While hash collisions are quite rare, the current state of the client
library carries a risk of merging two separate label values into a
single metric bucket. The effects are near impossible to detect and the
result will be missing or incorrect counters.
This changeset handles hash collisions by falling back to collision
resolution if multiple label values hash to the same value. This works
similar to separate chaining using a slice. Extra storage is minimized
to only the value key slice to that metrics can be differentiated
within a bucket.
In general, the cost of handling collisions is completely minimized
under normal operation. Performance does show slight increases in
certain areas, but these are more likely statistically anomalies. More
importantly, zero allocation behavior for metrics is preserved on the
fast path. Minimal allocations may be made during collision handling but
this has minimal effect.
Benchmark comparisons with and without collision resolution follow.
```
benchmark old ns/op new ns/op delta
BenchmarkCounterWithLabelValues-4 99.0 107 +8.08%
BenchmarkCounterWithLabelValuesConcurrent-4 79.6 91.0 +14.32%
BenchmarkCounterWithMappedLabels-4 518 542 +4.63%
BenchmarkCounterWithPreparedMappedLabels-4 127 137 +7.87%
BenchmarkCounterNoLabels-4 19.5 19.1 -2.05%
BenchmarkGaugeWithLabelValues-4 97.4 110 +12.94%
BenchmarkGaugeNoLabels-4 12.4 10.3 -16.94%
BenchmarkSummaryWithLabelValues-4 1204 915 -24.00%
BenchmarkSummaryNoLabels-4 936 847 -9.51%
BenchmarkHistogramWithLabelValues-4 147 147 +0.00%
BenchmarkHistogramNoLabels-4 50.6 49.3 -2.57%
BenchmarkHistogramObserve1-4 37.9 37.5 -1.06%
BenchmarkHistogramObserve2-4 122 137 +12.30%
BenchmarkHistogramObserve4-4 310 352 +13.55%
BenchmarkHistogramObserve8-4 691 729 +5.50%
BenchmarkHistogramWrite1-4 3374 3097 -8.21%
BenchmarkHistogramWrite2-4 5310 5051 -4.88%
BenchmarkHistogramWrite4-4 12094 10690 -11.61%
BenchmarkHistogramWrite8-4 19416 17755 -8.55%
BenchmarkHandler-4 11934304 13765894 +15.35%
BenchmarkSummaryObserve1-4 1119 1105 -1.25%
BenchmarkSummaryObserve2-4 3679 3430 -6.77%
BenchmarkSummaryObserve4-4 10678 7982 -25.25%
BenchmarkSummaryObserve8-4 22974 16689 -27.36%
BenchmarkSummaryWrite1-4 25962 14680 -43.46%
BenchmarkSummaryWrite2-4 38019 35073 -7.75%
BenchmarkSummaryWrite4-4 78027 56816 -27.18%
BenchmarkSummaryWrite8-4 117220 132248 +12.82%
BenchmarkMetricVecWithLabelValuesBasic-4 138 133 -3.62%
BenchmarkMetricVecWithLabelValues2Keys10ValueCardinality-4 150 144 -4.00%
BenchmarkMetricVecWithLabelValues4Keys10ValueCardinality-4 263 256 -2.66%
BenchmarkMetricVecWithLabelValues2Keys100ValueCardinality-4 145 155 +6.90%
BenchmarkMetricVecWithLabelValues10Keys100ValueCardinality-4 606 634 +4.62%
BenchmarkMetricVecWithLabelValues10Keys1000ValueCardinality-4 746 641 -14.08%
benchmark old allocs new allocs delta
BenchmarkCounterWithLabelValues-4 0 0 +0.00%
BenchmarkCounterWithLabelValuesConcurrent-4 0 0 +0.00%
BenchmarkCounterWithMappedLabels-4 2 2 +0.00%
BenchmarkCounterWithPreparedMappedLabels-4 0 0 +0.00%
BenchmarkCounterNoLabels-4 0 0 +0.00%
BenchmarkGaugeWithLabelValues-4 0 0 +0.00%
BenchmarkGaugeNoLabels-4 0 0 +0.00%
BenchmarkSummaryWithLabelValues-4 0 0 +0.00%
BenchmarkSummaryNoLabels-4 0 0 +0.00%
BenchmarkHistogramWithLabelValues-4 0 0 +0.00%
BenchmarkHistogramNoLabels-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValuesBasic-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues2Keys10ValueCardinality-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues4Keys10ValueCardinality-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues2Keys100ValueCardinality-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues10Keys100ValueCardinality-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues10Keys1000ValueCardinality-4 0 0 +0.00%
benchmark old bytes new bytes delta
BenchmarkCounterWithLabelValues-4 0 0 +0.00%
BenchmarkCounterWithLabelValuesConcurrent-4 0 0 +0.00%
BenchmarkCounterWithMappedLabels-4 336 336 +0.00%
BenchmarkCounterWithPreparedMappedLabels-4 0 0 +0.00%
BenchmarkCounterNoLabels-4 0 0 +0.00%
BenchmarkGaugeWithLabelValues-4 0 0 +0.00%
BenchmarkGaugeNoLabels-4 0 0 +0.00%
BenchmarkSummaryWithLabelValues-4 0 0 +0.00%
BenchmarkSummaryNoLabels-4 0 0 +0.00%
BenchmarkHistogramWithLabelValues-4 0 0 +0.00%
BenchmarkHistogramNoLabels-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValuesBasic-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues2Keys10ValueCardinality-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues4Keys10ValueCardinality-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues2Keys100ValueCardinality-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues10Keys100ValueCardinality-4 0 0 +0.00%
BenchmarkMetricVecWithLabelValues10Keys1000ValueCardinality-4 0 0 +0.00%
```
Signed-off-by: Stephen J Day <stephen.day@docker.com>
This rewrite had may backs and forths. In my git repository, it
consists of 35 commits which I cannot group or merge into reasonable
review buckets. Gerrit breaks fundamental git semantics, so I have to
squash the 35 commits into one for the review.
I'll push this not with refs/for/master, but with refs/for/next so
that we can transition after submission in a controlled fashion.
For the review, I recommend to start with looking at godoc and in
particular the many examples. After that, continue with a line-by-line
detailed review. (The big picture is hopefully as expected after
wrapping up the discussion earlier.)
Change-Id: Ib38cc46493a5139ca29d84020650929d94cac850
Stephen J Day