metrics: fast-path PB/JSON dump for empty histograms It turns out that a decent percentage of our histograms are empty in real life use cases. For example, some tablets might be cold enough that they have never had any action since the tserver started. Other histograms might correspond to features which aren't being used in a particular workload (eg commit-wait counters).
For example, in a test cluster running a cycling YCSB workload, about 20% of histograms are empty. In another test cluster running more of an analytic workload with a lot of very cold tablets, 81% of histograms were empty. We can fast-path various calculations on these histograms to just return 0 for a simple speedup on the /metricsjson endpoint. Change-Id: I0732e8299e2f27c7c6021ec70740984ab46a69c0 Reviewed-on: http://gerrit.cloudera.org:8080/9179 Tested-by: Kudu Jenkins Reviewed-by: Will Berkeley <wdberke...@gmail.com> Project: http://git-wip-us.apache.org/repos/asf/kudu/repo Commit: http://git-wip-us.apache.org/repos/asf/kudu/commit/afae5c75 Tree: http://git-wip-us.apache.org/repos/asf/kudu/tree/afae5c75 Diff: http://git-wip-us.apache.org/repos/asf/kudu/diff/afae5c75 Branch: refs/heads/master Commit: afae5c75112c9938ed9f2b335a75a92ea63f5587 Parents: 3e49fe2 Author: Todd Lipcon <t...@apache.org> Authored: Wed Jan 31 18:02:03 2018 -0800 Committer: Todd Lipcon <t...@apache.org> Committed: Sat Feb 3 01:08:32 2018 +0000 ---------------------------------------------------------------------- src/kudu/util/metrics.cc | 58 +++++++++++++++++++++++++++---------------- src/kudu/util/metrics.h | 2 +- 2 files changed, 38 insertions(+), 22 deletions(-) ---------------------------------------------------------------------- http://git-wip-us.apache.org/repos/asf/kudu/blob/afae5c75/src/kudu/util/metrics.cc ---------------------------------------------------------------------- diff --git a/src/kudu/util/metrics.cc b/src/kudu/util/metrics.cc index 4954bac..6565d97 100644 --- a/src/kudu/util/metrics.cc +++ b/src/kudu/util/metrics.cc @@ -627,34 +627,50 @@ Status Histogram::WriteAsJson(JsonWriter* writer, Status Histogram::GetHistogramSnapshotPB(HistogramSnapshotPB* snapshot_pb, const MetricJsonOptions& opts) const { - HdrHistogram snapshot(*histogram_); snapshot_pb->set_name(prototype_->name()); if (opts.include_schema_info) { snapshot_pb->set_type(MetricType::Name(prototype_->type())); snapshot_pb->set_label(prototype_->label()); snapshot_pb->set_unit(MetricUnit::Name(prototype_->unit())); snapshot_pb->set_description(prototype_->description()); - snapshot_pb->set_max_trackable_value(snapshot.highest_trackable_value()); - snapshot_pb->set_num_significant_digits(snapshot.num_significant_digits()); + snapshot_pb->set_max_trackable_value(histogram_->highest_trackable_value()); + snapshot_pb->set_num_significant_digits(histogram_->num_significant_digits()); } - snapshot_pb->set_total_count(snapshot.TotalCount()); - snapshot_pb->set_total_sum(snapshot.TotalSum()); - snapshot_pb->set_min(snapshot.MinValue()); - snapshot_pb->set_mean(snapshot.MeanValue()); - snapshot_pb->set_percentile_75(snapshot.ValueAtPercentile(75)); - snapshot_pb->set_percentile_95(snapshot.ValueAtPercentile(95)); - snapshot_pb->set_percentile_99(snapshot.ValueAtPercentile(99)); - snapshot_pb->set_percentile_99_9(snapshot.ValueAtPercentile(99.9)); - snapshot_pb->set_percentile_99_99(snapshot.ValueAtPercentile(99.99)); - snapshot_pb->set_max(snapshot.MaxValue()); - - if (opts.include_raw_histograms) { - RecordedValuesIterator iter(&snapshot); - while (iter.HasNext()) { - HistogramIterationValue value; - RETURN_NOT_OK(iter.Next(&value)); - snapshot_pb->add_values(value.value_iterated_to); - snapshot_pb->add_counts(value.count_at_value_iterated_to); + // Fast-path for a reasonably common case of an empty histogram. This occurs + // when a histogram is tracking some information about a feature not in + // use, for example. + if (histogram_->TotalCount() == 0) { + snapshot_pb->set_total_count(0); + snapshot_pb->set_total_sum(0); + snapshot_pb->set_min(0); + snapshot_pb->set_mean(0); + snapshot_pb->set_percentile_75(0); + snapshot_pb->set_percentile_95(0); + snapshot_pb->set_percentile_99(0); + snapshot_pb->set_percentile_99_9(0); + snapshot_pb->set_percentile_99_99(0); + snapshot_pb->set_max(0); + } else { + HdrHistogram snapshot(*histogram_); + snapshot_pb->set_total_count(snapshot.TotalCount()); + snapshot_pb->set_total_sum(snapshot.TotalSum()); + snapshot_pb->set_min(snapshot.MinValue()); + snapshot_pb->set_mean(snapshot.MeanValue()); + snapshot_pb->set_percentile_75(snapshot.ValueAtPercentile(75)); + snapshot_pb->set_percentile_95(snapshot.ValueAtPercentile(95)); + snapshot_pb->set_percentile_99(snapshot.ValueAtPercentile(99)); + snapshot_pb->set_percentile_99_9(snapshot.ValueAtPercentile(99.9)); + snapshot_pb->set_percentile_99_99(snapshot.ValueAtPercentile(99.99)); + snapshot_pb->set_max(snapshot.MaxValue()); + + if (opts.include_raw_histograms) { + RecordedValuesIterator iter(&snapshot); + while (iter.HasNext()) { + HistogramIterationValue value; + RETURN_NOT_OK(iter.Next(&value)); + snapshot_pb->add_values(value.value_iterated_to); + snapshot_pb->add_counts(value.count_at_value_iterated_to); + } } } return Status::OK(); http://git-wip-us.apache.org/repos/asf/kudu/blob/afae5c75/src/kudu/util/metrics.h ---------------------------------------------------------------------- diff --git a/src/kudu/util/metrics.h b/src/kudu/util/metrics.h index 60412c2..aa553c9 100644 --- a/src/kudu/util/metrics.h +++ b/src/kudu/util/metrics.h @@ -1009,7 +1009,7 @@ class Histogram : public Metric { const MetricJsonOptions& opts) const OVERRIDE; // Returns a snapshot of this histogram including the bucketed values and counts. - Status GetHistogramSnapshotPB(HistogramSnapshotPB* snapshot, + Status GetHistogramSnapshotPB(HistogramSnapshotPB* snapshot_pb, const MetricJsonOptions& opts) const; uint64_t CountInBucketForValueForTests(uint64_t value) const;
