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Diffstat (limited to 'drivers/md/bcache/bset.h')
| -rw-r--r-- | drivers/md/bcache/bset.h | 379 |
1 files changed, 379 insertions, 0 deletions
diff --git a/drivers/md/bcache/bset.h b/drivers/md/bcache/bset.h new file mode 100644 index 00000000000..57a9cff4154 --- /dev/null +++ b/drivers/md/bcache/bset.h @@ -0,0 +1,379 @@ +#ifndef _BCACHE_BSET_H +#define _BCACHE_BSET_H + +/* + * BKEYS: + * + * A bkey contains a key, a size field, a variable number of pointers, and some + * ancillary flag bits. + * + * We use two different functions for validating bkeys, bch_ptr_invalid and + * bch_ptr_bad(). + * + * bch_ptr_invalid() primarily filters out keys and pointers that would be + * invalid due to some sort of bug, whereas bch_ptr_bad() filters out keys and + * pointer that occur in normal practice but don't point to real data. + * + * The one exception to the rule that ptr_invalid() filters out invalid keys is + * that it also filters out keys of size 0 - these are keys that have been + * completely overwritten. It'd be safe to delete these in memory while leaving + * them on disk, just unnecessary work - so we filter them out when resorting + * instead. + * + * We can't filter out stale keys when we're resorting, because garbage + * collection needs to find them to ensure bucket gens don't wrap around - + * unless we're rewriting the btree node those stale keys still exist on disk. + * + * We also implement functions here for removing some number of sectors from the + * front or the back of a bkey - this is mainly used for fixing overlapping + * extents, by removing the overlapping sectors from the older key. + * + * BSETS: + * + * A bset is an array of bkeys laid out contiguously in memory in sorted order, + * along with a header. A btree node is made up of a number of these, written at + * different times. + * + * There could be many of them on disk, but we never allow there to be more than + * 4 in memory - we lazily resort as needed. + * + * We implement code here for creating and maintaining auxiliary search trees + * (described below) for searching an individial bset, and on top of that we + * implement a btree iterator. + * + * BTREE ITERATOR: + * + * Most of the code in bcache doesn't care about an individual bset - it needs + * to search entire btree nodes and iterate over them in sorted order. + * + * The btree iterator code serves both functions; it iterates through the keys + * in a btree node in sorted order, starting from either keys after a specific + * point (if you pass it a search key) or the start of the btree node. + * + * AUXILIARY SEARCH TREES: + * + * Since keys are variable length, we can't use a binary search on a bset - we + * wouldn't be able to find the start of the next key. But binary searches are + * slow anyways, due to terrible cache behaviour; bcache originally used binary + * searches and that code topped out at under 50k lookups/second. + * + * So we need to construct some sort of lookup table. Since we only insert keys + * into the last (unwritten) set, most of the keys within a given btree node are + * usually in sets that are mostly constant. We use two different types of + * lookup tables to take advantage of this. + * + * Both lookup tables share in common that they don't index every key in the + * set; they index one key every BSET_CACHELINE bytes, and then a linear search + * is used for the rest. + * + * For sets that have been written to disk and are no longer being inserted + * into, we construct a binary search tree in an array - traversing a binary + * search tree in an array gives excellent locality of reference and is very + * fast, since both children of any node are adjacent to each other in memory + * (and their grandchildren, and great grandchildren...) - this means + * prefetching can be used to great effect. + * + * It's quite useful performance wise to keep these nodes small - not just + * because they're more likely to be in L2, but also because we can prefetch + * more nodes on a single cacheline and thus prefetch more iterations in advance + * when traversing this tree. + * + * Nodes in the auxiliary search tree must contain both a key to compare against + * (we don't want to fetch the key from the set, that would defeat the purpose), + * and a pointer to the key. We use a few tricks to compress both of these. + * + * To compress the pointer, we take advantage of the fact that one node in the + * search tree corresponds to precisely BSET_CACHELINE bytes in the set. We have + * a function (to_inorder()) that takes the index of a node in a binary tree and + * returns what its index would be in an inorder traversal, so we only have to + * store the low bits of the offset. + * + * The key is 84 bits (KEY_DEV + key->key, the offset on the device). To + * compress