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mdbx: clarify page-merge strategy.

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Change-Id: I1bb843ab6bdfcd53deae6d219a5b164bd3a79fd3
This commit is contained in:
Leonid Yuriev 2020-10-07 14:57:06 +03:00
parent f5ce471ebb
commit b274fdd142
1 changed files with 21 additions and 23 deletions
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44
src/core.c
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@ -569,13 +569,6 @@ page_fill(const MDBX_env *env, const MDBX_page *mp) {
return page_used(env, mp) * 100.0 / page_space(env);
}
MDBX_NOTHROW_PURE_FUNCTION static __inline bool
page_fill_enough(const MDBX_page *mp, unsigned spaceleft_threshold,
unsigned minkeys_threshold) {
return page_room(mp) < spaceleft_threshold &&
page_numkeys(mp) >= minkeys_threshold;
}
/* The number of overflow pages needed to store the given size. */
MDBX_NOTHROW_PURE_FUNCTION static __always_inline pgno_t
number_of_ovpages(const MDBX_env *env, size_t bytes) {
@ -14645,40 +14638,45 @@ static void mdbx_cursor_copy(const MDBX_cursor *csrc, MDBX_cursor *cdst) {
* [in] mc Cursor pointing to the page where rebalancing should begin.
* Returns 0 on success, non-zero on failure. */
static int mdbx_rebalance(MDBX_cursor *mc) {
int rc;
mdbx_cassert(mc, mc->mc_snum > 0);
mdbx_cassert(mc, mc->mc_snum < mc->mc_db->md_depth ||
IS_LEAF(mc->mc_pg[mc->mc_db->md_depth - 1]));
const int pagetype = PAGETYPE(mc->mc_pg[mc->mc_top]);
const unsigned minkeys = (P_BRANCH == 1) ? (pagetype & P_BRANCH) + 1
: (pagetype & P_BRANCH) ? 2 : 1;
STATIC_ASSERT(P_BRANCH == 1);
const unsigned minkeys = (pagetype & P_BRANCH) + 1;
/* The threshold of minimum page fill factor, in form of a negative binary
* exponent, i.e. 2 means 1/(2**3) == 1/4 == 25%. Pages emptier than this
* are candidates for merging. */
* exponent, i.e. X = 2 means 1/(2**X) == 1/(2**2) == 1/4 == 25%.
* Pages emptier than this are candidates for merging. */
const unsigned threshold_fill_exp2 = 2;
/* The threshold of minimum page fill factor, as a number of free bytes on a
* page. Pages emptier than this are candidates for merging. */
const unsigned spaceleft_threshold =
const unsigned room_threshold =
page_space(mc->mc_txn->mt_env) -
(page_space(mc->mc_txn->mt_env) >> threshold_fill_exp2);
const MDBX_page *const tp = mc->mc_pg[mc->mc_top];
mdbx_debug("rebalancing %s page %" PRIaPGNO " (has %u keys, %.1f%% full)",
(pagetype & P_LEAF) ? "leaf" : "branch",
mc->mc_pg[mc->mc_top]->mp_pgno,
page_numkeys(mc->mc_pg[mc->mc_top]),
page_fill(mc->mc_txn->mt_env, mc->mc_pg[mc->mc_top]));
(pagetype & P_LEAF) ? "leaf" : "branch", tp->mp_pgno,
page_numkeys(tp), page_fill(mc->mc_txn->mt_env, tp));
if (page_fill_enough(mc->mc_pg[mc->mc_top], spaceleft_threshold, minkeys)) {
mdbx_debug("no need to rebalance page %" PRIaPGNO ", above fill threshold",
mc->mc_pg[mc->mc_top]->mp_pgno);
if (unlikely(page_numkeys(tp) < minkeys)) {
mdbx_debug("page %" PRIaPGNO " must be merged due keys < %u threshold",
tp->mp_pgno, minkeys);
} else if (unlikely(page_room(tp) > room_threshold)) {
mdbx_debug("page %" PRIaPGNO " should be merged due room %u > %u threshold",
tp->mp_pgno, page_room(tp), room_threshold);
} else {
mdbx_debug("no need to rebalance page %" PRIaPGNO
", room %u < %u threshold",
tp->mp_pgno, page_room(tp), room_threshold);
mdbx_cassert(mc, mc->mc_db->md_entries > 0);
return MDBX_SUCCESS;
}
int rc;
if (mc->mc_snum < 2) {
MDBX_page *const mp = mc->mc_pg[0];
const unsigned nkeys = page_numkeys(mp);
@ -14800,7 +14798,7 @@ static int mdbx_rebalance(MDBX_cursor *mc) {
const indx_t ki_top = mc->mc_ki[mc->mc_top];
const indx_t ki_pre_top = mn.mc_ki[pre_top];
const indx_t nkeys = (indx_t)page_numkeys(mn.mc_pg[mn.mc_top]);
if (left && page_room(left) > spaceleft_threshold &&
if (left && page_room(left) > room_threshold &&
(!right || page_room(right) < page_room(left))) {
/* try merge with left */
mdbx_cassert(mc, page_numkeys(left) >= minkeys);
@ -14819,7 +14817,7 @@ static int mdbx_rebalance(MDBX_cursor *mc) {
return rc;
}
}
if (right && page_room(right) > spaceleft_threshold) {
if (right && page_room(right) > room_threshold) {
/* try merge with right */
mdbx_cassert(mc, page_numkeys(right) >= minkeys);
mn.mc_pg[mn.mc_top] = right;