[GitHub] [ignite-3] denis-chudov commented on a diff in pull request #957: Ignite 17354

[GitBox <gi...@apache.org>]

denis-chudov commented on code in PR #957:
URL: https://github.com/apache/ignite-3/pull/957#discussion_r937679438


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modules/metrics/src/main/java/org/apache/ignite/internal/metrics/scalar/HitRateMetric.java:
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@@ -0,0 +1,208 @@
+/*
+ * 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.ignite.internal.metrics.scalar;
+
+import static org.apache.ignite.internal.util.IgniteUtils.currentTimeMillis;
+import static org.apache.ignite.internal.util.IgniteUtils.ensure;
+
+import java.util.concurrent.atomic.AtomicLongArray;
+import org.apache.ignite.internal.metrics.AbstractMetric;
+import org.jetbrains.annotations.Nullable;
+
+/**
+ * Accumulates approximate hit rate statistics.
+ * Calculates number of hits in last {@code rateTimeInterval} milliseconds.
+ * Algorithm is based on circular array of {@code size} hit counters, each is responsible for last corresponding time
+ * interval of {@code rateTimeInterval}/{@code size} milliseconds. Resulting number of hits is sum of all counters.
+ *
+ * <p>Implementation is nonblocking and protected from hits loss.
+ * Maximum relative error is 1/{@code size}.
+ * 2^55 - 1 hits per interval can be accumulated without numeric overflow.
+ */
+public class HitRateMetric extends AbstractMetric implements LongMetric {
+    /** Default counters array size. */
+    public static final int DFLT_SIZE = 10;
+
+    /** Bits that store actual hit count. */
+    private static final int TAG_OFFSET = 56;
+
+    /** Useful part mask. */
+    private static final long NO_TAG_MASK = ~(-1L << TAG_OFFSET);
+
+    /** Time interval when hits are counted to calculate rate, in milliseconds. */
+    private final long rateTimeInterval;
+
+    /** Counters array size. */
+    private final int size;
+
+    /** Tagged counters. */
+    private final AtomicLongArray taggedCounters;
+
+    /** Last hit times. */
+    private final AtomicLongArray lastHitTimes;
+
+    /**
+     * The constructor.
+     *
+     * @param name Name.
+     * @param desc Description.
+     * @param rateTimeInterval Rate time interval in milliseconds.
+     */
+    public HitRateMetric(String name, @Nullable String desc, long rateTimeInterval) {
+        this(name, desc, rateTimeInterval, DFLT_SIZE);
+    }
+
+    /**
+     * The constructor.
+     *
+     * @param name Name.
+     * @param desc Description.
+     * @param rateTimeInterval Rate time interval in milliseconds.
+     * @param size Counters array size.
+     */
+    public HitRateMetric(String name, @Nullable String desc, long rateTimeInterval, int size) {
+        super(name, desc);
+
+        ensure(rateTimeInterval > 0, "rateTimeInterval should be positive");
+        ensure(size > 1, "Minimum value for size is 2");
+
+        this.rateTimeInterval = rateTimeInterval;
+        this.size = size;
+        taggedCounters = new AtomicLongArray(size);
+        lastHitTimes = new AtomicLongArray(size);
+    }
+
+    /**
+     * Adds x to the metric.
+     *
+     * @param hits Value to be added.
+     */
+    public void add(long hits) {
+        long curTs = currentTimeMillis();
+
+        int curPos = position(curTs);
+
+        clearIfObsolete(curTs, curPos);
+
+        lastHitTimes.set(curPos, curTs);
+
+        // Order is important. Hit won't be cleared by concurrent #clearIfObsolete.
+        taggedCounters.addAndGet(curPos, hits);
+    }
+
+    /** Adds 1 to the metric. */
+    public void increment() {
+        add(1);
+    }
+
+    /** {@inheritDoc} */
+    @Override public long value() {
+        long curTs = currentTimeMillis();
+
+        long sum = 0;
+
+        for (int i = 0; i < size; i++) {
+            clearIfObsolete(curTs, i);
+
+            sum += untag(taggedCounters.get(i));
+        }
+
+        return sum;
+    }
+
+    /**
+     * Clear specific counter if obsolete.
+     *
+     * @param curTs Current timestamp.
+     * @param i Index.
+     */
+    private void clearIfObsolete(long curTs, int i) {
+        long cur = taggedCounters.get(i);
+
+        byte curTag = getTag(cur);
+
+        long lastTs = lastHitTimes.get(i);
+
+        if (isObsolete(curTs, lastTs)) {
+            if (taggedCounters.compareAndSet(i, cur, taggedLongZero(++curTag))) { // ABA problem prevention.
+                lastHitTimes.set(i, curTs);
+            }
+            // If CAS failed, counter is reset by another thread.
+        }
+    }
+
+    /**
+     * Whether the last hit time was too long ago.
+     *
+     * @param curTs Current timestamp.
+     * @param lastHitTime Last hit timestamp.
+     * @return True, is last hit time was too long ago.
+     */
+    private boolean isObsolete(long curTs, long lastHitTime) {
+        return curTs - lastHitTime > rateTimeInterval * (size - 1) / size;
+    }
+
+    /**
+     * Index of counter for given timestamp.
+     *
+     * @param time Timestamp.
+     * @return Index of counter for given timestamp.
+     */
+    private int position(long time) {
+        return (int) ((time % rateTimeInterval * size) / rateTimeInterval);
+    }
+
+    /**
+     * Create a zero value with tag byte.
+     *
+     * @param tag Tag byte.
+     * @return 0L with given tag byte.
+     */
+    private static long taggedLongZero(byte tag) {
+        return ((long) tag << TAG_OFFSET);
+    }
+
+    /**
+     * Long without tag byte.
+     *
+     * @param l Tagged long.
+     * @return Long without tag byte.
+     */
+    private static long untag(long l) {
+        return l & NO_TAG_MASK;
+    }
+
+    /**
+     * Tag byte.
+     *
+     * @param taggedLong Tagged long.
+     * @return Tag byte.
+     */
+    private static byte getTag(long taggedLong) {
+        return (byte) (taggedLong >> TAG_OFFSET);
+    }
+
+    /**
+     * Rate time interval in milliseconds.
+     *
+     * @return Rate time interval in milliseconds.
+     */
+    public long rateTimeInterval() {

Review Comment:
   It could be used later by exporters.



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