Reader: concurrent scan with a confined arena per worker thread

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java
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data, performance

调研方向

This is a design proposal, so the first step is deciding the open questions, not editing code. Read ScanIterator, Chunk and the existing confined-versus-shared arena handling, plus ADR 0003 for the exception contract on worker threads. Done means a recorded decision on shared versus per-worker mapping and ordered versus unordered delivery, then an A/B run with RealFileScanBenchmark.

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描述

enhancement perf

Parked design for a concurrent reader scan. Follow-up to #474 (writer side, shipped).

Reference behavior

  • Rust (vortex-layout/src/scan/scan_builder.rs): the scan is split into row-range tasks (SplitBy), each spawned onto the session runtime, up to concurrency * num_workers in flight (default 4 per worker). ordered (default) uses .buffered(n), otherwise .buffer_unordered(n). A per-split map function runs on the spawned task.
  • .NET (Vorticity): MaxDegreeOfParallelism per session or per scan, MaxConcurrentReads for I/O, shared SegmentCache, concurrent scans over one open file.
  • vortex-java today: ScanIterator is single-threaded, every arena is ofConfined.

Proposed shape

A confined Arena per worker thread, so the callback runs on the worker and the caller never touches decoded memory from another thread:

scan.parallel(executor)            // executor + bounded in-flight window on ScanOptions
    .map(chunk -> r)               // runs on the worker; chunk and its arena close on return
  • R must not reference chunk memory (aggregates, copied-out values); Javadoc states the contract.
  • Results delivered ordered (Rust buffered, default) or unordered (buffer_unordered).
  • The file mapping becomes ofShared (one close at the end), since workers must read it. Alternative: each worker maps its own view.
  • scan() / ScanIterator / Chunk lifetime rules stay unchanged.

Pros

  • Rust-shaped (map on the spawned task, ordered/unordered, bounded window), so parity with the reference.
  • Confined arenas keep the cheap alloc/close path; Arena.ofShared().close() needs a cross-thread handshake per chunk.
  • No change to the existing single-threaded API or its safety checks.
  • Same bounded-window idea as the writer's pending queue / MAX_IN_FLIGHT_SEGMENTS.
  • Overlaps decode with consumption; wide files and slow consumers benefit most.

Cons

  • No ordered, parallel, zero-copy Iterator<Chunk> for a consumer thread: a chunk decoded on a worker cannot be read elsewhere. That case would need a shared per-chunk arena (slower close, and relaxes the "second next() while a Chunk is open is rejected" rule).
  • Callback API is less ergonomic than an iterator; callers who must keep data have to copy out.
  • Mapping arena goes shared; any cross-thread use after close() becomes a hazard to test (ADR 0003 security contract still applies on worker threads: only VortexException may escape).
  • Worker exceptions must be propagated and in-flight work cancelled/drained on failure (the writer's discardPending equivalent).
  • Needs A/B on all 7 real files (RealFileScanBenchmark, interleaved): memory-bandwidth-bound scans may not scale, and oversubscribing the pool gains nothing.

Out of scope / not planned

  • Per-column parallel decode within a chunk: not a Rust pattern, so it would be a deliberate divergence needing an explicit decision.
  • Parallel I/O for VortexHttpReader (concurrent range requests): independent of this, worth its own ticket if pursued.
  • Writer follow-ups (parallel scheme trials in CascadingCompressor.chooseBest): comment on #474.
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