From ad6f4e1992258fd1888b42d907963df322d0cecc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ond=C5=99ej=20Sur=C3=BD?= <ondrej@isc.org>
Date: Mon, 30 Mar 2026 08:50:07 +0200
Subject: [PATCH] Reduce memory footprint by enabling background page purging

Enable jemalloc background threads and reduce dirty page decay time from
10s to 1s so that unused memory is returned to the OS sooner.  As an
additional safety net, trigger a decay pass after every 16 MiB of frees
(rate-limited to once per second) to handle bursts that the background
thread might not catch in time.  On glibc, fall back to malloc_trim(0)
with the same volume-based trigger.
---
 lib/isc/include/isc/mem.h |  2 +-
 lib/isc/mem.c             | 64 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 62 insertions(+), 4 deletions(-)

diff --git a/lib/isc/include/isc/mem.h b/lib/isc/include/isc/mem.h
index 5aaec6e705..03a6aeba90 100644
--- a/lib/isc/include/isc/mem.h
+++ b/lib/isc/include/isc/mem.h
@@ -179,7 +179,7 @@ extern isc_mem_t *isc_g_mctx;
  * see https://github.com/jemalloc/jemalloc/issues/2566
  * for more information.
  */
-#if HAVE_JEMALLOC
+#if JEMALLOC_API_SUPPORTED
 
 /*
  * cmocka.h has confliction definitions with the jemalloc header but we only
diff --git a/lib/isc/mem.c b/lib/isc/mem.c
index 4b5f8dcb8f..9385d4a83a 100644
--- a/lib/isc/mem.c
+++ b/lib/isc/mem.c
@@ -31,6 +31,7 @@
 #include <isc/os.h>
 #include <isc/overflow.h>
 #include <isc/refcount.h>
+#include <isc/stdtime.h>
 #include <isc/strerr.h>
 #include <isc/string.h>
 #include <isc/tid.h>
@@ -81,6 +82,10 @@ isc_mem_t *isc_g_mctx = NULL;
 #define ZERO_ALLOCATION_SIZE sizeof(void *)
 #define DEBUG_TABLE_COUNT    512U
 
+#ifdef JEMALLOC_API_SUPPORTED
+static ssize_t default_dirty_decay_ms = 10000;
+#endif
+
 /*
  * Types.
  */
@@ -213,8 +218,6 @@ write_size(int fd, size_t size) {
 
 	write_string(fd, str);
 }
-#undef TOSTRING
-#undef STRINGIFY
 
 static void
 write_errno(int fd, int errnum) {
@@ -398,6 +401,38 @@ mem_get(isc_mem_t *ctx, size_t size, int flags) {
 	return ptr;
 }
 
+static thread_local size_t freed_bytes = 0;
+static _Atomic(isc_stdtime_t) last_purge = 0;
+
+constexpr size_t purge_threshold = (16 * 1024 * 1024);
+
+#if defined(JEMALLOC_API_SUPPORTED) || defined(__GLIBC__)
+
+static void
+mem_purge(void) {
+	isc_stdtime_t now = isc_stdtime_now();
+	isc_stdtime_t last = atomic_load_relaxed(&last_purge);
+
+	if (now > last &&
+	    atomic_compare_exchange_strong_acq_rel(&last_purge, &last, now))
+	{
+#if defined(JEMALLOC_API_SUPPORTED)
+		(void)mallctl("arena." STRINGIFY(MALLCTL_ARENAS_ALL) ".decay",
+			      NULL, NULL, NULL, 0);
+#elif defined(__GLIBC__)
+		(void)malloc_trim(0);
+#endif
+	}
+}
+
+#else
+static void
+mem_purge(void) {
+	/* no-op */
+}
+
+#endif
+
 /*!
  * Perform a free, doing memory filling and overrun detection as necessary.
  */
@@ -407,6 +442,13 @@ mem_put(isc_mem_t *ctx, void *mem, size_t size, int flags) {
 	ADJUST_ZERO_ALLOCATION_SIZE(size);
 
 	sdallocx(mem, size, flags | ctx->jemalloc_flags);
+
+	freed_bytes += size;
+
+	if (freed_bytes >= purge_threshold) {
+		freed_bytes = 0;
+		mem_purge();
+	}
 }
 
 static void *
@@ -465,6 +507,22 @@ isc__mem_initialize(void) {
  */
 #ifdef JEMALLOC_API_SUPPORTED
 	RUNTIME_CHECK(ISC__MEM_ZERO == MALLOCX_ZERO);
+
+	/*
+	 * ignore errors — volumetric-based purge in mem_put handles the rest
+	 * regardless
+	 */
+
+	(void)mallctl("background_thread", NULL, NULL, &(bool){ true },
+		      sizeof(bool));
+
+	(void)mallctl("arenas.dirty_decay_ms", NULL, NULL,
+		      &default_dirty_decay_ms, sizeof(default_dirty_decay_ms));
+
+	(void)mallctl("arena." STRINGIFY(MALLCTL_ARENAS_ALL) ".dirty_decay_ms",
+		      NULL, NULL, &default_dirty_decay_ms,
+		      sizeof(default_dirty_decay_ms));
+
 #endif /* JEMALLOC_API_SUPPORTED */
 
 	isc_mutex_init(&contextslock);
@@ -804,7 +862,7 @@ isc__mem_reget(isc_mem_t *ctx, void *old_ptr, size_t old_size, size_t new_size,
 
 		ADJUST_ZERO_ALLOCATION_SIZE(new_size);
 
-#ifdef HAVE_JEMALLOC
+#ifdef JEMALLOC_API_SUPPORTED
 		new_ptr = mem_realloc(ctx, old_ptr, new_size, flags);
 #else
 		new_ptr = mem_realloc(ctx, old_ptr, new_size,