[PATCH 02/33] radixtree: look-aside cache

Wu Fengguang
2006-05-24T11:19:29Z

[-- Attachment #1: radixtree-lookaside-cache.patch --]
[-- Type: text/plain, Size: 9264 bytes --]

Introduce a set of lookup functions to radix tree for the read-ahead logic.
Other access patterns with high locality may also benefit from them.

- radix_tree_lookup_parent(root, index, level)
Perform partial lookup, return the @level'th parent of the slot at
@index.

- radix_tree_cache_xxx()
Init/Query the cache.
- radix_tree_cache_lookup(root, cache, index)
Perform lookup with the aid of a look-aside cache.
For sequential scans, it has a time complexity of 64*O(1) + 1*O(logN).

Typical usage:

void func() {
+ struct radix_tree_cache cache;
+
+ radix_tree_cache_init(&cache);
read_lock_irq(&mapping->tree_lock);
for(;;) {
- page = radix_tree_lookup(&mapping->page_tree, index);
+ page = radix_tree_cache_lookup(&mapping->page_tree, &cache, index);
}
read_unlock_irq(&mapping->tree_lock);
}

Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---

include/linux/radix-tree.h | 83 +++++++++++++++++++++++++++++++++++
lib/radix-tree.c | 104 ++++++++++++++++++++++++++++++++++-----------
2 files changed, 161 insertions(+), 26 deletions(-)

--- linux-2.6.17-rc4-mm3.orig/include/linux/radix-tree.h
+++ linux-2.6.17-rc4-mm3/include/linux/radix-tree.h
@@ -26,12 +26,29 @@
#define RADIX_TREE_MAX_TAGS 2

/* root tags are stored in gfp_mask, shifted by __GFP_BITS_SHIFT */
+#ifdef __KERNEL__
+#define RADIX_TREE_MAP_SHIFT (CONFIG_BASE_SMALL ? 4 : 6)
+#else
+#define RADIX_TREE_MAP_SHIFT 3 /* For more stressful testing */
+#endif
+
+#define RADIX_TREE_MAP_SIZE (1UL << RADIX_TREE_MAP_SHIFT)
+#define RADIX_TREE_MAP_MASK (RADIX_TREE_MAP_SIZE-1)
+
struct radix_tree_root {
unsigned int height;
gfp_t gfp_mask;
struct radix_tree_node *rnode;
};

+/*
+ * Lookaside cache to support access patterns with strong locality.
+ */
+struct radix_tree_cache {
+ unsigned long first_index;
+ struct radix_tree_node *tree_node;
+};
+
#define RADIX_TREE_INIT(mask) { \
.height = 0, \
.gfp_mask = (mask), \
@@ -49,9 +66,14 @@ do { \
} while (0)

int radix_tree_insert(struct radix_tree_root *, unsigned long, void *);
-void *radix_tree_lookup(struct radix_tree_root *, unsigned long);
-void **radix_tree_lookup_slot(struct radix_tree_root *, unsigned long);
+void *radix_tree_lookup_parent(struct radix_tree_root *, unsigned long,
+ unsigned int);
+void **radix_tree_lookup_slot(struct radix_tree_root *root, unsigned long);
void *radix_tree_delete(struct radix_tree_root *, unsigned long);
+unsigned int radix_tree_cache_count(struct radix_tree_cache *cache);
+void *radix_tree_cache_lookup_parent(struct radix_tree_root *root,
+ struct radix_tree_cache *cache,
+ unsigned long index, unsigned int level);
unsigned int
radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
unsigned long first_index, unsigned int max_items);
@@ -74,4 +96,61 @@ static inline void radix_tree_preload_en
preempt_enable();
}

+/**
+ * radix_tree_lookup - perform lookup operation on a radix tree
+ * @root: radix tree root
+ * @index: index key
+ *
+ * Lookup the item at the position @index in the radix tree @root.
+ */
+static inline void *radix_tree_lookup(struct radix_tree_root *root,
+ unsigned long index)
+{
+ return radix_tree_lookup_parent(root, index, 0);
+}
+
+/**
+ * radix_tree_cache_init - init a look-aside cache
+ * @cache: look-aside cache
+ *
+ * Init the radix tree look-aside cache @cache.
+ */
+static inline void radix_tree_cache_init(struct radix_tree_cache *cache)
+{
+ cache->first_index = RADIX_TREE_MAP_MASK;
+ cache->tree_node = NULL;
+}
+
+/**
+ * radix_tree_cache_lookup - cached lookup on a radix tree
+ * @root: radix tree root
+ * @cache: look-aside cache
+ * @index: index key
+ *
+ * Lookup the item at the position @index in the radix tree @root,
+ * and make use of @cache to speedup the lookup process.
+ */
+static inline void *radix_tree_cache_lookup(struct radix_tree_root *root,
+ struct radix_tree_cache *cache,
+ unsigned long index)
+{
+ return radix_tree_cache_lookup_parent(root, cache, index, 0);
+}
+
+static inline unsigned int radix_tree_cache_size(struct radix_tree_cache *cache)
+{
+ return RADIX_TREE_MAP_SIZE;
+}
+
+static inline int radix_tree_cache_full(struct radix_tree_cache *cache)
+{
+ return radix_tree_cache_count(cache) == radix_tree_cache_size(cache);
+}
+
+static inline unsigned long
+radix_tree_cache_first_index(struct radix_tree_cache *cache)
+{
+ return cache->first_index;
+}
+
#endif /* _LINUX_RADIX_TREE_H */
--- linux-2.6.17-rc4-mm3.orig/lib/radix-tree.c
+++ linux-2.6.17-rc4-mm3/lib/radix-tree.c
@@ -309,36 +309,90 @@ int radix_tree_insert(struct radix_tree_
}
EXPORT_SYMBOL(radix_tree_insert);

