batch: retain hottest arena chunk and grow chunk caps

[snissn]

Summary

• retain the largest reusable batch arena chunk on Reset instead of always retaining the first chunk
• add bounded geometric growth in ensureArenaChunk (up to batchArenaMaxRetainCap) to reduce repeated chunk allocations in large batches
• add unit tests for both invariants:
  • reset keeps largest reusable chunk and drops oversized-only sets
  • chunk capacities grow geometrically as chunks are exhausted

Why

batch.(*Batch).ensureArenaChunk remained one of the top allocators in write-heavy profiles. The old reset behavior repeatedly reverted pooled batches to a small first chunk, and chunk growth remained too incremental.

Validation

• go test ./TreeDB/batch ./TreeDB/caching
• time ./bin/unified-bench -dbs treedb -profile fast -keys 500000 -progress=false -treedb-index-outer-leaves-in-vlog=true -valsize=100 -treedb-force-value-pointers=false -checkpoint-between-tests -profile-dir=/home/mikers/tmp/perf-batch-geogrow-b-1773435448

Comparison baseline (PR #818 head): /home/mikers/tmp/perf-entry-lease32b-1773434944

alloc_space (batch_write_steady)

• total: 138.13MB -> 122.66MB
• batch.(*Batch).ensureArenaChunk: 48.93MB -> 31.27MB

ops/sec highlights

• batch_write_steady: 2,107,601 -> 2,231,000 (+5.9%)
• batch_write: 12,362,290 -> 12,610,767 (+2.0%)
• random_write: 5,039,465 -> 5,218,219 (+3.5%)
• batch_random: 8,377,934 -> 8,278,773 (-1.2%)
• dataset_write_sorted: 3,656,106 -> 3,634,457 (-0.6%)

[Copilot]

Pull request overview

This PR optimizes batch.Batch’s internal copy arena reuse/growth to reduce allocations on large or write-heavy workloads, and adds tests to lock in the new reuse/growth invariants.

Changes:

• Update resetArenaLocked to retain the largest reusable arena chunk (bounded by batchArenaMaxRetainCap) instead of always keeping the first.
• Add bounded geometric growth logic in ensureArenaChunk to reduce repeated chunk allocations for large batches.
• Add unit tests covering reset retention/drop behavior and geometric chunk growth.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
TreeDB/batch/batch.go	Adjust arena reset retention policy and implement geometric growth for subsequent arena chunks.
TreeDB/batch/batch_arena_test.go	Add tests validating retained chunk selection, oversized-chunk dropping, and geometric growth behavior.

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[chatgpt-codex-connector]

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[Copilot]

Pull request overview

This PR reduces allocations in TreeDB/batch by improving batch arena chunk reuse across Batch.Reset() and by introducing bounded geometric growth when allocating additional arena chunks during large batch writes.

Changes:

• Update Batch.resetArenaLocked() to retain the largest reusable arena chunk (bounded by batchArenaMaxRetainCap) instead of always retaining the first chunk.
• Update Batch.ensureArenaChunk() to grow subsequent chunk capacities geometrically (doubling, capped at batchArenaMaxRetainCap) to reduce repeated allocations in large batches.
• Add unit tests covering the new reset-retention invariant and geometric growth behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
TreeDB/batch/batch.go	Adjusts arena reset retention policy and implements bounded geometric growth for chunk allocation.
TreeDB/batch/batch_arena_test.go	Adds tests ensuring the largest reusable chunk is retained (or oversized-only sets dropped) and that chunk caps grow geometrically.

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