perf: Skip redundant __init__ assignments and remove dead attributes in ResponseFuture (-32ns/call - 4.7% improvement for the common case, +43ns (+4.5%) for the non-common case, - code cleanup also)

[mykaul]

Summary

• Remove 3 dead class attributes from ResponseFuture:
  • default_timeout — belongs to Session, never used in RF
  • _profile_manager — belongs to Session, never used in RF
  • _warned_timeout — never read or written anywhere in the codebase
• Skip 4 redundant STORE_ATTR operations in __init__ when parameters are None (matching the class-level default): _metrics, prepared_statement, _host, _continuous_paging_state
• Move prepared_statement and _continuous_paging_state class defaults (previously only set in __init__) to class level so the conditional skip works correctly

Thread Safety

All skipped attributes have class-level None defaults and are only set during __init__ or by the owning thread after construction. No lazy initialization of shared mutable state is introduced — the thread-safety invariants are preserved.

Benchmark

ResponseFuture.__init__ micro-benchmark, min() of 7 × 200k iterations:

Scenario	Before	After	Δ
Common case (all 4 params = None)	670 ns/call	638 ns/call	-32 ns/call (4.7%)
Non-None case (metrics + prepared_statement + host set)	957 ns/call	1001 ns/call	+43 ns/call (+4.5%)

The common case (simple queries without metrics/prepared statements/host pinning) is the dominant path. The non-None case overhead is from the added if checks, but these params are rarely all non-None simultaneously.

Additional optimization: isinstance dispatch order in _create_response_future

This PR also reorders the isinstance chain in Session._create_response_future (cassandra/cluster.py) so BoundStatement is checked before SimpleStatement. BoundStatement and SimpleStatement are sibling classes under Statement (no subclass relationship), so the reorder changes dispatch order only — no behavioral change.

For prepared-statement workloads (the perf-critical case, since BoundStatement is produced by PreparedStatement.bind()), BoundStatement is the most common query type reaching this dispatch, so checking it first avoids one wasted isinstance() call per query on the hot path.

A dedicated micro-benchmark, benchmarks/micro/bench_isinstance_dispatch.py, simulates a representative workload mix (80% BoundStatement, 15% SimpleStatement, 5% other) and measures dispatch order in isolation:

SimpleStatement first: 32.8 ns/dispatch
BoundStatement first:  23.2 ns/dispatch
Speedup: ~1.4-1.7x (~10-15 ns/dispatch saved)

Run it yourself with python benchmarks/micro/bench_isinstance_dispatch.py from the repository root.

Tests

All 645 unit tests pass (43 skipped), matching the origin/master baseline.

[mykaul]

Follow-up commit: reorder isinstance chain in _create_response_future

Commit: 40f45ecf5 — perf: reorder isinstance chain to check BoundStatement first in _create_response_future

Change

Swapped the isinstance check order so BoundStatement is checked before SimpleStatement in the _create_response_future dispatch chain. For prepared-statement workloads (the perf-critical case), this saves one wasted isinstance() call per dispatch.

BoundStatement and SimpleStatement are sibling classes under Statement — no subclass relationship — so reorder is safe with no behavioral change.

Benchmark results (benchmarks/bench_isinstance_dispatch.py)

Simulated workload mix: 80% BoundStatement, 15% SimpleStatement, 5% other:

Python 3.14.3
SimpleStatement first: 32.8 ns/dispatch
BoundStatement first:  23.2 ns/dispatch
Speedup: ~1.4–1.7x (~10–15 ns/dispatch saved)

Tests

607 unit tests passed, 0 failures.

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[Copilot]

Pull request overview

Optimizes response-future construction and statement dispatch while removing unused attributes.

Changes:

• Avoids redundant ResponseFuture assignments for None values.
• Removes dead attributes and adds class-level defaults.
• Prioritizes BoundStatement dispatch and adds a micro-benchmark.

Reviewed changes

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

File	Description
cassandra/cluster.py	Optimizes dispatch and ResponseFuture initialization.
benchmarks/micro/bench_isinstance_dispatch.py	Benchmarks statement dispatch ordering.

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[Copilot]

Pull request overview

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

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (2)

cassandra/c_shard_info.pyx:42

• This portability rewrite is unrelated to the ResponseFuture/dispatch optimization described by the PR, and the summary does not mention that shard-routing arithmetic is being changed. Please either document and justify this additional scope (including the Windows-build motivation) or move it to a separate PR so reviewers and release notes do not miss a correctness-sensitive routing change.

        # Compute (biased_token * shards_count) >> 64, i.e. the high 64 bits of the
        # 64x32-bit product, using only 64-bit arithmetic. This used to rely on the
        # GCC/Clang-only __uint128_t extension type, which MSVC does not support at
        # all (no 128-bit integer type), causing a compile error on Windows builds.

cassandra/c_shard_info.pyx:49

• The new multiply-high implementation has no boundary/equivalence coverage. The existing shard-aware test checks only five application tokens, so carry boundaries and token extrema could regress shard routing unnoticed. Add a parametrized test that imports the C implementation directly and compares it with _ShardingInfo for INT64_MIN, INT64_MAX, low-half carry boundaries, multiple shard counts, and sharding_ignore_msb values.

        cdef uint64_t low_product = (biased_token & <uint64_t>UINT32_MAX) * shards_count
        cdef uint64_t carry = low_product >> 32
        cdef uint64_t mid = (biased_token >> 32) * shards_count + carry
        cdef int shardId = <int>(mid >> 32);