-static inline void **__lookup_slot(struct radix_tree_root *root,
- unsigned long index)
+/**
+ * radix_tree_lookup_parent - low level lookup routine
+ * @root: radix tree root
+ * @index: index key
+ * @level: stop at that many levels from the tree leaf
+ *
+ * Lookup the @level'th parent of the slot at @index in radix tree @root.
+ * The return value is:
+ * @level == 0: page at @index;
+ * @level == 1: the corresponding bottom level tree node;
+ * @level < height: (@level-1)th parent node of the bottom node
+ * that contains @index;
+ * @level >= height: the root node.
+ */
+void *radix_tree_lookup_parent(struct radix_tree_root *root,
+ unsigned long index, unsigned int level)
{
unsigned int height, shift;
- struct radix_tree_node **slot;
+ struct radix_tree_node *slot;

height = root->height;

if (index > radix_tree_maxindex(height))
return NULL;

- if (height == 0 && root->rnode)
- return (void **)&root->rnode;
-
shift = (height-1) * RADIX_TREE_MAP_SHIFT;
- slot = &root->rnode;
+ slot = root->rnode;

- while (height > 0) {
- if (*slot == NULL)
+ while (height > level) {
+ if (slot == NULL)
return NULL;

- slot = (struct radix_tree_node **)
- ((*slot)->slots +
- ((index >> shift) & RADIX_TREE_MAP_MASK));
+ slot = slot->slots[(index >> shift) & RADIX_TREE_MAP_MASK];
shift -= RADIX_TREE_MAP_SHIFT;
height--;
}

- return (void **)slot;
+ return slot;
+}
+EXPORT_SYMBOL(radix_tree_lookup_parent);
+
+/**
+ * radix_tree_cache_lookup_parent - cached lookup node
+ * @root: radix tree root
+ * @cache: look-aside cache
+ * @index: index key
+ *
+ * Lookup the item at the position @index in the radix tree @root,
+ * and return the node @level levels from the bottom in the search path.
+ *
+ * @cache stores the last accessed upper level tree node by this
+ * function, and is always checked first before searching in the tree.
+ * It can improve speed for access patterns with strong locality.
+ *
+ * NOTE:
+ * - The cache becomes invalid on leaving the lock;
+ * - Do not intermix calls with different @level.
+ */
+void *radix_tree_cache_lookup_parent(struct radix_tree_root *root,
+ struct radix_tree_cache *cache,
+ unsigned long index, unsigned int level)
+{
+ struct radix_tree_node *node;
+ unsigned long i;
+ unsigned long mask;
+
+ if (level >= root->height)
+ return radix_tree_lookup_parent(root, index, level);
+
+ i = (index >> (level * RADIX_TREE_MAP_SHIFT)) & RADIX_TREE_MAP_MASK;
+ mask = (~0UL) << ((level + 1) * RADIX_TREE_MAP_SHIFT);
+
+ if ((index & mask) == cache->first_index)
+ return cache->tree_node->slots[i];
+
+ node = radix_tree_lookup_parent(root, index, level + 1);
+ if (!node)
+ return 0;
+
+ cache->tree_node = node;
+ cache->first_index = (index & mask);
+ return node->slots[i];
}
+EXPORT_SYMBOL(radix_tree_cache_lookup_parent);

/**
* radix_tree_lookup_slot - lookup a slot in a radix tree
@@ -350,25 +404,27 @@ static inline void **__lookup_slot(struc
*/
void **radix_tree_lookup_slot(struct radix_tree_root *root, unsigned long index)
{
- return __lookup_slot(root, index);
+ struct radix_tree_node *node;
+
+ node = radix_tree_lookup_parent(root, index, 1);
+ return node->slots + (index & RADIX_TREE_MAP_MASK);
}
EXPORT_SYMBOL(radix_tree_lookup_slot);

/**
- * radix_tree_lookup - perform lookup operation on a radix tree
- * @root: radix tree root
- * @index: index key
+ * radix_tree_cache_count - items in the cached node
+ * @cache: radix tree look-aside cache
*
- * Lookup the item at the position @index in the radix tree @root.
+ * Query the number of items contained in the cached node.
*/
-void *radix_tree_lookup(struct radix_tree_root *root, unsigned long index)
+unsigned int radix_tree_cache_count(struct radix_tree_cache *cache)
{
- void **slot;
-
- slot = __lookup_slot(root, index);
- return slot != NULL ? *slot : NULL;
+ if (!(cache->first_index & RADIX_TREE_MAP_MASK))
+ return cache->tree_node->count;
+ else
+ return 0;
}
-EXPORT_SYMBOL(radix_tree_lookup);
+EXPORT_SYMBOL(radix_tree_cache_count);

/**
* radix_tree_tag_set - set a tag on a radix tree node

--