Add member_order (fifo/lifo) + demand-history cull#110
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Introduces two related changes:
1. `member_order => fifo | lifo` pool config option (default `lifo`,
preserving existing stack behaviour). `fifo` uses an OTP `queue()`
as the free-member structure, producing round-robin dispatch where
the longest-idle worker is always handed out next. The free-member
abstraction (`free_push/pop/delete/fold/take_n/convert`) hides the
list-vs-queue difference from all call sites; `free_count` is now
exclusively managed by these helpers.
2. Pool-level demand-history cull replaces the per-worker `max_age`
idle-time check. A fixed-capacity OTP-queue ring buffer (head =
current bucket, capacity = ceil(max_age/bucket_size)) tracks peak
concurrent in-use count; at each cull tick the pool sizes to that
peak (floored at `init_count`). This gives LIFO and FIFO pools
identical shrinkage behaviour for the same config, and correctly
handles FIFO rotation where workers never accumulate idle time.
`max_age` now means "demand memory window"; `cull_interval` default
changed from {1,min} to {15,sec} so defaults satisfy CI <= MA.
Hot-upgrade (vsn 5->6) reconstructs the demand buffer from member
`time` timestamps.
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>
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Add
member_order => lifo | fifopool config (lifois the default and preservesall existing behaviour).
fifouses an OTP queue as the free-member structure,handing out the longest-idle worker first (round-robin rotation).
Replace the per-worker
max_ageidle-time cull gate with pool-level demandhistory: a fixed-capacity queue ring buffer (head = current bucket) tracks peak
concurrent in-use count over a rolling
max_agewindow. At each cull tick the poolsizes to that peak (floored at
init_count). This gives LIFO and FIFO identicalshrinkage behaviour for the same config — FIFO workers never go idle under rotation,
so the old per-worker check never fired for them.
max_agenow means "demand memory window" rather than "individual worker idle time".Practical behaviour for LIFO pools is essentially unchanged; FIFO pools now cull
correctly.
cull_intervaldefault changed from{1, min}to{15, sec}so thedefaults satisfy
cull_interval ≤ max_age.Fixed-size pools (
init_count == max_count) skip demand tracking entirely(
demand_buf = undefined) — cull never fires for them anyway.Hot upgrade v5 → v6:
demand_bufis reconstructed from membertimetimestamps.Benchmark (OTP 27.3, vs master baseline)
LIFO path overhead is within noise (0–5%) for all existing benchmarks; the cull-heavy
benchmark improves by ~8%. FIFO adds ~10–15% vs LIFO for single take/return
(queue vs list-head), negligible for drain-all and concurrent patterns.