JayajP opened a new pull request, #30769:
URL: https://github.com/apache/beam/pull/30769
Implement `LockFreeHistogram` and use it in Dataflow's PerWorkerMetrics.
Currently, we record histogram metrics in `HistogramData`; however, this
class locks the histogram on each metric update/extraction. I assume this is
done for historic reasons since `HistogramData` also supports computing
arbitrary percentiles on the recorded values and this computation would be
inconsistent without locking on reads/writes. For
PerWorkerMetrics/BigQueryMetrics we do not need percentiles so we can record
metrics in a lock free manner.
LockFreeHistogram stores the following attributes on the values recorded:
- Count of number of values recorded in each bucket for recorded values that
are in the bucket boundaries.
- Stored in a
[AtomicLongArray](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/atomic/AtomicLongArray.html)
- Count/Mean of values the values recorded that are below the bucket
boundaries (UnderflowStatistics)
- This is stored in an AtomicReference and updated with
`AtomicReference::compareAndSet` similar to how existing [Distribution
metrics](https://github.com/apache/beam/blob/45444209c776460ba67ac14a1c33e0775485266a/runners/core-java/src/main/java/org/apache/beam/runners/core/metrics/DistributionData.java#L33)
are recorded.
- Count/Mean of values the values recorded that are over the bucket
boundaries (OverflowStatistic)
- stored similarly to UnderflowStatistic.
LockFreeHistogram supports extracting a delta update with the
`getSnapshotAndReset` method. The delta update represents values that have been
recorded in this histogram since the last time this method was called.
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