diff --git a/sys/amd64/include/vmparam.h b/sys/amd64/include/vmparam.h
index 474ea4bd856d..bda972213c86 100644
--- a/sys/amd64/include/vmparam.h
+++ b/sys/amd64/include/vmparam.h
@@ -178,23 +178,16 @@
 #define	PHYS_TO_DMAP(x)		((x) | DMAP_MIN_ADDRESS)
 #define	DMAP_TO_PHYS(x)		((x) & ~DMAP_MIN_ADDRESS)
 
-/* virtual sizes (bytes) for various kernel submaps */
-#ifndef VM_KMEM_SIZE
-#define	VM_KMEM_SIZE		(12 * 1024 * 1024)
-#endif
-
 /*
- * How many physical pages per KVA page allocated.
- * min(max(max(VM_KMEM_SIZE, Physical memory/VM_KMEM_SIZE_SCALE),
- *     VM_KMEM_SIZE_MIN), VM_KMEM_SIZE_MAX)
- * is the total KVA space allocated for kmem_map.
+ * How many physical pages per kmem arena virtual page.
  */
 #ifndef VM_KMEM_SIZE_SCALE
 #define	VM_KMEM_SIZE_SCALE	(1)
 #endif
 
 /*
- * Ceiling on amount of kmem_map kva space.
+ * Optional ceiling (in bytes) on the size of the kmem arena: 60% of the
+ * kernel map.
  */
 #ifndef VM_KMEM_SIZE_MAX
 #define	VM_KMEM_SIZE_MAX	((VM_MAX_KERNEL_ADDRESS - \
diff --git a/sys/arm/include/vmparam.h b/sys/arm/include/vmparam.h
index 8fa6988472f5..a93a92680a33 100644
--- a/sys/arm/include/vmparam.h
+++ b/sys/arm/include/vmparam.h
@@ -165,17 +165,22 @@
 #define	VM_MAX_KERNEL_ADDRESS	(vm_max_kernel_address)
 
 /*
- * Virtual size (bytes) for various kernel submaps.
+ * How many physical pages per kmem arena virtual page.
  */
-#ifndef VM_KMEM_SIZE
-#define VM_KMEM_SIZE		(12*1024*1024)
-#endif
 #ifndef VM_KMEM_SIZE_SCALE
-#define VM_KMEM_SIZE_SCALE	(3)
+#define	VM_KMEM_SIZE_SCALE	(3)
 #endif
 
 /*
- * Ceiling on the size of the kmem submap: 40% of the kernel map.
+ * Optional floor (in bytes) on the size of the kmem arena.
+ */
+#ifndef VM_KMEM_SIZE_MIN
+#define	VM_KMEM_SIZE_MIN	(12 * 1024 * 1024)
+#endif
+
+/*
+ * Optional ceiling (in bytes) on the size of the kmem arena: 40% of the
+ * kernel map.
  */
 #ifndef VM_KMEM_SIZE_MAX
 #define	VM_KMEM_SIZE_MAX	((vm_max_kernel_address - \
diff --git a/sys/i386/include/vmparam.h b/sys/i386/include/vmparam.h
index 19cdd8e115ef..c086d76d49aa 100644
--- a/sys/i386/include/vmparam.h
+++ b/sys/i386/include/vmparam.h
@@ -164,24 +164,23 @@
 #define VM_MAX_ADDRESS		VADDR(PTDPTDI, PTDPTDI)
 #define VM_MIN_ADDRESS		((vm_offset_t)0)
 
-/* virtual sizes (bytes) for various kernel submaps */
-#ifndef VM_KMEM_SIZE
-#define VM_KMEM_SIZE		(12 * 1024 * 1024)
-#endif
-
 /*
- * How many physical pages per KVA page allocated.
- * min(max(max(VM_KMEM_SIZE, Physical memory/VM_KMEM_SIZE_SCALE),
- *     VM_KMEM_SIZE_MIN), VM_KMEM_SIZE_MAX)
- * is the total KVA space allocated for kmem_map.
+ * How many physical pages per kmem arena virtual page.
  */
 #ifndef VM_KMEM_SIZE_SCALE
 #define	VM_KMEM_SIZE_SCALE	(3)
 #endif
 
