belliottsmith commented on code in PR #4324: URL: https://github.com/apache/cassandra/pull/4324#discussion_r2340128171
########## src/java/org/apache/cassandra/metrics/ThreadLocalMetrics.java: ########## @@ -0,0 +1,377 @@ +/* + * 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.PhantomReference; +import java.lang.ref.ReferenceQueue; +import java.util.BitSet; +import java.util.Collections; +import java.util.List; +import java.util.Set; +import java.util.concurrent.ConcurrentHashMap; +import java.util.concurrent.CopyOnWriteArrayList; +import java.util.concurrent.TimeUnit; +import java.util.concurrent.TimeoutException; +import java.util.concurrent.atomic.AtomicInteger; +import java.util.concurrent.atomic.AtomicLongArray; +import java.util.concurrent.locks.Lock; +import java.util.concurrent.locks.ReadWriteLock; +import java.util.concurrent.locks.ReentrantReadWriteLock; + +import com.google.common.annotations.VisibleForTesting; + +import io.netty.util.concurrent.FastThreadLocal; +import org.apache.cassandra.concurrent.Shutdownable; + +import static com.google.common.collect.ImmutableList.of; +import static org.apache.cassandra.concurrent.ExecutorFactory.Global.executorFactory; +import static org.apache.cassandra.concurrent.InfiniteLoopExecutor.SimulatorSafe.UNSAFE; +import static org.apache.cassandra.utils.ExecutorUtils.shutdownAndWait; + +/** + * A thread-local counter implementation designed to use in metrics as an alternative to LongAdder used by Dropwizard metrics. + * This implementation has reduced write (increment) CPU usage costs in exchange for a higher read cost. + * We keep and increment parts of a counter locally for each thread. + * To reduce memory footprint per counter they are grouped together to a long[] array for each thread. + * A position of a counter value is the same for every thread for the same counter id. + * Piggyback volatile visibility is expected for readers who execute getCount method to see recent writes to thread local arrays. + * If a metric is not used anymore the position in the array is reused. Phantom references are used to track aliveness of metric users. + * When a thread died the counter values accumulated by it are transfered to a shared summaryValues collection. + * Threads death is tracked using 2 approaches: FastThreadLocal.onRemoval callback and phantom references to Thread objects. + */ +public class ThreadLocalMetrics +{ + private static final int INITIAL_COUNTERS_CAPACITY = 16; + + static final AtomicInteger idGenerator = new AtomicInteger(); + + private static final Object freeMetricIdSetGuard = new Object(); + + @VisibleForTesting + static final BitSet freeMetricIdSet = new BitSet(); + + private static final List<ThreadLocalMetrics> allThreadLocalMetrics = new CopyOnWriteArrayList<>(); + + /* the lock is used to coordinate the threads which: + * 1) transfer values from a dead thread to summaryValues + * 2) calculate a getCount value. + * Using this lock we want to avoid + * a value lost while moving it in getCount + * as well as a double-counting + */ + private static final ReadWriteLock summaryLock = new ReentrantReadWriteLock(); + + private static final FastThreadLocal<ThreadLocalMetrics> threadLocalMetricsCurrent = new FastThreadLocal<>() + { + @Override + protected ThreadLocalMetrics initialValue() + { + ThreadLocalMetrics result = new ThreadLocalMetrics(); + allThreadLocalMetrics.add(result); + destroyWhenUnreachable(Thread.currentThread(), result::release); + return result; + } + + // this method is invoked when a thread is going to finish, but it works only for FastThreadLocalThread + // so, we use phantom references for other cases + @Override + protected void onRemoval(ThreadLocalMetrics value) + { + value.release(); + } + }; + + private static volatile AtomicLongArray summaryValues = new AtomicLongArray(INITIAL_COUNTERS_CAPACITY); + + private static final Shutdownable cleaner; + private static final Set<PhantomReference<Object>> phantomReferences = Collections.newSetFromMap(new ConcurrentHashMap<>()); + private static final ReferenceQueue<Object> referenceQueue = new ReferenceQueue<>(); + + static + { + cleaner = executorFactory().infiniteLoop("ThreadLocalMetrics-Cleaner", ThreadLocalMetrics::cleanupOneReference, UNSAFE); + } + + // we assume that counterValues can be only extended + private long[] counterValues = new long[INITIAL_COUNTERS_CAPACITY]; + + private static void cleanupOneReference() throws InterruptedException + { + Object obj = referenceQueue.remove(100); + if (obj instanceof MetricIdReference) + { + ((MetricIdReference) obj).release(); + phantomReferences.remove(obj); + } + else if (obj instanceof MetricCleanerReference) + { + ((MetricCleanerReference) obj).release(); + phantomReferences.remove(obj); + } + } + + private static class MetricIdReference extends PhantomReference<Object> + { + private final int metricId; + + public MetricIdReference(Object referent, ReferenceQueue<? super Object> q, int metricId) + { + super(referent, q); + this.metricId = metricId; + } + + public void release() + { + recycleMetricId(metricId); + } + } + + private static class MetricCleanerReference extends PhantomReference<Object> + { + private final MetricCleaner metricCleaner; + + public MetricCleanerReference(Object referent, ReferenceQueue<? super Object> q, MetricCleaner metricCleaner) + { + super(referent, q); + this.metricCleaner = metricCleaner; + } + + public void release() + { + metricCleaner.clean(); + } + } + + interface MetricCleaner + { + void clean(); + } + + static void destroyWhenUnreachable(Object referent, int metricId) + { + phantomReferences.add(new MetricIdReference(referent, referenceQueue, metricId)); + } + + static