perf(build): set jemalloc narenas:1 to trim init metadata + RSS

[duncanista]

Jira: none yet — add before marking ready.

⚠️ DRAFT — stacked on top of #1271 (jordan.gonzalez/cold-start-instrumentation/feature). Base will be retargeted to main once #1271 merges. Needs benchmarking and was NOT docker-built locally.

Overview

Tune jemalloc at compile time to a single arena (narenas:1) in both compile Dockerfiles:

• images/Dockerfile.bottlecap.compile (GNU / AL2)
• images/Dockerfile.bottlecap.alpine.compile (musl / Alpine)

ENV JEMALLOC_SYS_WITH_MALLOC_CONF="narenas:1"

Why compile-time, and not a MALLOC_CONF env var

The binary allocates through tikv-jemallocator, which links jemalloc with the _rjem_ symbol prefix. With that prefix, jemalloc does not read the standard runtime MALLOC_CONF environment variable (nor the malloc_conf weak symbol under the unprefixed name), so setting MALLOC_CONF at runtime is silently ignored — a footgun. The robust way to configure a prefixed build is the compile-time env JEMALLOC_SYS_WITH_MALLOC_CONF, which jemalloc-sys bakes into the build. Hence the change lives in the Dockerfiles rather than in the Lambda runtime environment.

Mechanism / expected effect

• jemalloc pre-maps per-arena metadata at init; the default arena count scales with CPU count. Forcing narenas:1 cuts that to a single arena, reducing the metadata mapped at initialization and lowering steady-state RSS.
• The extension is not allocation-throughput-bound — it is an async I/O-bound telemetry forwarder — so the usual reason for many arenas (reducing cross-thread allocator contention) does not apply here. Arena contention is a non-issue for this workload.

Testing

• Benchmark cold-start init time and RSS before/after (primary validation — not yet done).
• Confirm both images build (GNU + Alpine). Not docker-built locally.

Dockerfile-only change — no Rust source touched, no cargo run required.

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⚠️ Warnings

🚦 9 Pipeline jobs failed

DataDog/datadog-lambda-extension | integration-suite: [oom]     

DataDog/datadog-lambda-extension | integration-suite: [payload-size]     

DataDog/datadog-lambda-extension | bottlecap (amd64, alpine)     

View all 9 failed jobs.

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