Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/7648#discussion_r35921186
--- Diff:
streaming/src/main/scala/org/apache/spark/streaming/scheduler/rate/PIDRateEstimator.scala
---
@@ -0,0 +1,109 @@
+/*
+ * 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.streaming.scheduler.rate
+
+/**
+ * Implements a proportional-integral-derivative (PID) controller which
acts on
+ * the speed of ingestion of elements into Spark Streaming. A PID
controller works
+ * by calculating an '''error''' between a measured output and a desired
value. In the
+ * case of Spark Streaming the error is the difference between the
measured processing
+ * rate (number of elements/processing delay) and the previous rate.
+ *
+ * @see https://en.wikipedia.org/wiki/PID_controller
+ *
+ * @param batchDurationMillis the batch duration, in milliseconds
+ * @param proportional how much the correction should depend on the current
+ * error. This term usually provides the bulk of correction. A
value too large would
+ * make the controller overshoot the setpoint, while a small value
would make the
+ * controller too insensitive. The default value is -1.
+ * @param integral how much the correction should depend on the
accumulation
+ * of past errors. This term accelerates the movement towards the
setpoint, but a large
+ * value may lead to overshooting. The default value is -0.2.
+ * @param derivative how much the correction should depend on a prediction
+ * of future errors, based on current rate of change. This term is
not used very often,
+ * as it impacts stability of the system. The default value is 0.
+ */
+private[streaming] class PIDRateEstimator(
+ batchIntervalMillis: Long,
+ proportional: Double = -1D,
+ integral: Double = -.2D,
+ derivative: Double = 0D)
+ extends RateEstimator {
+
+ private var firstRun: Boolean = true
+ private var latestTime: Long = -1L
+ private var latestRate: Double = -1D
+ private var latestError: Double = -1L
+
+ require(
+ batchIntervalMillis > 0,
+ s"Specified batch interval $batchIntervalMillis in PIDRateEstimator is
invalid.")
+
+ def compute(time: Long, // in milliseconds
+ elements: Long,
+ processingDelay: Long, // in milliseconds
+ schedulingDelay: Long // in milliseconds
+ ): Option[Double] = {
+
+ this.synchronized {
+ if (time > latestTime && processingDelay > 0 && batchIntervalMillis
> 0) {
+
+ // in seconds, should be close to batchDuration
+ val delaySinceUpdate = (time - latestTime).toDouble / 1000
+
+ // in elements/second
+ val processingRate = elements.toDouble / processingDelay * 1000
+
+ // in elements/second
+ val error = latestRate - processingRate
+
+ // The error integral, based on schedulingDelay as an indicator
for accumulated errors
+ // a scheduling delay s corresponds to s * processingRate
overflowing elements. Those
+ // are elements that couldn't be processed in previous batches,
leading to this delay.
+ // We assume the processingRate didn't change too much.
+ // from the number of overflowing elements we can calculate the
rate at which they would be
--- End diff --
This is good, but there are grammatical issues. Please revise. E.g. "from"
should be capitalized if it was to be start of a sentence.
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