void destroyWhenUnreachable(Object referent, MetricCleaner metricCleaner) + { + phantomReferences.add(new MetricCleanerReference(referent, referenceQueue, metricCleaner)); + } + + @VisibleForTesting + public static void shutdownCleaner(long timeout, TimeUnit unit) throws InterruptedException, TimeoutException + { + shutdownAndWait(timeout, unit, of(cleaner)); + } + + private void release() + { + // Using this lock while moving we want to avoid races with readers in getCount + // such races can cause a transfered value lost or its double-counting by a reader + Lock lock = summaryLock.writeLock(); + lock.lock(); + try + { + // we may try to release ThreadLocalMetrics 2 times: onRemoval and by PhantomReference + // so this if check is needed to avoid a potential double release + if (allThreadLocalMetrics.remove(this)) + { + for (int metricId = 0; metricId < counterValues.length; metricId++) + { + long value = counterValues[metricId]; + if (value != 0) + updateSummary(metricId, value); + } + } + } + finally + { + lock.unlock(); + } + } + + /** + * If we already have ThreadLocalMetrics instance looked up for the current thread + * we can use this method to avoid thread local lookup costs. + * It can be used if you need to update several counters at the same time. + * @param metricId metric to add a value + * @param n valuen to add, can be negative number as well + */ + public void addNonStatic(int metricId, long n) + { + getNonStatic(metricId)[metricId] += n; + } + + public static void add(int metricId, long n) + { + get(metricId)[metricId] += n; + } + + private static long getCount(int metricId, boolean resetToZero) + { + long result; + Lock readLock = summaryLock.readLock(); + readLock.lock(); + try + { + result = getSummaryValue(metricId); + for (ThreadLocalMetrics threadLocalMetrics : allThreadLocalMetrics) + { + long count = 0; + long[] currentCounterValues = threadLocalMetrics.counterValues; + // currentCounterValues is extended for a thread when a value for metricId is reported in the thread + if (metricId < currentCounterValues.length) + count = currentCounterValues[metricId]; + result += count; + } + if (resetToZero) + updateSummary(metricId, -result); // compensative reset without writing to thread local values + } + finally + { + readLock.unlock(); + } + return result; + } + + // must be executed under summaryLock + private static long getSummaryValue(int metricId) + { + return summaryValues.get(metricId); + } + + // must be executed under summaryLock + private static void updateSummary(int metricId, long value) + { + summaryValues.getAndAdd(metricId, value); + } + + public static long getCount(int metricId) + { + return getCount(metricId, false); + } + + public static long getCountAndReset(int metricId) + { + return getCount(metricId, true); + } + + public static ThreadLocalMetrics get() { + return threadLocalMetricsCurrent.get(); + } + + private static long[] get(int metricId) + { + ThreadLocalMetrics threadLocalMetrics = ThreadLocalMetrics.get(); + return threadLocalMetrics.getNonStatic(metricId); + } + + private long[] getNonStatic(int metricId) + { + long[] currentCounterValues = counterValues; + if (metricId < currentCounterValues.length) + return currentCounterValues; + + long[] newCounterValues = new long[calculateNewCapacity(metricId)]; + System.arraycopy(currentCounterValues, 0, newCounterValues, 0, currentCounterValues.length); + counterValues = newCounterValues; + return newCounterValues; + } + + private static int calculateNewCapacity(int metricId) + { + return Math.max(metricId + 1, (int)(metricId * 1.1) ); + } + + static int allocateMetricId() + { + int metricId; + synchronized (freeMetricIdSetGuard) + { + metricId = freeMetricIdSet.nextSetBit(0); + if (metricId >= 0) + freeMetricIdSet.clear(metricId); + } + if (metricId < 0) + metricId = idGenerator.getAndIncrement(); + + if (metricId >= summaryValues.length()) // double-checked locking + { + Lock lock = summaryLock.writeLock(); + lock.lock(); + try + { + if (metricId >= summaryValues.length()) + { + AtomicLongArray newSummaryValues = new AtomicLongArray(calculateNewCapacity(metricId)); + for (int i = 0; i < summaryValues.length(); i++) + newSummaryValues.set(i, summaryValues.get(i)); + summaryValues = newSummaryValues; + } + } + finally + { + lock.unlock(); + } + } + return metricId; + } + + static void recycleMetricId(int metricId) + { + // we use lock here to avoid potential issues when a metric is releasing and a thread is detected as dead at the same time + // in this case we may clean a summary value and later the thread removal logic may re-add a non-zero summary value + Lock lock = summaryLock.writeLock(); + lock.lock(); + try + { + for (ThreadLocalMetrics threadLocalMetrics : allThreadLocalMetrics) + { + long[] currentCounterValues = threadLocalMetrics.counterValues; + if (metricId < currentCounterValues.length) + currentCounterValues[metricId] = 0; Review Comment: I think we have a couple of race conditions here. The main one is when the threadLocalMetric resizes itself, which is done in without coordination wrt this operation, and may allow this reset to be lost. The less serious one is that there's no obvious happens-before relationship between this write and the read of the entry by the thread when it sees the metric id reused. Perhaps we can simply rely on the safe publication of the metric itself to that thread; this one doesn't overly worry me, anyway. The main problem can be addressed a few ways, the simplest being to synchronise on the instance object we're updating, and also when resizing. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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