 /*
- * Ceiling on the amount of kmem_map KVA space: 40% of the entire KVA space
- * rounded to the nearest multiple of the superpage size.
+ * Optional floor (in bytes) on the size of the kmem arena.
+ */
+#ifndef VM_KMEM_SIZE_MIN
+#define	VM_KMEM_SIZE_MIN	(12 * 1024 * 1024)
+#endif
+
+/*
+ * Optional ceiling (in bytes) on the size of the kmem arena: 40% of the
+ * kernel map rounded to the nearest multiple of the superpage size.
  */
 #ifndef VM_KMEM_SIZE_MAX
 #define	VM_KMEM_SIZE_MAX	(((((VM_MAX_KERNEL_ADDRESS - \
diff --git a/sys/ia64/include/vmparam.h b/sys/ia64/include/vmparam.h
index 1fda472a273e..edae4c4689f7 100644
--- a/sys/ia64/include/vmparam.h
+++ b/sys/ia64/include/vmparam.h
@@ -189,19 +189,11 @@
 #define	USRSTACK		VM_MAXUSER_ADDRESS
 #define	IA64_BACKINGSTORE	(USRSTACK - (2 * MAXSSIZ) - PAGE_SIZE)
 
-/* virtual sizes (bytes) for various kernel submaps */
-#ifndef VM_KMEM_SIZE
-#define VM_KMEM_SIZE		(12 * 1024 * 1024)
-#endif
-
 /*
- * How many physical pages per KVA page allocated.
- * min(max(max(VM_KMEM_SIZE, Physical memory/VM_KMEM_SIZE_SCALE),
- *     VM_KMEM_SIZE_MIN), VM_KMEM_SIZE_MAX)
- * is the total KVA space allocated for kmem_map.
+ * How many physical pages per kmem arena virtual page.
  */
 #ifndef VM_KMEM_SIZE_SCALE
-#define	VM_KMEM_SIZE_SCALE	(4) /* XXX 8192 byte pages */
+#define	VM_KMEM_SIZE_SCALE	(4)
 #endif
 
 /* initial pagein size of beginning of executable file */
diff --git a/sys/kern/kern_malloc.c b/sys/kern/kern_malloc.c
index bab745623d09..a5d25ea55032 100644
--- a/sys/kern/kern_malloc.c
+++ b/sys/kern/kern_malloc.c
@@ -684,41 +684,47 @@ kmem_reclaim(vmem_t *vm, int flags)
 	pagedaemon_wakeup();
 }
 
+CTASSERT(VM_KMEM_SIZE_SCALE >= 1);
+
 /*
- * Initialize the kernel memory arena.
+ * Initialize the kernel memory (kmem) arena.
  */
 void
 kmeminit(void)
 {
 	u_long mem_size, tmp;
- 
+
 	/*
-	 * Try to auto-tune the kernel memory size, so that it is
-	 * more applicable for a wider range of machine sizes.  The
-	 * VM_KMEM_SIZE_MAX is dependent on the maximum KVA space
-	 * available.
+	 * Calculate the amount of kernel virtual address (KVA) space that is
+	 * preallocated to the kmem arena.  In order to support a wide range
+	 * of machines, it is a function of the physical memory size,
+	 * specifically,
 	 *
-	 * Note that the kmem arena is also used by the zone allocator,
-	 * so make sure that there is enough space.
+	 *	min(max(physical memory size / VM_KMEM_SIZE_SCALE,
+	 *	    VM_KMEM_SIZE_MIN), VM_KMEM_SIZE_MAX)
+	 *
+	 * Every architecture must define an integral value for
+	 * VM_KMEM_SIZE_SCALE.  However, the definitions of VM_KMEM_SIZE_MIN
+	 * and VM_KMEM_SIZE_MAX, which represent respectively the floor and
+	 * ceiling on this preallocation, are optional.  Typically,
+	 * VM_KMEM_SIZE_MAX is itself a function of the available KVA space on
+	 * a given architecture.
 	 */
-	vm_kmem_size = VM_KMEM_SIZE;
 	mem_size = cnt.v_page_count;
 
