[storage/qmdb/bench] chained-growth benchmark

[roberto-bayardo]

Summary

Add a chained_growth benchmark to the qmdb suite that exercises the pipelined-speculation workload: applying a parent batch while a child batch is still alive, repeatedly, to exercise Current QMDB's committed-bitmap parent chain.

Benchmark shape

1. Setup (untimed): seed NUM_KEYS=1_000_000 keys, then grow a chain of PREBUILT_CHAIN=10_000 batches applying each parent while the child is alive. Commit and sync, so buffered seed data doesn't leak into the timed region's fsync.
2. Timed: do batches=100 more merkleize + apply iterations on top of the pre-built chain, then drop the final parent.

Each batch contains a single random key update (UPDATES_PER_BATCH=1) so that each batch's chunk overlay covers a tiny fraction of the total chunks. Combined with the deep pre-built chain, this forces each chunk read during merkleize to walk the chain before finding a matching layer — exposing the asymptotic cost of chain growth that shallower-coverage workloads mask.

Only Current variants are exercised (the chain-growth behavior is specific to Current's bitmap batching) with chunk sizes of 32 & 256. Variants are limited to fixed and MMB since value type and merkle family aren't relevant for this bench.

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[Copilot]

Pull request overview

Adds a new QMDB benchmark focused on “chained-growth” merkleization behavior in Current variants, intended to stress the committed-bitmap parent-chain walk cost under a deep chain with very sparse overlays.

Changes:

• Introduces chained_growth Criterion benchmark that seeds a large DB, pre-builds a deep speculative/commit chain, then measures additional merkleize+apply iterations.
• Adds the new benchmark module to the QMDB benches entrypoint so it runs as part of the suite.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
storage/src/qmdb/benches/chained_growth.rs	New chained-growth benchmark implementation for Current fixed+MMB variants at chunk sizes 32 and 256.
storage/src/qmdb/benches/bench.rs	Registers the new chained_growth benchmark group in criterion_main!.

[Copilot]

Pull request overview

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

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.81%. Comparing base (c6a805b) to head (aa59e3e).
⚠️ Report is 3 commits behind head on main.

@@            Coverage Diff             @@
##             main    #3633      +/-   ##
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- Coverage   95.81%   95.81%   -0.01%     
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  Files         440      440              
  Lines      167922   167922              
  Branches     3922     3922              
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- Hits       160900   160891       -9     
- Misses       5800     5806       +6     
- Partials     1222     1225       +3     

see 3 files with indirect coverage changes

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