README

This bench sends a single pre-formatted trace through the encoder many times with a null writer, so all I/O is excluded and the cost being measured is the encoder itself.

The trace shape (trace-fixture.js) mirrors a typical Node.js HTTP-service request: one root Express server span plus a fan of internal middleware spans, Postgres / Redis client spans, a few outbound HTTP client spans, DNS lookups, and one error-bearing span with error.message / error.stack. The default trace has 30 spans; TRACE_SPANS=<n> scales the composition proportionally.

Strings reuse the same keys (span.kind, component, runtime-id, …) and the same hot values (GET, server, client, internal, javascript, …) across spans, mirroring what the encoder's string cache sees in production.

tickTrace(trace, iteration) runs before every encode and rewrites the per-request dynamic fields in place: start nanos and duration advance, the low half of every ID buffer is rewritten, db.row_count on the Postgres spans jitters, the root span rotates through eight coherent request shapes (route / URL / resource / status / client IP), and the error span rotates through four type/message/stack variants (each stack ~1.5 KB). Without that, every iteration encodes byte-identical data and V8 collapses the integer magnitude branches plus the stale-string cache hits the encoder is meant to exercise. attachFreshEvents does the same for span_events (legacy path).

The + iteration * 4096 step on span.start is well above the IEEE-754 double's ULP at the ~1.7e18 nano-timestamp magnitude (256 nanos); fixing that precision loss properly needs the span to carry start as a BigInt instead of a Number, which is a separate change on the tracer side, not here.

Variants:

• 0.4 / 0.5 — wire-format variants of the agent encoder.
• 0.4-events-native / 0.4-events-legacy / 0.5-events-legacy — exercise the span-event encoding paths (WITH_SPAN_EVENTS).