btree: Implement faster binary search algorithm

This implements a binary search algorithm for B-Trees that reduces
branching to the absolute minimum necessary for a binary search
algorithm. It also enables the compiler to inlines the comparator to
ensure that the only slowdown when doing binary search is from waiting
for memory accesses. Additionally, it instructs the compiler to unroll
the loop, which gives an additional 40% improve with Clang and 8%
improvement with GCC.

Consumers must opt into using the faster algorithm. At present, only
B-Trees used inside kernel code have been modified to use the faster
algorithm.

Micro-benchmarks suggest that this can improve binary search performance
by up to 3.5 times when compiling with Clang 16 and up to 1.9 times when
compiling with GCC 12.2.

Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>

cmd/zdb/zdb.c

@@ -326,7 +326,7 @@ sublivelist_verify_func(void *args, dsl_deadlist_entry_t *dle)
 	int err;
 	struct sublivelist_verify *sv = args;
 
-	zfs_btree_create(&sv->sv_pair, sublivelist_block_refcnt_compare,
+	zfs_btree_create(&sv->sv_pair, sublivelist_block_refcnt_compare, NULL,
 	    sizeof (sublivelist_verify_block_refcnt_t));
 
 	err = bpobj_iterate_nofree(&dle->dle_bpobj, sublivelist_verify_blkptr,
@@ -390,7 +390,7 @@ sublivelist_verify_lightweight(void *args, dsl_deadlist_entry_t *dle)
 {
 	(void) args;
 	sublivelist_verify_t sv;
-	zfs_btree_create(&sv.sv_leftover, livelist_block_compare,
+	zfs_btree_create(&sv.sv_leftover, livelist_block_compare, NULL,
 	    sizeof (sublivelist_verify_block_t));
 	int err = sublivelist_verify_func(&sv, dle);
 	zfs_btree_clear(&sv.sv_leftover);
@@ -682,7 +682,7 @@ livelist_metaslab_validate(spa_t *spa)
 	(void) printf("Verifying deleted livelist entries\n");
 
 	sublivelist_verify_t sv;
-	zfs_btree_create(&sv.sv_leftover, livelist_block_compare,
+	zfs_btree_create(&sv.sv_leftover, livelist_block_compare, NULL,
 	    sizeof (sublivelist_verify_block_t));
 	iterate_deleted_livelists(spa, livelist_verify, &sv);
 
@@ -716,7 +716,7 @@ livelist_metaslab_validate(spa_t *spa)
 			mv.mv_start = m->ms_start;
 			mv.mv_end = m->ms_start + m->ms_size;
 			zfs_btree_create(&mv.mv_livelist_allocs,
-			    livelist_block_compare,
+			    livelist_block_compare, NULL,
 			    sizeof (sublivelist_verify_block_t));
 
 			mv_populate_livelist_allocs(&mv, &sv);

include/sys/btree.h

@@ -105,8 +105,13 @@ typedef struct zfs_btree_index {
 	boolean_t	bti_before;
 } zfs_btree_index_t;
 
-typedef struct btree {
+typedef struct btree zfs_btree_t;
+typedef void * (*bt_find_in_buf_f) (zfs_btree_t *, uint8_t *, uint32_t,
+    const void *, zfs_btree_index_t *);
+
+struct btree {
 	int (*bt_compar) (const void *, const void *);
+	bt_find_in_buf_f	bt_find_in_buf;
 	size_t			bt_elem_size;
 	size_t			bt_leaf_size;
 	uint32_t		bt_leaf_cap;
@@ -115,7 +120,42 @@ typedef struct btree {
 	uint64_t		bt_num_nodes;
 	zfs_btree_hdr_t		*bt_root;
 	zfs_btree_leaf_t	*bt_bulk; // non-null if bulk loading
-} zfs_btree_t;
+};
+
+/*
+ * Implementation of Shar's algorithm designed to accelerate binary search by
+ * eliminating impossible to predict branches. For optimality, this should be
+ * used to generate the search function in the same file as the comparator and
+ * the comparator must be marked `__attribute__((always_inline) inline` so that
+ * the compiler can inline the search function.
+ */
+#define	ZFS_BTREE_FIND_IN_BUF_FUNC(NAME, T, COMP)			\
+_Pragma("GCC diagnostic push")						\
+_Pragma("GCC diagnostic ignored \"-Wunknown-pragmas\"")			\
+static void *								\
+NAME(zfs_btree_t *tree, uint8_t *buf, uint32_t nelems,			\
+    const void *value, zfs_btree_index_t *where)			\
+{									\
+	T *i = (T *)buf;						\
+	(void) tree;							\
+	_Pragma("GCC unroll 9")						\
+	while (nelems > 1) {						\
+		uint32_t half = nelems / 2;				\
+		nelems -= half;						\
+		i += (COMP(&i[half - 1], value) < 0) * half;		\
+	}								\
+									\
+	int comp = COMP(i, value);					\
+	where->bti_offset = (i - (T *)buf) + (comp < 0);		\
+	where->bti_before = (comp != 0);				\
+									\
+	if (comp == 0) {						\
+		return (i);						\
+	}								\
+									\
+	return (NULL);							\
+}									\
+_Pragma("GCC diagnostic pop")
 
