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Reduce memory footprint by enabling background page purging

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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.
This commit is contained in:
Ondřej Surý 2026-03-30 08:50:07 +02:00
parent e99b5f80be
commit ad6f4e1992
2 changed files with 62 additions and 4 deletions
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2
lib/isc/include/isc/mem.h
View file

@ -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

64
lib/isc/mem.c
View file

@ -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,