diff --git a/lib/isc/mem.c b/lib/isc/mem.c
index 5d00eb6154..12136fc366 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/overflow.h>
+#include <isc/random.h>
 #include <isc/refcount.h>
 #include <isc/strerr.h>
 #include <isc/string.h>
@@ -131,7 +132,6 @@ struct isc_mem {
 	char name[16];
 	atomic_size_t inuse;
 	atomic_bool hi_called;
-	atomic_bool is_overmem;
 	atomic_size_t hi_water;
 	atomic_size_t lo_water;
 	ISC_LIST(isc_mempool_t) pools;
@@ -570,7 +570,6 @@ mem_create(isc_mem_t **ctxp, unsigned int debugging, unsigned int 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);
 
 	ISC_LIST_INIT(ctx->pools);
 
@@ -1017,48 +1016,30 @@ bool
 isc_mem_isovermem(isc_mem_t *ctx) {
 	REQUIRE(VALID_CONTEXT(ctx));
 
-	bool is_overmem = atomic_load_relaxed(&ctx->is_overmem);
-
-	if (!is_overmem) {
-		/* We are not overmem, check whether we should be? */
-		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 false;
-		}
-
-		if ((isc_mem_debugging & ISC_MEM_DEBUGUSAGE) != 0) {
-			fprintf(stderr,
-				"overmem mctx %p inuse %zu hi_water %zu\n", ctx,
-				inuse, hiwater);
-		}
-
-		atomic_store_relaxed(&ctx->is_overmem, true);
-		return true;
-	} else {
-		/* We are overmem, check whether we should not be? */
-		size_t lowater = atomic_load_relaxed(&ctx->lo_water);
-		if (lowater == 0) {
-			return false;
-		}
-
-		size_t inuse = atomic_load_relaxed(&ctx->inuse);
-		if (inuse >= lowater) {
-			return true;
-		}
-
-		if ((isc_mem_debugging & ISC_MEM_DEBUGUSAGE) != 0) {
-			fprintf(stderr,
-				"overmem mctx %p inuse %zu lo_water %zu\n", ctx,
-				inuse, lowater);
-		}
-		atomic_store_relaxed(&ctx->is_overmem, false);
+	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/qpdb_test.c b/tests/dns/qpdb_test.c
index c1888101d1..260b6c1dc3 100644
--- a/tests/dns/qpdb_test.c
+++ b/tests/dns/qpdb_test.c
@@ -137,7 +137,6 @@ ISC_LOOP_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.
@@ -188,7 +187,6 @@ ISC_LOOP_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 long name.
diff --git a/tests/isc/mem_test.c b/tests/isc/mem_test.c
index 7754488fb0..2c26922c8d 100644
--- a/tests/isc/mem_test.c
+++ b/tests/isc/mem_test.c
@@ -291,6 +291,17 @@ ISC_RUN_TEST_IMPL(isc_mem_reallocate) {
 	isc_mem_free(mctx, data);
 }
 
+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;
+}
+
 ISC_RUN_TEST_IMPL(isc_mem_overmem) {
 	isc_mem_t *omctx = NULL;
 	isc_mem_create(&omctx);
@@ -298,27 +309,27 @@ ISC_RUN_TEST_IMPL(isc_mem_overmem) {
 
 	isc_mem_setwater(omctx, 1024, 512);
 
-	/* inuse < lo_water */
+	/* inuse <= lo_water is always false */
 	void *data1 = isc_mem_allocate(omctx, 256);
 	assert_false(isc_mem_isovermem(omctx));
 
-	/* lo_water < inuse < hi_water */
+	/* lo_water < inuse < hi_water might be true or false */
 	void *data2 = isc_mem_allocate(omctx, 512);
-	assert_false(isc_mem_isovermem(omctx));
+	assert_true(at_least_one_overmem(omctx));
 
-	/* hi_water < inuse */
+	/* hi_water <= inuse is always true */
 	void *data3 = isc_mem_allocate(omctx, 512);
 	assert_true(isc_mem_isovermem(omctx));
 
-	/* lo_water < inuse < hi_water */
+	/* lo_water < inuse < hi_water might be true or false */
 	isc_mem_free(omctx, data2);
-	assert_true(isc_mem_isovermem(omctx));
+	assert_true(at_least_one_overmem(omctx));
 
-	/* inuse < lo_water */
+	/* inuse <= lo_water is always false */
 	isc_mem_free(omctx, data3);
 	assert_false(isc_mem_isovermem(omctx));
 
-	/* inuse == 0 */
+	/* inuse == 0 is always false */
 	isc_mem_free(omctx, data1);
 	assert_false(isc_mem_isovermem(omctx));