Index: sys/cddl/compat/opensolaris/kern/opensolaris_kmem.c
===================================================================
--- sys/cddl/compat/opensolaris/kern/opensolaris_kmem.c	(revision 271017)
+++ sys/cddl/compat/opensolaris/kern/opensolaris_kmem.c	(working copy)
@@ -132,12 +132,25 @@ kmem_size(void)
 }
 
 uint64_t
-kmem_used(void)
+kmem_map_used(void)
 {
 
 	return (kmem_map->size);
 }
 
+uint64_t
+kmem_map_free(void)
+{
+	uint64_t size;
+
+	vm_map_lock_read(kmem_map);
+	size = kmem_map->root != NULL ? kmem_map->root->max_free :
+	    kmem_map->max_offset - kmem_map->min_offset;
+	vm_map_unlock_read(kmem_map);
+
+	return (size);
+}
+
 static int
 kmem_std_constructor(void *mem, int size __unused, void *private, int flags)
 {
Index: sys/cddl/compat/opensolaris/sys/kmem.h
===================================================================
--- sys/cddl/compat/opensolaris/sys/kmem.h	(revision 271017)
+++ sys/cddl/compat/opensolaris/sys/kmem.h	(working copy)
@@ -66,7 +66,8 @@ typedef struct kmem_cache {
 void *zfs_kmem_alloc(size_t size, int kmflags);
 void zfs_kmem_free(void *buf, size_t size);
 uint64_t kmem_size(void);
-uint64_t kmem_used(void);
+uint64_t kmem_map_used(void);
+uint64_t kmem_map_free(void);
 kmem_cache_t *kmem_cache_create(char *name, size_t bufsize, size_t align,
     int (*constructor)(void *, void *, int), void (*destructor)(void *, void *),
     void (*reclaim)(void *) __unused, void *private, vmem_t *vmp, int cflags);
@@ -78,6 +79,8 @@ void kmem_reap(void);
 int kmem_debugging(void);
 void *calloc(size_t n, size_t s);
 
+#define	freemem				(cnt.v_free_count + cnt.v_cache_count)
+#define	minfree				cnt.v_free_min
 #define	kmem_alloc(size, kmflags)	zfs_kmem_alloc((size), (kmflags))
 #define	kmem_zalloc(size, kmflags)	zfs_kmem_alloc((size), (kmflags) | M_ZERO)
 #define	kmem_free(buf, size)		zfs_kmem_free((buf), (size))
Index: sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
===================================================================
--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c	(revision 271017)
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c	(working copy)
@@ -193,9 +193,6 @@ extern int zfs_prefetch_disable;
  */
 static boolean_t arc_warm;
 
-/*
- * These tunables are for performance analysis.
- */
 uint64_t zfs_arc_max;
 uint64_t zfs_arc_min;
 uint64_t zfs_arc_meta_limit = 0;
@@ -203,7 +200,20 @@ int zfs_arc_grow_retry = 0;
 int zfs_arc_shrink_shift = 0;
 int zfs_arc_p_min_shift = 0;
 int zfs_disable_dup_eviction = 0;
+u_int zfs_arc_free_target = (1 << 19); /* default before pagedaemon init only */
 
+static int sysctl_vfs_zfs_arc_free_target(SYSCTL_HANDLER_ARGS);
+
+#ifdef _KERNEL
+static void
+arc_free_target_init(void *unused __unused)
+{
+
+	zfs_arc_free_target = cnt.v_free_reserved + cnt.v_cache_min;
+}
+SYSINIT(arc_free_target_init, SI_SUB_KTHREAD_PAGE, SI_ORDER_ANY,
+    arc_free_target_init, NULL);
+
 TUNABLE_QUAD("vfs.zfs.arc_max", &zfs_arc_max);
 TUNABLE_QUAD("vfs.zfs.arc_min", &zfs_arc_min);
 TUNABLE_QUAD("vfs.zfs.arc_meta_limit", &zfs_arc_meta_limit);
@@ -214,6 +224,37 @@ SYSCTL_UQUAD(_vfs_zfs, OID_AUTO, arc_min, CTLFLAG_
     "Minimum ARC size");
 