 /*
  * Allocate and deallocate caches for btree nodes.
@@ -129,13 +169,19 @@ void zfs_btree_fini(void);
  * tree   - the tree to be initialized
  * compar - function to compare two nodes, it must return exactly: -1, 0, or +1
  *          -1 for <, 0 for ==, and +1 for >
+ * find   - optional function to accelerate searches inside B-Tree nodes
+ *          through Shar's algorithm and comparator inlining. Setting this to
+ *          NULL will use a generic function. The function should be created
+ *          using ZFS_BTREE_FIND_IN_BUF_FUNC() in the same file as compar.
+ *          compar should be marked `__attribute__((always_inline)) inline` or
+ *          performance is unlikely to improve very much.
  * size   - the value of sizeof(struct my_type)
  * lsize  - custom leaf size
  */
 void zfs_btree_create(zfs_btree_t *, int (*) (const void *, const void *),
-    size_t);
+    bt_find_in_buf_f, size_t);
 void zfs_btree_create_custom(zfs_btree_t *, int (*)(const void *, const void *),
-    size_t, size_t);
+    bt_find_in_buf_f, size_t, size_t);
 
 /*
  * Find a node with a matching value in the tree. Returns the matching node

module/Kbuild.in

@@ -34,6 +34,20 @@ ifeq ($(CONFIG_KASAN),y)
 ZFS_MODULE_CFLAGS += -Wno-error=frame-larger-than=
 endif
 
+# Generated binary search code is particularly bad with this optimization.
+# Oddly, range_tree.c is not affected when unrolling is not done and dsl_scan.c
+# is not affected when unrolling is done.
+# Disable it on the other two until the following upstream issue is resolved:
+# https://github.com/llvm/llvm-project/issues/62790
+ifeq ($(CONFIG_X86_64),y)
+ifeq ($(CONFIG_CC_IS_CLANG),y)
+CFLAGS_zfs/dsl_scan.o += -mllvm -x86-cmov-converter=false
+CFLAGS_zfs/metaslab.o += -mllvm -x86-cmov-converter=false
+CFLAGS_zfs/range_tree.o += -mllvm -x86-cmov-converter=false
+CFLAGS_zfs/zap_micro.o += -mllvm -x86-cmov-converter=false
+endif
+endif
+
 ifneq ($(KBUILD_EXTMOD),)
 @CONFIG_QAT_TRUE@ZFS_MODULE_CFLAGS += -I@QAT_SRC@/include
 @CONFIG_QAT_TRUE@KBUILD_EXTRA_SYMBOLS += @QAT_SYMBOLS@

module/Makefile.bsd

@@ -400,6 +400,18 @@ beforeinstall:
 
 .include <bsd.kmod.mk>
 
+# Generated binary search code is particularly bad with this optimization.
+# Oddly, range_tree.c is not affected when unrolling is not done and dsl_scan.c
+# is not affected when unrolling is done.
+# Disable it on the other two until the following upstream issue is resolved:
+# https://github.com/llvm/llvm-project/issues/62790
+.if ${CC} == "clang"
+CFLAGS.dsl_scan.c= -mllvm -x86-cmov-converter=false
+CFLAGS.metaslab.c= -mllvm -x86-cmov-converter=false
+CFLAGS.range_tree.c= -mllvm -x86-cmov-converter=false
+CFLAGS.zap_micro.c= -mllvm -x86-cmov-converter=false
+.endif
+
 CFLAGS.sysctl_os.c= -include ../zfs_config.h
 CFLAGS.xxhash.c+= -include ${SYSDIR}/sys/_null.h
 

module/zfs/btree.c

@@ -193,21 +193,29 @@ zfs_btree_leaf_free(zfs_btree_t *tree, void *ptr)
 
 void
 zfs_btree_create(zfs_btree_t *tree, int (*compar) (const void *, const void *),
-    size_t size)
+    bt_find_in_buf_f bt_find_in_buf, size_t size)
 {
-	zfs_btree_create_custom(tree, compar, size, BTREE_LEAF_SIZE);
+	zfs_btree_create_custom(tree, compar, bt_find_in_buf, size,
+	    BTREE_LEAF_SIZE);
 }
 
