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