Index: sys/conf/NOTES
===================================================================
--- sys/conf/NOTES
+++ sys/conf/NOTES
@@ -2984,6 +2984,7 @@
 # If neither is present, then the Fortuna algorithm is used.
 options 	RANDOM_YARROW	# Yarrow CSPRNG (old default)
 #options 	RANDOM_DUMMY	# Dummy CSPRNG that always blocks
+options 	RANDOM_ENABLE_UMA	# slab allocator
 # For developers.
 options 	RANDOM_DEBUG	# Extra debugging messages
 
Index: sys/conf/options
===================================================================
--- sys/conf/options
+++ sys/conf/options
@@ -950,6 +950,9 @@
 # These are mutually exclusive. With neither, Fortuna is selected.
 RANDOM_DUMMY	opt_global.h
 RANDOM_YARROW	opt_random.h
+# This turns on high-rate and potentially expensive harvesting in
+# the uma slab allocator.
+RANDOM_ENABLE_UMA	opt_global.h
 
 # Intel em(4) driver
 EM_MULTIQUEUE	opt_em.h
Index: sys/sys/random.h
===================================================================
--- sys/sys/random.h
+++ sys/sys/random.h
@@ -1,5 +1,5 @@
 /*-
- * Copyright (c) 2000-2013 Mark R. V. Murray
+ * Copyright (c) 2000-2015 Mark R. V. Murray
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -86,6 +86,12 @@
 void random_harvest_direct(const void *, u_int, u_int, enum random_entropy_source);
 #endif /* defined(RANDOM_DUMMY) */
 
+#if defined(RANDOM_ENABLE_UMA)
+#define random_harvest_fast_uma(a, b, c, d)	random_harvest_fast(a, b, c, d)
+#else /* !defined(RANDOM_ENABLE_UMA) */
+#define random_harvest_fast_uma(a, b, c, d)	do {} while (0)
+#endif /* defined(RANDOM_ENABLE_UMA) */
+
 #endif /* _KERNEL */
 
 #endif /* _SYS_RANDOM_H_ */
Index: sys/vm/uma_core.c
===================================================================
--- sys/vm/uma_core.c
+++ sys/vm/uma_core.c
@@ -2135,8 +2135,11 @@
 	int lockfail;
 	int cpu;
 
-	/* XXX: FIX? The entropy here is desirable, but the harvesting may be expensive */
-	random_harvest_fast(&zone, sizeof(zone), 1, RANDOM_FAST);
+	/*
+	 * This is only enabled by kernel option.
+	 * If not enabled, it is a null macro.
+	 */
+	random_harvest_fast_uma(&zone, sizeof(zone), 1, RANDOM_FAST);
 
 	/* This is the fast path allocation */
 #ifdef UMA_DEBUG_ALLOC_1
@@ -2677,8 +2680,11 @@
 	int lockfail;
 	int cpu;
 
-	/* XXX: FIX? The entropy here is desirable, but the harvesting may be expensive */
-	random_harvest_fast(&zone, sizeof(zone), 1, RANDOM_FAST);
+	/*
+	 * This is only enabled by kernel option.
+	 * If not enabled, it is a null macro.
+	 */
+	random_harvest_fast_uma(&zone, sizeof(zone), 1, RANDOM_FAST);
 
 #ifdef UMA_DEBUG_ALLOC_1
 	printf("Freeing item %p to %s(%p)\n", item, zone->uz_name, zone);