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/thread_event.c

@@ -69,6 +69,11 @@ peak_dalloc_fetch_elapsed(tsd_t *tsd) {
 	return TE_INVALID_ELAPSED;
 }
 
+static uint64_t
+prof_threshold_fetch_elapsed(tsd_t *tsd) {
+	return TE_INVALID_ELAPSED;
+}
+
 /* Per event facilities done. */
 
 static bool