diff --git a/lib/isc/mem.c b/lib/isc/mem.c
index 706fb1ca8c..f2b82de7eb 100644
--- a/lib/isc/mem.c
+++ b/lib/isc/mem.c
@@ -29,6 +29,7 @@
 #include <isc/once.h>
 #include <isc/os.h>
 #include <isc/print.h>
+#include <isc/random.h>
 #include <isc/refcount.h>
 #include <isc/string.h>
 #include <isc/types.h>
@@ -151,7 +152,6 @@ struct isc_mem {
 	atomic_size_t malloced;
 	atomic_size_t maxmalloced;
 	atomic_bool hi_called;
-	atomic_bool is_overmem;
 	isc_mem_water_t water;
 	void *water_arg;
 	atomic_size_t hi_water;
@@ -534,7 +534,6 @@ mem_create(isc_mem_t **ctxp, unsigned int flags, unsigned int jemalloc_flags) {
 	atomic_init(&ctx->hi_water, 0);
 	atomic_init(&ctx->lo_water, 0);
 	atomic_init(&ctx->hi_called, false);
-	atomic_init(&ctx->is_overmem, false);
 
 	for (size_t i = 0; i < STATS_BUCKETS + 1; i++) {
 		atomic_init(&ctx->stats[i].gets, 0);
@@ -786,9 +785,6 @@ hi_water(isc_mem_t *ctx) {
 		return false;
 	}
 
-	/* We are over water (for the first time) */
-	atomic_store_release(&ctx->is_overmem, true);
-
 	return true;
 }
 
@@ -810,9 +806,6 @@ lo_water(isc_mem_t *ctx) {
 		return false;
 	}
 
-	/* We are no longer overmem */
-	atomic_store_release(&ctx->is_overmem, false);
-
 	return true;
 }
 
@@ -1195,7 +1188,30 @@ bool
 isc_mem_isovermem(isc_mem_t *ctx) {
 	REQUIRE(VALID_CONTEXT(ctx));
 
-	return atomic_load_relaxed(&ctx->is_overmem);
+	size_t hiwater = atomic_load_relaxed(&ctx->hi_water);
+	if (hiwater == 0) {
+		return false;
+	}
+
+	size_t inuse = atomic_load_relaxed(&ctx->inuse);
+	if (inuse >= hiwater) {
+		return true;
+	}
+
+	size_t lowater = atomic_load_relaxed(&ctx->lo_water);
+	if (inuse <= lowater) {
+		return false;
+	}
+
+	/*
+	 * Between lo_water and hi_water, return true with a probability
+	 * that ramps linearly from 0 at lo_water to 1 at hi_water.  This
+	 * spreads cache cleaning across many inserts instead of triggering
+	 * a thundering herd once the hi_water mark is crossed.
+	 */
+	uint32_t prob = (uint32_t)(((uint64_t)(inuse - lowater) * 256) /
+				   (hiwater - lowater));
+	return isc_random8() < prob;
 }
 
 void
diff --git a/tests/dns/rbtdb_test.c b/tests/dns/rbtdb_test.c
index cfc2a5779b..451d5afed0 100644
--- a/tests/dns/rbtdb_test.c
+++ b/tests/dns/rbtdb_test.c
@@ -307,7 +307,6 @@ ISC_RUN_TEST_IMPL(overmempurge_bigrdata) {
 	for (i = 0; !isc_mem_isovermem(mctx2) && i < (maxcache / 10); i++) {
 		overmempurge_addrdataset(db, now, i, 50053, 0, false);
 	}
-	assert_true(isc_mem_isovermem(mctx2));
 
 	/*
 	 * Then try to add the same number of entries, each has very large data.
@@ -353,7 +352,6 @@ ISC_RUN_TEST_IMPL(overmempurge_longname) {
 	for (i = 0; !isc_mem_isovermem(mctx2) && i < (maxcache / 10); i++) {
 		overmempurge_addrdataset(db, now, i, 50053, 0, false);
 	}
-	assert_true(isc_mem_isovermem(mctx2));
 
 	/*
 	 * Then try to add the same number of entries, each has very large data.
diff --git a/tests/isc/mem_test.c b/tests/isc/mem_test.c
index c2540df729..ca23960880 100644
--- a/tests/isc/mem_test.c
+++ b/tests/isc/mem_test.c
@@ -290,6 +290,57 @@ ISC_RUN_TEST_IMPL(isc_mem_reget) {
 	isc_mem_put(mctx, data, REGET_SHRINK_SIZE);
 }
 
+static bool
+at_least_one_overmem(isc_mem_t *omctx) {
+	for (size_t i = 0; i < UINT16_MAX; i++) {
+		/* The overmem is probability based in this range */
+		if (isc_mem_isovermem(omctx)) {
+			return true;
+		}
+	}
+	return false;
+}
+
+static void
+water(void *arg, int mark) {
+	UNUSED(arg);
+	UNUSED(mark);
+}
+
+ISC_RUN_TEST_IMPL(isc_mem_overmem) {
+	isc_mem_t *omctx = NULL;
+	isc_mem_create(&omctx);
+	assert_non_null(omctx);
+
+	isc_mem_setwater(omctx, water, NULL, 1024, 512);
+
+	/* inuse <= lo_water is always false */
+	void *data1 = isc_mem_allocate(omctx, 256);
+	assert_false(isc_mem_isovermem(omctx));
+
+	/* lo_water < inuse < hi_water might be true or false */
+	void *data2 = isc_mem_allocate(omctx, 512);
+	assert_true(at_least_one_overmem(omctx));
+
+	/* hi_water <= inuse is always true */
+	void *data3 = isc_mem_allocate(omctx, 512);
+	assert_true(isc_mem_isovermem(omctx));
+
+	/* lo_water < inuse < hi_water might be true or false */
+	isc_mem_free(omctx, data2);
+	assert_true(at_least_one_overmem(omctx));
+
+	/* inuse <= lo_water is always false */
+	isc_mem_free(omctx, data3);
+	assert_false(isc_mem_isovermem(omctx));
+
+	/* inuse == 0 is always false */
+	isc_mem_free(omctx, data1);
+	assert_false(isc_mem_isovermem(omctx));
+
+	isc_mem_destroy(&omctx);
+}
+
 #if ISC_MEM_TRACKLINES
 
 /* test mem with no flags */
@@ -501,6 +552,7 @@ ISC_TEST_ENTRY(isc_mem_total)
 ISC_TEST_ENTRY(isc_mem_inuse)
 ISC_TEST_ENTRY(isc_mem_zeroget)
 ISC_TEST_ENTRY(isc_mem_reget)
+ISC_TEST_ENTRY(isc_mem_overmem)
 
 #if !defined(__SANITIZE_THREAD__)
 ISC_TEST_ENTRY(isc_mem_benchmark)