diff --git a/src/tilemaker.cpp b/src/tilemaker.cpp
index 7a7ae1e5..c737981e 100644
--- a/src/tilemaker.cpp
+++ b/src/tilemaker.cpp
@@ -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