perf: use pre-allocated constant for null sentinel in collection serialization (10's of ns if null elements are used)#763
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Replace io.BytesIO() buffer pattern with list accumulation + b''.join() in serialize_safe methods for ListType, SetType, MapType, TupleType, and UserType. Also pre-compute _INT32_NULL = int32_pack(-1) as a module-level constant to avoid repeated packing of the null sentinel. Buffer assembly micro-benchmarks (isolating the BytesIO overhead from per-element to_binary() cost): Scenario Before (us) After (us) Speedup List 100 elements 9.0 8.6 1.05x List 10 elements 1.1 0.9 1.18x List 10 all-null 0.8 0.4 2.23x Map 10 entries 2.0 1.6 1.24x The all-null case benefits most from the pre-computed _INT32_NULL constant, which eliminates repeated int32_pack(-1) calls. Note: PR scylladb#763 on this repo adds only the _INT32_NULL constant; this commit is a superset that also replaces BytesIO with b''.join() across all four collection/composite type serializers.
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…alization Replace per-call int32_pack(-1) with a module-level _INT32_NULL constant in serialize methods of ListType, SetType, MapType, TupleType, and UserType. This avoids a struct.pack call on every null element during collection serialization. Affected sites: - _SimpleParameterizedType.serialize_safe (ListType/SetType) - MapType.serialize_safe (null key + null value) - TupleType.serialize_safe - UserType.serialize_safe
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Summary
int32_pack(-1)with a module-level_INT32_NULLconstant in 5 collection serialize methodsstruct.pack()call on every null element during collection serializationDetails
The CQL protocol represents
nullcollection elements as a 4-byteint32with value-1. Previously, every null element triggered a freshstruct.pack('>i', -1)call. This PR pre-computes the result once at module load time and reuses thebytesobject.Affected sites
ListType/SetType_SimpleParameterizedType.serialize_safeMapTypeMapType.serialize_safeTupleTypeTupleType.serialize_safeUserTypeUserType.serialize_safeBenchmark results
All benchmarks: CPython 3.14, median of 3-5 runs, results in nanoseconds.
Isolated operation (
int32_pack(-1)call vs_INT32_NULLconstant lookup, 10M iterations, median of 5 runs):int32_pack(-1)(before)_INT32_NULLlookup (after)End-to-end
serialize()— before vs after (200K iterations, median of 3 runs):Each collection contains ~50% None elements to exercise the null path.
ListType(10 elements, 5 nulls)SetType(10 elements, 5 nulls)MapType(5 entries, 3 null values)TupleType(5 fields, 2 nulls)UserType(5 fields, 2 nulls)End-to-end improvement is within measurement noise (~1-6%) because the null sentinel write is a small fraction of total serialization cost (which includes
to_binary()calls,int32_pack(len(...))for non-null elements,BytesIOwrites, etc.). The benefit scales with null density — workloads with many sparse/null columns will see a larger improvement.Analysis
The isolated benchmark shows a clear 7.4x speedup for the null-write operation itself (31 ns saved per null element). In end-to-end serialization, the improvement is diluted by the much larger cost of serializing non-null elements. The optimization is:
Testing
CollectionNullSentinelTestswith 5 round-trip tests covering List, Set, Map, Tuple, and UserType withNoneelements