Unlike prof_sample which is supported only with profiling mode active, prof_threshold is intended to be an always-supported allocation callback with much less overhead. The usage of the threshold allows performance critical callers to change program execution based on the callback: e.g. drop caches when memory becomes high or to predict the program is about to OOM ahead of time using peak memory watermarks.

src/prof_threshold.c

@@ -0,0 +1,57 @@
+#include "jemalloc/internal/jemalloc_preamble.h"
+#include "jemalloc/internal/jemalloc_internal_includes.h"
+
+#include "jemalloc/internal/activity_callback.h"
+#include "jemalloc/internal/prof_threshold.h"
+
+#include "jemalloc/internal/prof_externs.h"
+
+/*
+ * Update every 128MB by default.
+ */
+#define PROF_THRESHOLD_LG_WAIT_DEFAULT 27
+
+/* Logically a prof_threshold_hook_t. */
+static atomic_p_t prof_threshold_hook;
+size_t opt_experimental_lg_prof_threshold = PROF_THRESHOLD_LG_WAIT_DEFAULT;
+
+void
+prof_threshold_hook_set(prof_threshold_hook_t hook) {
+	atomic_store_p(&prof_threshold_hook, hook, ATOMIC_RELEASE);
+}
+
+prof_threshold_hook_t
+prof_threshold_hook_get(void) {
+	return (prof_threshold_hook_t)atomic_load_p(&prof_threshold_hook,
+	    ATOMIC_ACQUIRE);
+}
+
+/* Invoke callback for threshold reached */
+static void
+prof_threshold_update(tsd_t *tsd) {
+	prof_threshold_hook_t prof_threshold_hook = prof_threshold_hook_get();
+	if (prof_threshold_hook == NULL) {
+		return;
+        }
+	uint64_t alloc = tsd_thread_allocated_get(tsd);
+	uint64_t dalloc = tsd_thread_deallocated_get(tsd);
+	peak_t *peak = tsd_peakp_get(tsd);
+	pre_reentrancy(tsd, NULL);
+	prof_threshold_hook(alloc, dalloc, peak->cur_max);
+	post_reentrancy(tsd);
+}
+
+uint64_t
+prof_threshold_new_event_wait(tsd_t *tsd) {
+	return 1 << opt_experimental_lg_prof_threshold;
+}
+
+uint64_t
+prof_threshold_postponed_event_wait(tsd_t *tsd) {
+	return TE_MIN_START_WAIT;
+}
+
+void
+prof_threshold_event_handler(tsd_t *tsd, uint64_t elapsed) {
+	prof_threshold_update(tsd);
+}