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https://issues.apache.org/jira/browse/CASSANDRA-14436?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16569012#comment-16569012
]
ASF GitHub Bot commented on CASSANDRA-14436:
--------------------------------------------
Github user dineshjoshi commented on a diff in the pull request:
https://github.com/apache/cassandra/pull/244#discussion_r207697430
--- Diff: src/java/org/apache/cassandra/metrics/FrequencySampler.java ---
@@ -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.cassandra.metrics;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.stream.Collectors;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import com.clearspring.analytics.stream.StreamSummary;
+
+/**
+ * Find the most frequent sample. A sample adds to the sum of its key ie
+ * <p>add("x", 10); and add("x", 20); will result in "x" = 30</p> This
uses StreamSummary to only store the
+ * approximate cardinality (capacity) of keys. If the number of distinct
keys exceed the capacity, the error of the
+ * sample may increase depending on distribution of keys among the total
set.
+ *
+ * @param <T>
+ */
+public abstract class FrequencySampler<T> extends Sampler<T>
+{
+ private static final Logger logger =
LoggerFactory.getLogger(FrequencySampler.class);
+ private boolean enabled = false;
+
+ private StreamSummary<T> summary;
+
+ /**
+ * Start to record samples
+ *
+ * @param capacity
+ * Number of sample items to keep in memory, the lower this
is
+ * the less accurate results are. For best results use value
+ * close to cardinality, but understand the memory trade
offs.
+ */
+ public synchronized void beginSampling(int capacity)
+ {
+ if (!enabled)
+ {
+ summary = new StreamSummary<T>(capacity);
+ enabled = true;
+ }
+ }
+
+ /**
+ * Call to stop collecting samples, and gather the results
+ * @param count Number of most frequent items to return
+ */
+ public synchronized List<Sample<T>> finishSampling(int count)
+ {
+ List<Sample<T>> results = Collections.EMPTY_LIST;
+ if (enabled)
+ {
+ enabled = false;
+ results = summary.topK(count)
+ .stream()
+ .map(c -> new Sample<T>(c.getItem(),
c.getCount(), c.getError()))
+ .collect(Collectors.toList());
+ }
+ return results;
+ }
+
+ protected synchronized void insert(final T item, final long value)
+ {
+ // samplerExecutor is single threaded but still need
+ // synchronization against jmx calls to finishSampling
+ if (enabled && value > 0)
+ {
+ try
+ {
+ summary.offer(item, (int) Math.min(value,
Integer.MAX_VALUE));
+ } catch (Exception e)
+ {
+ logger.trace("Failure to offer sample", e);
+ }
+ }
+ }
+
+ public boolean isEnabled()
+ {
+ return enabled;
+ }
+
+ public void setEnabled(boolean enabled)
+ {
+ this.enabled = enabled;
--- End diff --
This allows the user of the class to enable the `FrequencySampler` without
actually initializing the `summary` variable. This will cause an NPE.
> Add sampler for query time and expose with nodetool
> ---------------------------------------------------
>
> Key: CASSANDRA-14436
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14436
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Chris Lohfink
> Assignee: Chris Lohfink
> Priority: Major
>
> Create a new {{nodetool profileload}} that functions just like toppartitions
> but with more data, returning the slowest local reads and writes on the host
> during a given duration and highest frequency touched partitions (same as
> {{nodetool toppartitions}}). Refactor included to extend use of the sampler
> for uses outside of top frequency (max instead of total sample values).
> Future work to this is to include top cpu and allocations by query and
> possibly tasks/cpu/allocations by stage during time window.
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