-#if defined(VM_KMEM_SIZE_SCALE)
 	vm_kmem_size_scale = VM_KMEM_SIZE_SCALE;
-#endif
 	TUNABLE_INT_FETCH("vm.kmem_size_scale", &vm_kmem_size_scale);
-	if (vm_kmem_size_scale > 0 &&
-	    (mem_size / vm_kmem_size_scale) > (vm_kmem_size / PAGE_SIZE))
-		vm_kmem_size = (mem_size / vm_kmem_size_scale) * PAGE_SIZE;
+	if (vm_kmem_size_scale < 1)
+		vm_kmem_size_scale = VM_KMEM_SIZE_SCALE;
+
+	vm_kmem_size = (mem_size / vm_kmem_size_scale) * PAGE_SIZE;
 
 #if defined(VM_KMEM_SIZE_MIN)
 	vm_kmem_size_min = VM_KMEM_SIZE_MIN;
 #endif
 	TUNABLE_ULONG_FETCH("vm.kmem_size_min", &vm_kmem_size_min);
-	if (vm_kmem_size_min > 0 && vm_kmem_size < vm_kmem_size_min) {
+	if (vm_kmem_size_min > 0 && vm_kmem_size < vm_kmem_size_min)
 		vm_kmem_size = vm_kmem_size_min;
-	}
 
 #if defined(VM_KMEM_SIZE_MAX)
 	vm_kmem_size_max = VM_KMEM_SIZE_MAX;
@@ -727,15 +733,17 @@ kmeminit(void)
 	if (vm_kmem_size_max > 0 && vm_kmem_size >= vm_kmem_size_max)
 		vm_kmem_size = vm_kmem_size_max;
 
-	/* Allow final override from the kernel environment */
-	TUNABLE_ULONG_FETCH("vm.kmem_size", &vm_kmem_size);
-
 	/*
-	 * Limit kmem virtual size to twice the physical memory.
-	 * This allows for kmem map sparseness, but limits the size
-	 * to something sane.  Be careful to not overflow the 32bit
-	 * ints while doing the check or the adjustment.
+	 * Alternatively, the amount of KVA space that is preallocated to the
+	 * kmem arena can be set statically at compile-time or manually
+	 * through the kernel environment.  However, it is still limited to
+	 * twice the physical memory size, which has been sufficient to handle
+	 * the most severe cases of external fragmentation in the kmem arena. 
 	 */
+#if defined(VM_KMEM_SIZE)
+	vm_kmem_size = VM_KMEM_SIZE;
+#endif
+	TUNABLE_ULONG_FETCH("vm.kmem_size", &vm_kmem_size);
 	if (vm_kmem_size / 2 / PAGE_SIZE > mem_size)
 		vm_kmem_size = 2 * mem_size * PAGE_SIZE;
 
diff --git a/sys/mips/include/vmparam.h b/sys/mips/include/vmparam.h
index 4e1f03c55185..8922924faa76 100644
--- a/sys/mips/include/vmparam.h
+++ b/sys/mips/include/vmparam.h
@@ -108,22 +108,23 @@
 #define	VM_NRESERVLEVEL		0
 #endif
 
-/* virtual sizes (bytes) for various kernel submaps */
-#ifndef VM_KMEM_SIZE
-#define	VM_KMEM_SIZE		(12 * 1024 * 1024)
-#endif
-
 /*
- * How many physical pages per KVA page allocated.
- * min(max(VM_KMEM_SIZE, Physical memory/VM_KMEM_SIZE_SCALE), VM_KMEM_SIZE_MAX)
- * is the total KVA space allocated for kmem_map.
+ * How many physical pages per kmem arena virtual page.
  */
 #ifndef VM_KMEM_SIZE_SCALE
 #define	VM_KMEM_SIZE_SCALE	(3)
 #endif
 