+static void *
+zfs_btree_find_in_buf(zfs_btree_t *tree, uint8_t *buf, uint32_t nelems,
+    const void *value, zfs_btree_index_t *where);
+
 void
 zfs_btree_create_custom(zfs_btree_t *tree,
     int (*compar) (const void *, const void *),
+    bt_find_in_buf_f bt_find_in_buf,
     size_t size, size_t lsize)
 {
 	size_t esize = lsize - offsetof(zfs_btree_leaf_t, btl_elems);
 
 	ASSERT3U(size, <=, esize / 2);
 	memset(tree, 0, sizeof (*tree));
 	tree->bt_compar = compar;
+	tree->bt_find_in_buf = (bt_find_in_buf == NULL) ?
+	    zfs_btree_find_in_buf : bt_find_in_buf;
 	tree->bt_elem_size = size;
 	tree->bt_leaf_size = lsize;
 	tree->bt_leaf_cap = P2ALIGN(esize / size, 2);
@@ -303,7 +311,7 @@ zfs_btree_find(zfs_btree_t *tree, const void *value, zfs_btree_index_t *where)
 			 * element in the last leaf, it's in the last leaf or
 			 * it's not in the tree.
 			 */
-			void *d = zfs_btree_find_in_buf(tree,
+			void *d = tree->bt_find_in_buf(tree,
 			    last_leaf->btl_elems +
 			    last_leaf->btl_hdr.bth_first * size,
 			    last_leaf->btl_hdr.bth_count, value, &idx);
@@ -327,7 +335,7 @@ zfs_btree_find(zfs_btree_t *tree, const void *value, zfs_btree_index_t *where)
 	for (node = (zfs_btree_core_t *)tree->bt_root; depth < tree->bt_height;
 	    node = (zfs_btree_core_t *)node->btc_children[child], depth++) {
 		ASSERT3P(node, !=, NULL);
-		void *d = zfs_btree_find_in_buf(tree, node->btc_elems,
+		void *d = tree->bt_find_in_buf(tree, node->btc_elems,
 		    node->btc_hdr.bth_count, value, &idx);
 		EQUIV(d != NULL, !idx.bti_before);
 		if (d != NULL) {
@@ -347,7 +355,7 @@ zfs_btree_find(zfs_btree_t *tree, const void *value, zfs_btree_index_t *where)
 	 */
 	zfs_btree_leaf_t *leaf = (depth == 0 ?
 	    (zfs_btree_leaf_t *)tree->bt_root : (zfs_btree_leaf_t *)node);
-	void *d = zfs_btree_find_in_buf(tree, leaf->btl_elems +
+	void *d = tree->bt_find_in_buf(tree, leaf->btl_elems +
 	    leaf->btl_hdr.bth_first * size,
 	    leaf->btl_hdr.bth_count, value, &idx);
 
@@ -671,7 +679,7 @@ zfs_btree_insert_into_parent(zfs_btree_t *tree, zfs_btree_hdr_t *old_node,
 	zfs_btree_hdr_t *par_hdr = &parent->btc_hdr;
 	zfs_btree_index_t idx;
 	ASSERT(zfs_btree_is_core(par_hdr));
-	VERIFY3P(zfs_btree_find_in_buf(tree, parent->btc_elems,
+	VERIFY3P(tree->bt_find_in_buf(tree, parent->btc_elems,
 	    par_hdr->bth_count, buf, &idx), ==, NULL);
 	ASSERT(idx.bti_before);
 	uint32_t offset = idx.bti_offset;
@@ -897,7 +905,7 @@ zfs_btree_find_parent_idx(zfs_btree_t *tree, zfs_btree_hdr_t *hdr)
 	}
 	zfs_btree_index_t idx;
 	zfs_btree_core_t *parent = hdr->bth_parent;
-	VERIFY3P(zfs_btree_find_in_buf(tree, parent->btc_elems,
+	VERIFY3P(tree->bt_find_in_buf(tree, parent->btc_elems,
 	    parent->btc_hdr.bth_count, buf, &idx), ==, NULL);
 	ASSERT(idx.bti_before);
 	ASSERT3U(idx.bti_offset, <=, parent->btc_hdr.bth_count);

module/zfs/dsl_scan.c

@@ -4877,6 +4877,7 @@ scan_exec_io(dsl_pool_t *dp, const blkptr_t *bp, int zio_flags,
  * with single operation.  Plus it makes scrubs more sequential and reduces
  * chances that minor extent change move it within the B-tree.
  */
+__attribute__((always_inline)) inline
 static int
 ext_size_compare(const void *x, const void *y)
 {
@@ -4885,13 +4886,17 @@ ext_size_compare(const void *x, const void *y)
 	return (TREE_CMP(*a, *b));
 }
 
