourceRef(resource: NativeResource,
+                                       val resourceDeAllocator: CPtrAddress => Int)
+        extends PhantomReference[NativeResource](resource, NativeResourceRef.refQ) {}
+
+private[mxnet] object NativeResourceRef {
+
+  private[mxnet] val refQ: ReferenceQueue[NativeResource]
+                = new ReferenceQueue[NativeResource]
+
+  private[mxnet] val refMap = new ConcurrentHashMap[NativeResourceRef, CPtrAddress]()
 
 Review comment:
   do we really need this Map? can we store resource.nativeAddress (not whole NativeResource.resource) as a member of NativeResourceRef?
   My major concern is, given two thread-safe containers cooperate with each other, a small carelessness can result in concurrency disaster. `isDeAllocated` is not really necessary - if you're worry about `WarnIfNotDisposed`, can we simply remove it? since now everything is guaranteed to be disposed finally (we invented that because we removed `dispose()` in `finalize` at that time).

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