that, we take advantage of the fact that when we're traversing the + * search tree at every iteration we know that both our search key and the key + * we're looking for lie within some range - bounded by our previous + * comparisons. (We special case the start of a search so that this is true even + * at the root of the tree). + * + * So we know the key we're looking for is between a and b, and a and b don't + * differ higher than bit 50, we don't need to check anything higher than bit + * 50. + * + * We don't usually need the rest of the bits, either; we only need enough bits + * to partition the key range we're currently checking. Consider key n - the + * key our auxiliary search tree node corresponds to, and key p, the key + * immediately preceding n. The lowest bit we need to store in the auxiliary + * search tree is the highest bit that differs between n and p. + * + * Note that this could be bit 0 - we might sometimes need all 80 bits to do the + * comparison. But we'd really like our nodes in the auxiliary search tree to be + * of fixed size. + * + * The solution is to make them fixed size, and when we're constructing a node + * check if p and n differed in the bits we needed them to. If they don't we + * flag that node, and when doing lookups we fallback to comparing against the + * real key. As long as this doesn't happen to often (and it seems to reliably + * happen a bit less than 1% of the time), we win - even on failures, that key + * is then more likely to be in cache than if we were doing binary searches all + * the way, since we're touching so much less memory. + * + * The keys in the auxiliary search tree are stored in (software) floating + * point, with an exponent and a mantissa. The exponent needs to be big enough + * to address all the bits in the original key, but the number of bits in the + * mantissa is somewhat arbitrary; more bits just gets us fewer failures. + * + * We need 7 bits for the exponent and 3 bits for the key's offset (since keys + * are 8 byte aligned); using 22 bits for the mantissa means a node is 4 bytes. + * We need one node per 128 bytes in the btree node, which means the auxiliary + * search trees take up 3% as much memory as the btree itself. + * + * Constructing these auxiliary search trees is moderately expensive, and we + * don't want to be constantly rebuilding the search tree for the last set + * whenever we insert another key into it. For the unwritten set, we use a much + * simpler lookup table - it's just a flat array, so index i in the lookup table + * corresponds to the i range of BSET_CACHELINE bytes in the set. Indexing + * within each byte range works the same as with the auxiliary search trees. + * + * These are much easier to keep up to date when we insert a key - we do it + * somewhat lazily; when we shift a key up we usually just increment the pointer + * to it, only when it would overflow do we go to the trouble of finding the + * first key in that range of bytes again. + */ + +/* Btree key comparison/iteration */ + +struct btree_iter { + size_t size, used; + struct btree_iter_set { + struct bkey *k, *end; + } data[MAX_BSETS]; +}; + +struct bset_tree { + /* + * We construct a binary tree in an array as if the array + * started at 1, so that things line up on the same cachelines + * better: see comments in bset.c at cacheline_to_bkey() for + * details + */ + + /* size of the binary tree and prev array */ + unsigned size; + + /* function of size - precalculated for to_inorder() */ + unsigned extra; + + /* copy of the last key in the set */ + struct bkey end; + struct bkey_float *tree; + + /* + * The nodes in the bset tree point to specific keys - this + * array holds the sizes of the previous key. + * + * Conceptually it's a member of struct bkey_float, but we want + * to keep bkey_float to 4 bytes and prev isn't used in the fast + * path. + */ + uint8_t *prev; + + /* The actual btree node, with pointers to each sorted set */ + struct bset *data; +}; + +static __always_inline int64_t bkey_cmp(const struct bkey *l, + const struct bkey *r) +{ + return unlikely(KEY_INODE(l) != KEY_INODE(r)) + ? (int64_t) KEY_INODE(l) - (int64_t) KEY_INODE(r) + : (int64_t) KEY_OFFSET(l) - (int64_t) KEY_OFFSET(r); +} + +static inline size_t bkey_u64s(const struct bkey *k) +{ + BUG_ON(KEY_CSUM(k) > 1); + return 2 + KEY_PTRS(k) + (KEY_CSUM(k) ? 