+ZFS_BTREE_FIND_IN_BUF_FUNC(ext_size_find_in_buf, uint64_t,
+    ext_size_compare)
+
 static void
 ext_size_create(range_tree_t *rt, void *arg)
 {
 	(void) rt;
 	zfs_btree_t *size_tree = arg;
 
-	zfs_btree_create(size_tree, ext_size_compare, sizeof (uint64_t));
+	zfs_btree_create(size_tree, ext_size_compare, ext_size_find_in_buf,
+	    sizeof (uint64_t));
 }
 
 static void

module/zfs/metaslab.c

@@ -1342,6 +1342,7 @@ metaslab_group_allocatable(metaslab_group_t *mg, metaslab_group_t *rotor,
  * Comparison function for the private size-ordered tree using 32-bit
  * ranges. Tree is sorted by size, larger sizes at the end of the tree.
  */
+__attribute__((always_inline)) inline
 static int
 metaslab_rangesize32_compare(const void *x1, const void *x2)
 {
@@ -1352,16 +1353,15 @@ metaslab_rangesize32_compare(const void *x1, const void *x2)
 	uint64_t rs_size2 = r2->rs_end - r2->rs_start;
 
 	int cmp = TREE_CMP(rs_size1, rs_size2);
-	if (likely(cmp))
-		return (cmp);
 
-	return (TREE_CMP(r1->rs_start, r2->rs_start));
+	return (cmp + !cmp * TREE_CMP(r1->rs_start, r2->rs_start));
 }
 
 /*
  * Comparison function for the private size-ordered tree using 64-bit
  * ranges. Tree is sorted by size, larger sizes at the end of the tree.
  */
+__attribute__((always_inline)) inline
 static int
 metaslab_rangesize64_compare(const void *x1, const void *x2)
 {
@@ -1372,11 +1372,10 @@ metaslab_rangesize64_compare(const void *x1, const void *x2)
 	uint64_t rs_size2 = r2->rs_end - r2->rs_start;
 
 	int cmp = TREE_CMP(rs_size1, rs_size2);
-	if (likely(cmp))
-		return (cmp);
 
-	return (TREE_CMP(r1->rs_start, r2->rs_start));
+	return (cmp + !cmp * TREE_CMP(r1->rs_start, r2->rs_start));
 }
+
 typedef struct metaslab_rt_arg {
 	zfs_btree_t *mra_bt;
 	uint32_t mra_floor_shift;
@@ -1412,6 +1411,13 @@ metaslab_size_tree_full_load(range_tree_t *rt)
 	range_tree_walk(rt, metaslab_size_sorted_add, &arg);
 }
 
+
+ZFS_BTREE_FIND_IN_BUF_FUNC(metaslab_rt_find_rangesize32_in_buf,
+    range_seg32_t, metaslab_rangesize32_compare)
+
+ZFS_BTREE_FIND_IN_BUF_FUNC(metaslab_rt_find_rangesize64_in_buf,
+    range_seg64_t, metaslab_rangesize64_compare)
+
 /*
  * Create any block allocator specific components. The current allocators
  * rely on using both a size-ordered range_tree_t and an array of uint64_t's.
@@ -1424,19 +1430,22 @@ metaslab_rt_create(range_tree_t *rt, void *arg)
 
 	size_t size;
 	int (*compare) (const void *, const void *);
+	bt_find_in_buf_f bt_find;
 	switch (rt->rt_type) {
 	case RANGE_SEG32:
 		size = sizeof (range_seg32_t);
 		compare = metaslab_rangesize32_compare;
+		bt_find = metaslab_rt_find_rangesize32_in_buf;
 		break;
 	case RANGE_SEG64:
 		size = sizeof (range_seg64_t);
 		compare = metaslab_rangesize64_compare;
+		bt_find = metaslab_rt_find_rangesize64_in_buf;
 		break;
 	default:
 		panic("Invalid range seg type %d", rt->rt_type);
 	}
-	zfs_btree_create(size_tree, compare, size);
+	zfs_btree_create(size_tree, compare, bt_find, size);
 	mrap->mra_floor_shift = metaslab_by_size_min_shift;
 }