git/cbtree.c

168 lines
4.2 KiB
C
Raw Normal View History

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>
2021-07-07 23:10:19 +00:00
/*
* crit-bit tree implementation, does no allocations internally
* For more information on crit-bit trees: https://cr.yp.to/critbit.html
* Based on Adam Langley's adaptation of Dan Bernstein's public domain code
* git clone https://github.com/agl/critbit.git
*/
#include "cbtree.h"
static struct cb_node *cb_node_of(const void *p)
{
return (struct cb_node *)((uintptr_t)p - 1);
}
/* locate the best match, does not do a final comparision */
static struct cb_node *cb_internal_best_match(struct cb_node *p,
const uint8_t *k, size_t klen)
{
while (1 & (uintptr_t)p) {
struct cb_node *q = cb_node_of(p);
uint8_t c = q->byte < klen ? k[q->byte] : 0;
size_t direction = (1 + (q->otherbits | c)) >> 8;
p = q->child[direction];
}
return p;
}
/* returns NULL if successful, existing cb_node if duplicate */
struct cb_node *cb_insert(struct cb_tree *t, struct cb_node *node, size_t klen)
{
size_t newbyte, newotherbits;
uint8_t c;
int newdirection;
struct cb_node **wherep, *p;
assert(!((uintptr_t)node & 1)); /* allocations must be aligned */
if (!t->root) { /* insert into empty tree */
t->root = node;
return NULL; /* success */
}
/* see if a node already exists */
p = cb_internal_best_match(t->root, node->k, klen);
/* find first differing byte */
for (newbyte = 0; newbyte < klen; newbyte++) {
if (p->k[newbyte] != node->k[newbyte])
goto different_byte_found;
}
return p; /* element exists, let user deal with it */
different_byte_found:
newotherbits = p->k[newbyte] ^ node->k[newbyte];
newotherbits |= newotherbits >> 1;
newotherbits |= newotherbits >> 2;
newotherbits |= newotherbits >> 4;
newotherbits = (newotherbits & ~(newotherbits >> 1)) ^ 255;
c = p->k[newbyte];
newdirection = (1 + (newotherbits | c)) >> 8;
node->byte = newbyte;
node->otherbits = newotherbits;
node->child[1 - newdirection] = node;
/* find a place to insert it */
wherep = &t->root;
for (;;) {
struct cb_node *q;
size_t direction;
p = *wherep;
if (!(1 & (uintptr_t)p))
break;
q = cb_node_of(p);
if (q->byte > newbyte)
break;
if (q->byte == newbyte && q->otherbits > newotherbits)
break;
c = q->byte < klen ? node->k[q->byte] : 0;
direction = (1 + (q->otherbits | c)) >> 8;
wherep = q->child + direction;
}
node->child[newdirection] = *wherep;
*wherep = (struct cb_node *)(1 + (uintptr_t)node);
return NULL; /* success */
}
struct cb_node *cb_lookup(struct cb_tree *t, const uint8_t *k, size_t klen)
{
struct cb_node *p = cb_internal_best_match(t->root, k, klen);
return p && !memcmp(p->k, k, klen) ? p : NULL;
}
struct cb_node *cb_unlink(struct cb_tree *t, const uint8_t *k, size_t klen)
{
struct cb_node **wherep = &t->root;
struct cb_node **whereq = NULL;
struct cb_node *q = NULL;
size_t direction = 0;
uint8_t c;
struct cb_node *p = t->root;
if (!p) return NULL; /* empty tree, nothing to delete */
/* traverse to find best match, keeping link to parent */
while (1 & (uintptr_t)p) {
whereq = wherep;
q = cb_node_of(p);
c = q->byte < klen ? k[q->byte] : 0;
direction = (1 + (q->otherbits | c)) >> 8;
wherep = q->child + direction;
p = *wherep;
}
if (memcmp(p->k, k, klen))
return NULL; /* no match, nothing unlinked */
/* found an exact match */
if (whereq) /* update parent */
*whereq = q->child[1 - direction];
else
t->root = NULL;
return p;
}
static enum cb_next cb_descend(struct cb_node *p, cb_iter fn, void *arg)
{
if (1 & (uintptr_t)p) {
struct cb_node *q = cb_node_of(p);
enum cb_next n = cb_descend(q->child[0], fn, arg);
return n == CB_BREAK ? n : cb_descend(q->child[1], fn, arg);
} else {
return fn(p, arg);
}
}
void cb_each(struct cb_tree *t, const uint8_t *kpfx, size_t klen,
cb_iter fn, void *arg)
{
struct cb_node *p = t->root;
struct cb_node *top = p;
size_t i = 0;
if (!p) return; /* empty tree */
/* Walk tree, maintaining top pointer */
while (1 & (uintptr_t)p) {
struct cb_node *q = cb_node_of(p);
uint8_t c = q->byte < klen ? kpfx[q->byte] : 0;
size_t direction = (1 + (q->otherbits | c)) >> 8;
p = q->child[direction];
if (q->byte < klen)
top = p;
}
for (i = 0; i < klen; i++) {
if (p->k[i] != kpfx[i])
return; /* "best" match failed */
}
cb_descend(top, fn, arg);
}