(improvement) (cython only) cache deserializer instances in find_deserializer and m…

[mykaul]

Cache find_deserializer() and make_deserializers() results in Cython cdef dict caches keyed on cqltype objects to avoid repeated class lookups and Deserializer object creation on every result set.

Using cqltype objects (not id()) as cache keys holds strong references, preventing GC/id-reuse correctness issues with parameterized types.

Motivation

On every result set, make_deserializers(coltypes) is called from row_parser.pyx:37, which in turn calls find_deserializer() for each column type. These functions perform class name lookups and issubclass() chains, then create fresh Deserializer objects -- all redundant work when the same column types appear repeatedly (which is always the case for prepared statements).

Benchmark results

Benchmarks compare the original code (Before) against the new cached implementation (After).

find_deserializer (single type lookup) -- 6.6x faster:

Variant	Min	Mean	Median	Ops/sec	Speedup
Before (original)	367.0 ns	416.7 ns	403.0 ns	2.4 Mops/s	--
After (with cache)	54.9 ns	62.9 ns	60.5 ns	15.9 Mops/s	6.6x

make_deserializers (5 types) -- 29x faster:

Variant	Min	Mean	Median	Ops/sec	Speedup
Before (original)	2,983 ns	3,766 ns	3,620 ns	266 Kops/s	--
After (with cache)	110.7 ns	128.4 ns	118.6 ns	7,790 Kops/s	29x

make_deserializers (10 types) -- 40x faster:

Variant	Min	Mean	Median	Ops/sec	Speedup
Before (original)	4,946 ns	5,830 ns	5,864 ns	172 Kops/s	--
After (with cache)	127.7 ns	147.3 ns	140.9 ns	6,787 Kops/s	40x

Design notes

• Caches are cdef dict (C-level, not accessible from Python) for minimal overhead
• Cache keys are the cqltype objects themselves, not id(cqltype) -- holds strong references preventing GC and id() reuse
• For prepared statements (the hot path), cache hit rate is effectively 100%
• Caches are bounded to 256 entries; on overflow the entire cache is cleared (simple and sufficient for real workloads where the number of distinct column-type signatures is typically dozens to low hundreds)
• The returned array from make_deserializers may be a shared cached object; callers must not modify it
• Runtime Des* class overrides (e.g. DesBytesType = DesBytesTypeByteArray) require a call to clear_deserializer_caches() to flush stale entries
• A clear_deserializer_caches() API and get_deserializer_cache_sizes() diagnostic helper are provided

Tests

All existing unit tests pass. New tests cover:

• Cache hit/miss behaviour for both find_deserializer and make_deserializers
• Bounded eviction (cache does not exceed 256 entries)
• clear_deserializer_caches() invalidation
• get_deserializer_cache_sizes() diagnostic helper
• Benchmark correctness and performance comparison vs uncached baseline

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[Copilot]

Pull request overview

This PR improves performance in the Cython fast-path by caching deserializer lookups and per-column deserializer arrays, avoiding repeated class-resolution work and repeated Deserializer object creation across result sets.

Changes:

• Add module-level Cython dict caches for find_deserializer() and make_deserializers() keyed by cqltype objects / tuples of cqltype objects.
• Update find_deserializer() / make_deserializers() to consult and populate these caches.
• Add a pytest-based benchmark module to compare cached vs uncached behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
cassandra/deserializers.pyx	Adds caching for deserializer instance resolution and for per-column deserializer arrays.
benchmarks/test_deserializer_cache_benchmark.py	Introduces correctness checks + pytest-benchmark-style benchmarks for the new caching behavior.

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