 /*
- * Ceiling on the amount of kmem_map KVA space: 40% of the entire KVA space.
+ * Optional floor (in bytes) on the size of the kmem arena.
+ */
+#ifndef VM_KMEM_SIZE_MIN
+#define	VM_KMEM_SIZE_MIN	(12 * 1024 * 1024)
+#endif
+
+/*
+ * Optional ceiling (in bytes) on the size of the kmem arena: 40% of the
+ * kernel map.
  */
 #ifndef VM_KMEM_SIZE_MAX
 #define	VM_KMEM_SIZE_MAX	((VM_MAX_KERNEL_ADDRESS - \
diff --git a/sys/powerpc/include/vmparam.h b/sys/powerpc/include/vmparam.h
index e8e22cd76bcb..2d50f94c9dcd 100644
--- a/sys/powerpc/include/vmparam.h
+++ b/sys/powerpc/include/vmparam.h
@@ -172,21 +172,23 @@ struct pmap_physseg {
 #define	SGROWSIZ	(128UL*1024)		/* amount to grow stack */
 #endif
 
-#ifndef VM_KMEM_SIZE
-#define	VM_KMEM_SIZE		(12 * 1024 * 1024)
-#endif
-
 /*
- * How many physical pages per KVA page allocated.
- * min(max(VM_KMEM_SIZE, Physical memory/VM_KMEM_SIZE_SCALE), VM_KMEM_SIZE_MAX)
- * is the total KVA space allocated for kmem_map.
+ * How many physical pages per kmem arena virtual page.
  */
 #ifndef VM_KMEM_SIZE_SCALE
-#define VM_KMEM_SIZE_SCALE	(3)
+#define	VM_KMEM_SIZE_SCALE	(3)
 #endif
 
 /*
- * Ceiling on the amount of kmem_map KVA space: 40% of the entire KVA space.
+ * Optional floor (in bytes) on the size of the kmem arena.
+ */
+#ifndef VM_KMEM_SIZE_MIN
+#define	VM_KMEM_SIZE_MIN	(12 * 1024 * 1024)
+#endif
+
+/*
+ * Optional ceiling (in bytes) on the size of the kmem arena: 40% of the
+ * usable KVA space.
  */
 #ifndef VM_KMEM_SIZE_MAX
 #define VM_KMEM_SIZE_MAX	((VM_MAX_SAFE_KERNEL_ADDRESS - \
diff --git a/sys/sparc64/include/vmparam.h b/sys/sparc64/include/vmparam.h
index 3cd744cb912c..b1c90d290a8b 100644
--- a/sys/sparc64/include/vmparam.h
+++ b/sys/sparc64/include/vmparam.h
@@ -198,24 +198,22 @@
 #define	USRSTACK		(VM_MAX_USER_ADDRESS)
 
 /*
- * Virtual size (bytes) for various kernel submaps.
- */
-#ifndef	VM_KMEM_SIZE
-#define	VM_KMEM_SIZE		(16*1024*1024)
-#endif
-
-/*
- * How many physical pages per KVA page allocated.
- * min(max(max(VM_KMEM_SIZE, Physical memory/VM_KMEM_SIZE_SCALE),
- *     VM_KMEM_SIZE_MIN), VM_KMEM_SIZE_MAX)
- * is the total KVA space allocated for kmem_map.
+ * How many physical pages per kmem arena virtual page.
  */
 #ifndef VM_KMEM_SIZE_SCALE
 #define	VM_KMEM_SIZE_SCALE	(tsb_kernel_ldd_phys == 0 ? 3 : 2)
 #endif
 
 /*
- * Ceiling on amount of kmem_map kva space.
+ * Optional floor (in bytes) on the size of the kmem arena.
+ */
+#ifndef VM_KMEM_SIZE_MIN
+#define	VM_KMEM_SIZE_MIN	(16 * 1024 * 1024)
+#endif
+
+/*
+ * Optional ceiling (in bytes) on the size of the kmem arena: 60% of the
+ * kernel map.
  */
 #ifndef VM_KMEM_SIZE_MAX
 #define	VM_KMEM_SIZE_MAX	((VM_MAX_KERNEL_ADDRESS - \