netudima commented on code in PR #4324: URL: https://github.com/apache/cassandra/pull/4324#discussion_r2317052439
########## src/java/org/apache/cassandra/metrics/ThreadLocalMeter.java: ########## @@ -0,0 +1,372 @@ +/* + * 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.cassandra.metrics; + +import java.lang.ref.WeakReference; +import java.util.ArrayList; +import java.util.BitSet; +import java.util.List; +import java.util.concurrent.CopyOnWriteArrayList; +import java.util.concurrent.TimeUnit; +import java.util.concurrent.atomic.AtomicInteger; + +import com.google.common.annotations.VisibleForTesting; + +import com.codahale.metrics.Clock; +import org.apache.cassandra.concurrent.ScheduledExecutors; +import org.apache.cassandra.utils.MonotonicClock; +import org.apache.cassandra.utils.ReflectionUtils; + +import static java.lang.Math.exp; + +/** + * An alternative to Dropwizard Meter which implements the same kind of API. + * it has more efficent mark operations and consumes less memory. + * Only exponential decaying moving average is supported for 1/5/15-minutes rate values. + * Tick logic is moved out from a mark operation and always executed in a background thread. + * For better cache locality rate values are extracted to a common non thread-local array + * updated by a background thread in bulk. + * Counter logic inside is implemented using @see ThreadLocalMetrics functionality. + * NOTE: Dropwizard Meter is a class and there is no an interface for Dropwizard Meter logic, + * so we have to create an alternative hierarchy. + */ +public class ThreadLocalMeter extends com.codahale.metrics.Meter implements Meter +{ + private static final int INTERVAL_SEC = 5; + private static final long TICK_INTERVAL_NS = TimeUnit.SECONDS.toNanos(INTERVAL_SEC); + private static final double SECONDS_PER_MINUTE = 60.0; + private static final int ONE_MINUTE = 1; + private static final int FIVE_MINUTES = 5; + private static final int FIFTEEN_MINUTES = 15; + private static final double M1_ALPHA = 1 - exp(-INTERVAL_SEC / SECONDS_PER_MINUTE / ONE_MINUTE); + private static final double M5_ALPHA = 1 - exp(-INTERVAL_SEC / SECONDS_PER_MINUTE / FIVE_MINUTES); + private static final double M15_ALPHA = 1 - exp(-INTERVAL_SEC / SECONDS_PER_MINUTE / FIFTEEN_MINUTES); + + // each meter instance stores rate stats using 3 consecutive cells in rates array + private static final int M1_RATE_OFFSET = 0; + private static final int M5_RATE_OFFSET = 1; + private static final int M15_RATE_OFFSET = 2; + private static final int RATES_COUNT = 3; + private static final double NON_INITIALIZED = Double.MIN_VALUE; + + private static final int BACKGROUND_TICK_INTERVAL_SEC = INTERVAL_SEC; + private static final List<WeakReference<ThreadLocalMeter>> allMeters = new CopyOnWriteArrayList<>(); + + /** + * CASSANDRA-19332 + * If ticking would reduce even Long.MAX_VALUE in the 15 minute EWMA below this target then don't bother + * ticking in a loop and instead reset all the EWMAs. + */ + private static final double maxTickZeroTarget = 0.0001; + private static final int maxTicks; + + static + { + int m3Ticks = 1; + double emulatedM15Rate = 0.0; + emulatedM15Rate = tickFifteenMinuteEWMA(emulatedM15Rate, Long.MAX_VALUE); + do + { + emulatedM15Rate = tickFifteenMinuteEWMA(emulatedM15Rate, 0); + m3Ticks++; + } + while (getRatePerSecond(emulatedM15Rate) > maxTickZeroTarget); + maxTicks = m3Ticks; + } + + static + { + ScheduledExecutors.scheduledTasks.scheduleWithFixedDelay(ThreadLocalMeter::tickAll, + BACKGROUND_TICK_INTERVAL_SEC, + BACKGROUND_TICK_INTERVAL_SEC, + TimeUnit.SECONDS); + } + + private static final Object ratesArrayGuard = new Object(); + + // writes should be synchonized using ratesArrayGuard + // 1) write during a copy when we need to extend rates array when a new Meter is created + // 2) write during a tick + private static volatile double[] rates = new double[RATES_COUNT * 16]; + static final AtomicInteger rateGroupIdGenerator = new AtomicInteger(); + + private static final Object freeRateGroupIdSetGuard = new Object(); + + // we recycle and reuse rate group IDs + // a set bit means the correspondent rate group id is available to use + private static final BitSet freeRateGroupIdSet = new BitSet(); + + private static int allocateRateGroupOffset() + { + int rateGroupId; + synchronized (freeRateGroupIdSetGuard) + { + rateGroupId = freeRateGroupIdSet.nextSetBit(0); + if (rateGroupId >= 0) + freeRateGroupIdSet.clear(rateGroupId); + } + if (rateGroupId < 0) + { + rateGroupId = rateGroupIdGenerator.getAndAdd(RATES_COUNT); + } + synchronized (ratesArrayGuard) + { + if (rates.length < rateGroupId + RATES_COUNT) + { + double[] newRates = new double[(int) (rateGroupId + RATES_COUNT)]; Review Comment: yes, it is int, I do not remember why do I have this logic :-), probably it is an artefact from an old alternative draft how to extend the array. -- This is an automated message from the Apache Git Service. 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