1 : 0); +} + +static inline size_t bkey_bytes(const struct bkey *k) +{ + return bkey_u64s(k) * sizeof(uint64_t); +} + +static inline void bkey_copy(struct bkey *dest, const struct bkey *src) +{ + memcpy(dest, src, bkey_bytes(src)); +} + +static inline void bkey_copy_key(struct bkey *dest, const struct bkey *src) +{ + if (!src) + src = &KEY(0, 0, 0); + + SET_KEY_INODE(dest, KEY_INODE(src)); + SET_KEY_OFFSET(dest, KEY_OFFSET(src)); +} + +static inline struct bkey *bkey_next(const struct bkey *k) +{ + uint64_t *d = (void *) k; + return (struct bkey *) (d + bkey_u64s(k)); +} + +/* Keylists */ + +struct keylist { + struct bkey *top; + union { + uint64_t *list; + struct bkey *bottom; + }; + + /* Enough room for btree_split's keys without realloc */ +#define KEYLIST_INLINE 16 + uint64_t d[KEYLIST_INLINE]; +}; + +static inline void bch_keylist_init(struct keylist *l) +{ + l->top = (void *) (l->list = l->d); +} + +static inline void bch_keylist_push(struct keylist *l) +{ + l->top = bkey_next(l->top); +} + +static inline void bch_keylist_add(struct keylist *l, struct bkey *k) +{ + bkey_copy(l->top, k); + bch_keylist_push(l); +} + +static inline bool bch_keylist_empty(struct keylist *l) +{ + return l->top == (void *) l->list; +} + +static inline void bch_keylist_free(struct keylist *l) +{ + if (l->list != l->d) + kfree(l->list); +} + +void bch_keylist_copy(struct keylist *, struct keylist *); +struct bkey *bch_keylist_pop(struct keylist *); +int bch_keylist_realloc(struct keylist *, int, struct cache_set *); + +void bch_bkey_copy_single_ptr(struct bkey *, const struct bkey *, + unsigned); +bool __bch_cut_front(const struct bkey *, struct bkey *); +bool __bch_cut_back(const struct bkey *, struct bkey *); + +static inline bool bch_cut_front(const struct bkey *where, struct bkey *k) +{ + BUG_ON(bkey_cmp(where, k) > 0); + return __bch_cut_front(where, k); +} + +static inline bool bch_cut_back(const struct bkey *where, struct bkey *k) +{ + BUG_ON(bkey_cmp(where, &START_KEY(k)) < 0); + return __bch_cut_back(where, k); +} + +const char *bch_ptr_status(struct cache_set *, const struct bkey *); +bool __bch_ptr_invalid(struct cache_set *, int level, const struct bkey *); +bool bch_ptr_bad(struct btree *, const struct bkey *); + +static inline uint8_t gen_after(uint8_t a, uint8_t b) +{ + uint8_t r = a - b; + return r > 128U ? 0 : r; +} + +static inline uint8_t ptr_stale(struct cache_set *c, const struct bkey *k, + unsigned i) +{ + return gen_after(PTR_BUCKET(c, k, i)->gen, PTR_GEN(k, i)); +} + +static inline bool ptr_available(struct cache_set *c, const struct bkey *k, + unsigned i) +{ + return (PTR_DEV(k, i) < MAX_CACHES_PER_SET) && PTR_CACHE(c, k, i); +} + + +typedef bool (*ptr_filter_fn)(struct btree *, const struct bkey *); + +struct bkey *bch_next_recurse_key(struct btree *, struct bkey *); +struct bkey *bch_btree_iter_next(struct btree_iter *); +struct bkey *bch_btree_iter_next_filter(struct btree_iter *, + struct btree *, ptr_filter_fn); + +void bch_btree_iter_push(struct btree_iter *, struct bkey *, struct bkey *); +struct bkey *__bch_btree_iter_init(struct btree *, struct btree_iter *, + struct bkey *, struct bset_tree *); + +/* 32 bits total: */ +#define BKEY_MID_BITS 3 +#define BKEY_EXPONENT_BITS 7 +#define BKEY_MANTISSA_BITS 22 +#define BKEY_MANTISSA_MASK ((1 << BKEY_MANTISSA_BITS) - 1) + +struct bkey_float { + unsigned exponent:BKEY_EXPONENT_BITS; + unsigned m:BKEY_MID_BITS; + unsigned mantissa:BKEY_MANTISSA_BITS; +} __packed; + +/* + * BSET_CACHELINE was originally intended to match the hardware cacheline size - + * it used to be 64, but I realized the lookup code would touch slightly less + * memory if it was 128. + * + * It definites the number of bytes (in struct bset) per struct bkey_float in + * the auxiliar search tree - when we're done searching the bset_float tree we + * have this many bytes left that we do a linear search over. + * + * Since (after level 5) every level of the bset_tree is on a new cacheline, + * we're touching one fewer cacheline in the bset tree in exchange for one more + * cacheline in the linear search - but the linear search might stop before it + * gets to the second cacheline. + */ + +#define BSET_CACHELINE 128 +#define bset_tree_space(b) (btree_data_space(b) / BSET_CACHELINE) + +#define bset_tree_bytes(b) (bset_tree_space(b) * sizeof(struct bkey_float)) +#define bset_prev_bytes(b) (bset_tree_space(b) * sizeof(uint8_t)) + +void bch_bset_init_next(struct btree *); + +void bch_bset_fix_invalidated_key(struct btree *, struct bkey *); +void bch_bset_fix_lookup_table(struct btree *, struct bkey *); + +struct bkey *__bch_bset_search(struct btree *, struct bset_tree *, + const struct bkey *); + +static inline struct bkey *bch_bset_search(struct btree *b, struct bset_tree *t, + const struct bkey *search) +{ + return search ? __bch_bset_search(b, t, search) : t->data->start; +} + +bool bch_bkey_try_merge(struct btree *, struct bkey *, struct bkey *); +void bch_btree_sort_lazy(struct btree *); +void bch_btree_sort_into(struct btree *, struct btree *); +void bch_btree_sort_and_fix_extents(struct btree *, struct btree_iter *); +void bch_btree_sort_partial(struct btree *, unsigned); + +static inline void bch_btree_sort(struct btree *b) +{ + bch_btree_sort_partial(b, 0); +} + +int bch_bset_print_stats(struct cache_set *, char *); + +#endif |