Distilled from #776's 132460f5. This project sets arena_eager_commit=0 +
purge_decommits=1 + purge_delay=0 in cbm_mem_init to reduce upfront memory, so
mimalloc's committed-page counter reads low. On Linux mi_process_info() never
sets current_rss (vendored/mimalloc/src/prim/unix/prim.c only fills peak_rss),
so current_rss defaults to that low committed counter — cbm_mem_rss() returned a
few MB while true RSS was multiple GB, leaving cbm_mem_over_budget() backpressure,
the memory ceiling, and the host RAM tier blind on Linux.
Prefer os_rss() (/proc/self/statm) as the primary source on Linux; macOS and
Windows are unchanged (their mi_process_info current_rss is accurate via
task_info / GetProcessMemoryInfo). cbm_mem_peak_rss() is untouched. Complements
#752 by making the RAM tiers actually bite on Linux.
Reproduce-first: mem_rss_reflects_external_resident_memory pins mimalloc's
committed counter low with a live mi_malloc, then grows true RSS via a raw 256MB
mmap (invisible to mimalloc); unfixed Linux returns the ~few-MB committed counter
(RED), fixed Linux returns /proc RSS (GREEN). macOS/Windows pass either way, so
the RED manifests on the Linux CI leg.
Co-authored-by: petercoxphoto <info@petercox.ie>
Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>
