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git/oidtree.h

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oidtree: a crit-bit tree for odb_loose_cache This saves 8K per `struct object_directory', meaning it saves around 800MB in my case involving 100K alternates (half or more of those alternates are unlikely to hold loose objects). This is implemented in two parts: a generic, allocation-free `cbtree' and the `oidtree' wrapper on top of it. The latter provides allocation using alloc_state as a memory pool to improve locality and reduce free(3) overhead. Unlike oid-array, the crit-bit tree does not require sorting. Performance is bound by the key length, for oidtree that is fixed at sizeof(struct object_id). There's no need to have 256 oidtrees to mitigate the O(n log n) overhead like we did with oid-array. Being a prefix trie, it is natively suited for expanding short object IDs via prefix-limited iteration in `find_short_object_filename'. On my busy workstation, p4205 performance seems to be roughly unchanged (+/-8%). Startup with 100K total alternates with no loose objects seems around 10-20% faster on a hot cache. (800MB in memory savings means more memory for the kernel FS cache). The generic cbtree implementation does impose some extra overhead for oidtree in that it uses memcmp(3) on "struct object_id" so it wastes cycles comparing 12 extra bytes on SHA-1 repositories. I've not yet explored reducing this overhead, but I expect there are many places in our code base where we'd want to investigate this. More information on crit-bit trees: https://cr.yp.to/critbit.html Signed-off-by: Eric Wong <e@80x24.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>
1 year ago
#ifndef OIDTREE_H
#define OIDTREE_H
#include "cbtree.h"
#include "hash.h"
#include "mem-pool.h"
struct oidtree {
struct cb_tree tree;
struct mem_pool mem_pool;
};
void oidtree_init(struct oidtree *);
void oidtree_clear(struct oidtree *);
void oidtree_insert(struct oidtree *, const struct object_id *);
int oidtree_contains(struct oidtree *, const struct object_id *);
typedef enum cb_next (*oidtree_iter)(const struct object_id *, void *data);
void oidtree_each(struct oidtree *, const struct object_id *,
size_t oidhexsz, oidtree_iter, void *data);
#endif /* OIDTREE_H */