 /*
+ * We don't have a tunable for arc_free_target due to the dependency on
+ * pagedaemon initialisation.
+ */
+SYSCTL_PROC(_vfs_zfs, OID_AUTO, arc_free_target,
+    CTLTYPE_UINT | CTLFLAG_MPSAFE | CTLFLAG_RW, 0, sizeof(u_int),
+    sysctl_vfs_zfs_arc_free_target, "IU",
+    "Desired number of free pages below which ARC triggers reclaim");
+
+static int
+sysctl_vfs_zfs_arc_free_target(SYSCTL_HANDLER_ARGS)
+{
+	u_int val;
+	int err;
+
+	val = zfs_arc_free_target;
+	err = sysctl_handle_int(oidp, &val, 0, req);
+	if (err != 0 || req->newptr == NULL)
+		 return (err);
+
+	if (val < minfree)
+		 return (EINVAL);
+	if (val > cnt.v_page_count)
+		 return (EINVAL);
+
+	zfs_arc_free_target = val;
+
+	return (0);
+}
+#endif
+
+/*
  * Note that buffers can be in one of 6 states:
  *	ARC_anon	- anonymous (discussed below)
  *	ARC_mru		- recently used, currently cached
@@ -2405,9 +2446,12 @@ arc_flush(spa_t *spa)
 void
 arc_shrink(void)
 {
+
 	if (arc_c > arc_c_min) {
 		uint64_t to_free;
 
+		DTRACE_PROBE2(arc__shrink, uint64_t, arc_c, uint64_t,
+			arc_c_min);
 #ifdef _KERNEL
 		to_free = arc_c >> arc_shrink_shift;
 #else
@@ -2427,8 +2471,11 @@ arc_shrink(void)
 		ASSERT((int64_t)arc_p >= 0);
 	}
 
-	if (arc_size > arc_c)
+	if (arc_size > arc_c) {
+		DTRACE_PROBE2(arc__shrink_adjust, uint64_t, arc_size,
+			uint64_t, arc_c);
 		arc_adjust();
+	}
 }
 
 static int needfree = 0;
@@ -2439,15 +2486,20 @@ arc_reclaim_needed(void)
 
 #ifdef _KERNEL
 
-	if (needfree)
+	if (needfree) {
+		DTRACE_PROBE(arc__reclaim_needfree);
 		return (1);
+	}
 
 	/*
 	 * Cooperate with pagedaemon when it's time for it to scan
 	 * and reclaim some pages.
 	 */
-	if (vm_paging_needed())
+	if (freemem < zfs_arc_free_target) {
+		DTRACE_PROBE2(arc__reclaim_freetarget, uint64_t,
+		    freemem, uint64_t, zfs_arc_free_target);
 		return (1);
+	}
 
 #ifdef sun
 	/*
@@ -2487,19 +2539,41 @@ arc_reclaim_needed(void)
 	 * heap is allocated.  (Or, in the calculation, if less than 1/4th is
 	 * free)
 	 */
-	if (btop(vmem_size(heap_arena, VMEM_FREE)) <
-	    (btop(vmem_size(heap_arena, VMEM_FREE | VMEM_ALLOC)) >> 2))
+	if (vmem_size(heap_arena, VMEM_FREE) <
+	    (vmem_size(heap_arena, VMEM_FREE | VMEM_ALLOC) >> 2))
 		return (1);
 #endif
+
+	/*
+	 * If zio data pages are being allocated out of a separate heap segment,
+	 * then enforce that the size of available vmem for this arena remains
+	 * above about 1/16th free.
+	 *
+	 * Note: The 1/16th arena free requirement was put in place
+	 * to aggressively evict memory from the arc in order to avoid
+	 * memory fragmentation issues.
+	 */
+	if (zio_arena != NULL &&
+	    vmem_size(zio_arena, VMEM_FREE) <
+	    (vmem_size(zio_arena, VMEM_ALLOC) >> 4))
+		return (1);
 #else	/* !sun */
-	if (kmem_used() > (kmem_size() * 3) / 4)
+#ifndef UMA_MD_SMALL_ALLOC
+	/* i386 has KVA limits that the raw page counts above don't consider */
+	if (kmem_map_used() > (kmem_size() * 3) / 4) {
+		DTRACE_PROBE2(arc__reclaim_used, uint64_t,
+		kmem_map_used(), uint64_t, (kmem_size() * 3) / 4);
 		return (1);
+	}
+#endif
 #endif	/* sun */
 
-#else
+#else	/* !_KERNEL */
 	if (spa_get_random(100) == 0)
 		return (1);
-#endif
+#endif	/* _KERNEL */
+	DTRACE_PROBE(arc__reclaim_no);
+
 	return (0);
 }
 
