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Track histogram of transport handling times #80581
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DaveCTurner
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DaveCTurner:2021-11-05-blocked-time-histogram
Nov 29, 2021
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e48ee01
Track histogram of transport handling times
DaveCTurner b7b2ec8
Merge branch 'master' into 2021-11-05-blocked-time-histogram
DaveCTurner 0d29128
Use raw times for better granularity
DaveCTurner a1ce073
Finer-grained buckets
DaveCTurner b527e82
Bucket count & bounds are known, no need to send over the wire
DaveCTurner c775e4e
Separate inbound & outbound histograms
DaveCTurner 6a967b9
Fix docs
DaveCTurner cd415c1
Merge branch 'master' into 2021-11-05-blocked-time-histogram
DaveCTurner 0e0ae90
Merge branch 'master' into 2021-11-05-blocked-time-histogram
DaveCTurner 8419a2d
Less magic
DaveCTurner 3ef1eff
Merge branch 'master' into 2021-11-05-blocked-time-histogram
DaveCTurner 90d2ef9
Merge branch 'master' of github.com:elastic/elasticsearch into 2021-1…
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65 changes: 65 additions & 0 deletions
65
server/src/main/java/org/elasticsearch/common/network/HandlingTimeTracker.java
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,65 @@ | ||
| /* | ||
| * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one | ||
| * or more contributor license agreements. Licensed under the Elastic License | ||
| * 2.0 and the Server Side Public License, v 1; you may not use this file except | ||
| * in compliance with, at your election, the Elastic License 2.0 or the Server | ||
| * Side Public License, v 1. | ||
| */ | ||
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| package org.elasticsearch.common.network; | ||
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| import java.util.concurrent.atomic.LongAdder; | ||
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| /** | ||
| * Tracks how long message handling takes on a transport thread as a histogram with fixed buckets. | ||
| */ | ||
| public class HandlingTimeTracker { | ||
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| public static int[] getBucketUpperBounds() { | ||
| int[] bounds = new int[17]; | ||
| for (int i = 0; i < bounds.length; i++) { | ||
| bounds[i] = 1 << i; | ||
| } | ||
| return bounds; | ||
| } | ||
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| private static int getBucket(long handlingTimeMillis) { | ||
| if (handlingTimeMillis <= 0) { | ||
| return 0; | ||
| } else if (LAST_BUCKET_LOWER_BOUND <= handlingTimeMillis) { | ||
| return BUCKET_COUNT - 1; | ||
| } else { | ||
| return Long.SIZE - Long.numberOfLeadingZeros(handlingTimeMillis); | ||
| } | ||
| } | ||
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| public static final int BUCKET_COUNT = getBucketUpperBounds().length + 1; | ||
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| private static final long LAST_BUCKET_LOWER_BOUND = getBucketUpperBounds()[BUCKET_COUNT - 2]; | ||
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| private final LongAdder[] buckets; | ||
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| public HandlingTimeTracker() { | ||
| buckets = new LongAdder[BUCKET_COUNT]; | ||
| for (int i = 0; i < BUCKET_COUNT; i++) { | ||
| buckets[i] = new LongAdder(); | ||
| } | ||
| } | ||
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| public void addHandlingTime(long handlingTimeMillis) { | ||
| buckets[getBucket(handlingTimeMillis)].increment(); | ||
| } | ||
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| /** | ||
| * @return An array of frequencies of handling times in buckets with upper bounds as returned by {@link #getBucketUpperBounds()}, plus | ||
| * an extra bucket for handling times longer than the longest upper bound. | ||
| */ | ||
| public long[] getHistogram() { | ||
| final long[] histogram = new long[BUCKET_COUNT]; | ||
| for (int i = 0; i < BUCKET_COUNT; i++) { | ||
| histogram[i] = buckets[i].longValue(); | ||
| } | ||
| return histogram; | ||
| } | ||
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| } |
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This may be spurious since it counts time spent waiting for the channel to become writeable (cf #77838). Should we track it separately from the inbound time tracking?
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I don't think it's going to be possible to differentiate between waiting for writable and time spent actually writing really (I know I promised differently a while back sorry about that). Definitely not easily. If you think about it, you could run into a non-writable channel and start counting from there. But then once it becomes writable again, you might not be the first in line to get your bytes flushed because some other write comes before you and may turn the channel not-writable again. And then that other write takes CPU for TLS and such, making it really hard to cleanly define what time was spent waiting.
I really like the current number for the simple fact that it gives an indication for the latency overall on a transport thread (while the inbound handler check indicates individual per message slowness). I don't really see how we could cleanly identify the fact that a channel isn't writable for an extended period of time and pinpoint that on the network.
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Yeah acking that we can't easily compute exactly what we want, but I still worry that we're putting two different numbers into the one histogram. Should we have two histograms, one for inbound things (which is purely handling-time) and one for outbound things (which potentially includes channel-blocked time)?
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Yea that would be optimal actually. Can we do that here? :)
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👍 done in c775e4e.
Another related observation is that we're not tracking outbound time for HTTP responses AFAICT. Should we? Can we?
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We could. I guess it would be nice to have but probably not all that valuable. The distribution on the outbound side for HTTP will be the same as that for sending transport messages. For REST outbound I'm almost thinking I'd rather like the distribution of serialization times there, because those have historically been the problem and it would give us more information when going on the hunt for why we have a bad distribution of outbound handling times.
IMO that'd be a worthwhile follow-up.