feat: add gather_time and coalesce_time metrics to shuffle write#3734
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andygrove wants to merge 2 commits intoapache:mainfrom
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feat: add gather_time and coalesce_time metrics to shuffle write#3734andygrove wants to merge 2 commits intoapache:mainfrom
andygrove wants to merge 2 commits intoapache:mainfrom
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Pass coalesce_time to BufBatchWriter::write and flush calls across all callers. Add gather_time instrumentation around PartitionedBatchIterator iteration in both multi_partition shuffle_write_partition and partition_writer spill to measure interleave_record_batch time.
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Which issue does this PR close?
No issue. This adds observability to the shuffle write path.
Rationale for this change
The shuffle write path has existing metrics for
repart_time(partition ID computation),encode_time(IPC + compression), andwrite_time(disk I/O), but a large portion of shuffle write time was unaccounted for. Profiling TPC-H 100GB revealed thatinterleave_record_batch(the gather step that pulls rows from buffered batches into per-partition output) accounts for 55% of total shuffle write time, but this was invisible in the existing metrics.What changes are included in this PR?
Adds two new timing metrics to the shuffle write path:
gather_time: Time spent ininterleave_record_batch, which gathers rows from buffered batches into per-partition batches during the write phase. This is the most expensive step in the shuffle write path (55.4% of total time in TPC-H 100GB benchmarks).coalesce_time: Time spent inBatchCoalescer, which merges small batches before IPC serialization (1.4% of total time).These metrics are wired through from the Rust native code to Spark SQL metrics, appearing alongside existing shuffle metrics in the Spark UI.
TPC-H 100GB Shuffle Write Breakdown (with new metrics)
The timing overhead is negligible (~0.3s out of 1268s total, using
Instant::now()which is ~50ns per call).How are these changes tested?
Existing shuffle tests (
CometNativeShuffleSuite,CometExecSuite) continue to pass. The metrics are additive — they don't change any behavior, only add timing instrumentation to existing code paths. Verified with TPC-H 100GB benchmark that metrics are correctly reported and sum to expected totals.