Github user andrewor14 commented on a diff in the pull request:
https://github.com/apache/spark/pull/1165#discussion_r14810031
--- Diff:
core/src/main/scala/org/apache/spark/util/collection/SizeTracker.scala ---
@@ -0,0 +1,105 @@
+/*
+ * 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.spark.util.collection
+
+import scala.collection.mutable.ArrayBuffer
+
+import org.apache.spark.util.SizeEstimator
+
+/**
+ * A general interface for collections to keep track of their estimated
sizes in bytes.
+ * We sample with a slow exponential back-off using the SizeEstimator to
amortize the time,
+ * as each call to SizeEstimator is somewhat expensive (order of a few
milliseconds).
+ */
+private[spark] trait SizeTracker {
+
+ import SizeTracker._
+
+ /**
+ * Controls the base of the exponential which governs the rate of
sampling.
+ * E.g., a value of 2 would mean we sample at 1, 2, 4, 8, ... elements.
+ */
+ private val SAMPLE_GROWTH_RATE = 1.1
+
+ /** Samples taken since last resetSamples(). Only the last two are kept
for extrapolation. */
+ private val samples = new ArrayBuffer[Sample]
+
+ /** The average number of bytes per update between our last two samples.
*/
+ private var bytesPerUpdate: Double = _
+
+ /** Total number of insertions and updates into the map since the last
resetSamples(). */
+ private var numUpdates: Long = _
+
+ /** The value of 'numUpdates' at which we will take our next sample. */
+ private var nextSampleNum: Long = _
+
+ resetSamples()
+
+ /**
+ * Reset samples collected so far.
+ * This should be called after the collection undergoes a dramatic
change in size.
+ */
+ protected def resetSamples(): Unit = {
+ numUpdates = 1
+ nextSampleNum = 1
+ samples.clear()
+ takeSample()
+ }
+
+ /**
+ * Callback to be invoked after every update.
+ */
+ protected def afterUpdate(): Unit = {
+ numUpdates += 1
+ if (nextSampleNum == numUpdates) {
+ takeSample()
+ }
+ }
+
+ /**
+ * Take a new sample of the current collection's size.
+ */
+ private def takeSample(): Unit = {
+ samples += Sample(SizeEstimator.estimate(this), numUpdates)
+ // Only use the last two samples to extrapolate
+ if (samples.size > 2) {
+ samples.remove(0)
+ }
+ val bytesDelta = samples.toSeq.reverse match {
+ case latest :: previous :: tail =>
+ (latest.size - previous.size).toDouble / (latest.numUpdates -
previous.numUpdates)
+ // If fewer than 2 samples, assume no change
+ case _ => 0
+ }
+ bytesPerUpdate = math.max(0, bytesDelta)
+ nextSampleNum = math.ceil(numUpdates * SAMPLE_GROWTH_RATE).toLong
+ }
+
+ /**
+ * Estimate the current size of the collection in bytes. O(1) time.
+ */
+ def estimateSize(): Long = {
+ assert(samples.nonEmpty)
--- End diff --
First, because `samples` can never be empty (`resetSamples` always takes at
least one sample). Second, having 0 samples does not mean we have no data in
the underlying collection; it just means we don't have any information about
the current size.
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