@@ -2697,20 +2771,6 @@ arc_evict_needed(arc_buf_contents_t type)
 	if (type == ARC_BUFC_METADATA && arc_meta_used >= arc_meta_limit)
 		return (1);
 
-#ifdef sun
-#ifdef _KERNEL
-	/*
-	 * If zio data pages are being allocated out of a separate heap segment,
-	 * then enforce that the size of available vmem for this area remains
-	 * above about 1/32nd free.
-	 */
-	if (type == ARC_BUFC_DATA && zio_arena != NULL &&
-	    vmem_size(zio_arena, VMEM_FREE) <
-	    (vmem_size(zio_arena, VMEM_ALLOC) >> 5))
-		return (1);
-#endif
-#endif	/* sun */
-
 	if (arc_reclaim_needed())
 		return (1);
 
@@ -3876,8 +3936,7 @@ static int
 arc_memory_throttle(uint64_t reserve, uint64_t txg)
 {
 #ifdef _KERNEL
-	uint64_t available_memory =
-	    ptoa((uintmax_t)cnt.v_free_count + cnt.v_cache_count);
+	uint64_t available_memory = ptob(freemem);
 	static uint64_t page_load = 0;
 	static uint64_t last_txg = 0;
 
@@ -3886,10 +3945,13 @@ arc_memory_throttle(uint64_t reserve, uint64_t txg
 	available_memory =
 	    MIN(available_memory, vmem_size(heap_arena, VMEM_FREE));
 #endif
+#else	/* sun */
+#ifndef UMA_MD_SMALL_ALLOC
+	available_memory = MIN(available_memory, kmem_map_free());
+#endif
 #endif	/* sun */
 
-	if (cnt.v_free_count + cnt.v_cache_count >
-	    (uint64_t)physmem * arc_lotsfree_percent / 100)
+	if (freemem > (uint64_t)physmem * arc_lotsfree_percent / 100)
 		return (0);
 
 	if (txg > last_txg) {
@@ -3902,7 +3964,7 @@ arc_memory_throttle(uint64_t reserve, uint64_t txg
 	 * continue to let page writes occur as quickly as possible.
 	 */
 	if (curproc == pageproc) {
-		if (page_load > available_memory / 4)
+		if (page_load > MAX(ptob(minfree), available_memory) / 4)
 			return (SET_ERROR(ERESTART));
 		/* Note: reserve is inflated, so we deflate */
 		page_load += reserve / 8;
Index: sys/vm/vm_pageout.c
===================================================================
--- sys/vm/vm_pageout.c	(revision 271017)
+++ sys/vm/vm_pageout.c	(working copy)
@@ -112,9 +112,13 @@ __FBSDID("$FreeBSD$");
 
 /* the kernel process "vm_pageout"*/
 static void vm_pageout(void);
+static void vm_pageout_init(void);
 static int vm_pageout_clean(vm_page_t);
 static void vm_pageout_scan(int pass);
 
+SYSINIT(pagedaemon_init, SI_SUB_KTHREAD_PAGE, SI_ORDER_FIRST, vm_pageout_init,
+    NULL);
+
 struct proc *pageproc;
 
 static struct kproc_desc page_kp = {
@@ -122,7 +126,7 @@ static struct kproc_desc page_kp = {
 	vm_pageout,
 	&pageproc
 };
-SYSINIT(pagedaemon, SI_SUB_KTHREAD_PAGE, SI_ORDER_FIRST, kproc_start,
+SYSINIT(pagedaemon, SI_SUB_KTHREAD_PAGE, SI_ORDER_SECOND, kproc_start,
     &page_kp);
 
 #if !defined(NO_SWAPPING)
@@ -1506,13 +1510,11 @@ vm_pageout_page_stats()
 }
 
 /*
- *	vm_pageout is the high level pageout daemon.
+ *	vm_pageout_init initialises basic pageout daemon settings.
  */
 static void
-vm_pageout()
+vm_pageout_init()
 {
-	int error, pass;
-
 	/*
 	 * Initialize some paging parameters.
 	 */
@@ -1579,7 +1581,16 @@ static void
 		vm_pageout_stats_interval = 5;
 	if (vm_pageout_full_stats_interval == 0)
 		vm_pageout_full_stats_interval = vm_pageout_stats_interval * 4;
+}
 
+/*
+ *	vm_pageout is the high level pageout daemon.
+ */
+static void
+vm_pageout()
+{
+	int error, pass;
+
 	swap_pager_swap_init();
 	pass = 0;
 	/*