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perf: set malloc top pad to 16 MB and mmap threshold to 8 MB instead of the trim threshold #971

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perf: set malloc top pad to 16 MB and mmap threshold to 8 MB instead of the trim threshold #971

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6587856
perf: set malloc top pad to 16 MB instead of the trim threshold
Oct 31, 2026
32d9bd9
perf: also set the mmap threshold to 8 MB
Nov 2, 2026
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18 changes: 13 additions & 5 deletions src/tilemaker.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -97,11 +97,19 @@ int main(const int argc, const char* argv[]) {
#ifdef __GLIBC__
// glibc raises its mmap threshold dynamically (up to 32/64 MB) once a large
// mmapped chunk is freed; multi-MB buffers then land in per-thread arenas
// that are never trimmed, inflating peak RSS. Setting the trim threshold
// explicitly also disables that dynamic adjustment (the mmap threshold
// stays at its 128 KB default), unless the user tunes malloc via the environment.
if (!getenv("MALLOC_TRIM_THRESHOLD_") && !getenv("MALLOC_MMAP_THRESHOLD_") && !getenv("GLIBC_TUNABLES")) {
mallopt(M_TRIM_THRESHOLD, 4 << 20);
// that are never trimmed, inflating peak RSS. Setting malloc parameters
// explicitly turns that dynamic adjustment off, so both are set here:
// - a 16 MB top pad makes the heap grow and shrink in large steps; with many
// threads allocating, the default 128 KB steps cost many brk() calls and
// page faults
// - a fixed 8 MB mmap threshold: otherwise it would stay at 128 KB, and every
// larger buffer would be a fresh mmap with zeroed pages. Boost's validity
// check on a huge polygon grows such buffers over and over (one relation
// with 1.9M points took 3x as long).
// Users who tune malloc via the environment keep their settings.
if (!getenv("MALLOC_TRIM_THRESHOLD_") && !getenv("MALLOC_MMAP_THRESHOLD_") && !getenv("MALLOC_TOP_PAD_") && !getenv("GLIBC_TUNABLES")) {
mallopt(M_TOP_PAD, 16 << 20);
mallopt(M_MMAP_THRESHOLD, 8 << 20);
}
#endif

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