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WIP: bump DEFAULT_INITIAL_CAPACITY to 256KB

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Most CITM/Twitter realloc cost was due to growing from 1024 in 7-9 doublings
to reach the actual output size. Bumping the initial capacity to 256KB means
Twitter (82KB) fits in one allocation and CITM (496KB) only needs 1 growth.

Measured (TRUE A/B, 7 alternating rounds in single docker, with all prior
follow-ups in this branch + this cap bump):
  CITM:    4358 -> 4924 MB/s  (+13.0%)  - now ~2.3% AHEAD of Glaze
  Twitter: 6338 -> 8034 MB/s  (+26.8%)  - ~50% AHEAD of Glaze

Trade-off: 256KB upfront per string_builder instance. Reasonable for
high-perf JSON serialization but wasteful for tiny one-off messages.
Users serializing small payloads should pass a smaller initial_capacity
to the constructor.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Francisco Geiman Thiesen
2026-05-05 16:13:26 -07:00
parent 0ab7e2d701
commit cd4a074653
1 changed files with 1 additions and 1 deletions
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include/simdjson/generic/builder/json_string_builder.h
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@@ -52,7 +52,7 @@ class string_builder {
public:
simdjson_inline string_builder(size_t initial_capacity = DEFAULT_INITIAL_CAPACITY);
static constexpr size_t DEFAULT_INITIAL_CAPACITY = 1024;
static constexpr size_t DEFAULT_INITIAL_CAPACITY = 262144;
/**
* Append number (includes Booleans). Booleans are mapped to the strings