perf: increase Dune's default minor heap - #15729
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Set a 512K-word default before the native OCaml runtime starts, while preserving explicit OCAMLRUNPARAM and CAMLRUNPARAM settings. Remove the temporary setting from Dune's process and child environments after startup. Signed-off-by: Rudi Grinberg <me@rgrinberg.com>
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Dune's no-op self-builds allocate enough short-lived data to collect the
standard 256K-word minor heap almost a thousand times. A 512K-word heap reduces
that GC work and lowers promotion.
Set the default before the native OCaml runtime starts, while treating either
OCAMLRUNPARAMorCAMLRUNPARAMas an explicit user choice. The temporaryenvironment setting is removed after startup and filtered from
Env.initial,so commands run by Dune do not inherit it.
Across 40 alternating warmed pairs, task-clock improved by 1.4% on
@installand 2.8% on
@check; minor collections fell by about 48% and promoted words byabout 1.9%. Maximum RSS increased by approximately 2%. This is internal runtime
tuning and does not require a changelog entry.