feat(fpm-tune): expose cpu_ceiling + cpu_headroom in the embedded runtime - #142
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…untime The tuner sizes pools to the memory budget; on CPU-bound workloads that oversubscribes the CPU and costs throughput. Benchmarked on a 2-CPU / 1 GiB container running a pure-CPU request (6k chained sha256 rounds), worker count vs throughput is monotonic downward: workers 2 4 6 8 12 21 rps 403 394 383 370 311 310 The memory budget allowed 21 workers (310 rps); the CPU fill with fpm-tune's default 2.0 headroom picks 4 (394 rps, +27%). fpm-tune has always had the mechanism - the standalone --cpu flag and serve.Config.CPUCeiling/CPUHeadroom - but the embedded runtime never mapped it. Now: fpm_tune.cpu_ceiling (bool) and fpm_tune.cpu_headroom (float, 0 = fpm-tune default 2.0), env-overridable via the generic CBOX_INIT_GLOBAL_FPM_TUNE_CPU_CEILING / _CPU_HEADROOM path, negative headroom rejected by validation. Example config documents the trade.
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The embedded tuner sizes pools to the memory budget. On CPU-bound workloads that oversubscribes the CPU: on a 2-CPU/1 GiB container running a pure-CPU request (6k chained sha256 rounds,
wrk -t4 -c64), throughput falls monotonically with worker count —The memory budget picked 21 workers (310 rps). The CPU fill × fpm-tune's default 2.0 headroom picks 4 (394 rps, +27%).
fpm-tune has always had the mechanism (standalone
--cpu,serve.Config.CPUCeiling/CPUHeadroom) — the embedded runtime just never mapped it. This addsfpm_tune.cpu_ceiling+fpm_tune.cpu_headroom(0 = default 2.0), automatically env-overridable asCBOX_INIT_GLOBAL_FPM_TUNE_CPU_CEILING/_CPU_HEADROOM, with validation (negative headroom rejected) and tests for yaml load, env override, and rejection. Example config documents the trade-off.Verified end-to-end in the php-baseimages benchmark harness: an image with this build,
CBOX_FPM_TUNE=true+CBOX_INIT_GLOBAL_FPM_TUNE_CPU_CEILING=true, converges to the CPU-sized pool at runtime. Gate: full tests, vet, gofmt, golangci-lint v2.12.2 0 issues.