Files
scylladb/test/unit/tree_test_key.hh
Pavel Emelyanov 2f7c03d84c utils: Intrusive B-tree (with tests)
The design of the tree goes from the row-cache needs, which are

1. Insert/Remove do not invalidate iterators
2. Elements are LSA-manageable
3. Low key overhead
4. External tri-comparator
5. As little actions on insert/remove as possible

With the above the design is

Two types of nodes -- inner and leaf. Both types keep pointer on parent nodes
and N pointers on keys (not keys themselves). Two differences: inner nodes have
array of pointers on kids, leaf nodes keep pointer on the tree (to update left-
and rightmost tree pointers on node move).

Nodes do not keep pointers/references on trees, thus we have O(1) move of any
object, but O(logN) to get the tree size. Fortunately, with big keys-per-node
value this won't result in too many steps.

In turn, the tree has 3 pointers -- root, left- and rightmost leaves. The latter
is for constant-time begin() and end().

Keys are managed by user with the help of embeddable member_hook instance,
which is 1 pointer in size.

The code was copied from the B+ tree one, then heavily reworked, the internal
algorythms turned out to differ quite significantly.

For the sake of mutation_partition::apply_monotonically(), which needs to move
an element from one tree into another, there's a key_grabber helping wrapper
that allows doing this move respecting the exception-safety requirement.

As measured by the perf_collections test the B-tree with 8 keys is faster, than
the std::set, but slower than the B+tree:

            vs set        vs b+tree
   fill:     +13%           -6%
   find:     +23%          -35%

Another neat thing is that 1-key insertion-removal is ~40% faster than
for BST (the same number of allocations, but the key object is smaller,
less pointers to set-up and less instructions to execute when linking
node with root).

v4:
- equip insertion methods with on_alloc_point() calls to catch
  potential exception guarantees violations eariler

- add unlink_leftmost_without_rebalance. The method is borrowed from
  boost intrusive set, and is added to kill two birds -- provide it,
  as it turns out to be popular, and use a bit faster step-by-step
  tree destruction than plain begin+erase loop

v3:
- introduce "inline" root node that is embedded into tree object and in
  which the 1st key is inserted. This greatly improves the 1-key-tree
  performance, which is pretty common case for rows cache

v2:
- introduce "linear" root leaf that grows on demand

  This improves the memory consumption for small trees. This linear node may
  and should over-grow the NodeSize parameter. This comes from the fact that
  there are two big per-key memory spikes on small trees -- 1-key root leaf
  and the first split, when the tree becomes 1-key root with two half-filled
  leaves. If the linear extention goes above NodeSize it can flatten even the
  2nd peak

- mitigate the keys indirection a bit

  Prefetching the keys while doing the intra-node linear scan and the nodes
  while descending the tree gives ~+5% of fill and find

- generalize stress tests for B and B+ trees

- cosmetic changes

TODO:

- fix few inefficincies in the core code (walks the sub-tree twice sometimes)
- try to optimize the leaf nodes, that are not lef-/righmost not to carry
  unused tree pointer on board

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
2021-02-02 09:30:29 +03:00

126 lines
3.5 KiB
C++

/*
* Copyright (C) 2020 ScyllaDB
*/
/*
* This file is part of Scylla.
*
* Scylla is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* Scylla is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with Scylla. If not, see <http://www.gnu.org/licenses/>.
*/
#pragma once
/*
* Helper class that helps to check that tree
* - works with keys without default contstuctor
* - moves the keys around properly
*/
class tree_test_key_base {
int _val;
int* _cookie;
int* _p_cookie;
public:
int cookie() const noexcept { return *_cookie; }
bool is_alive() const {
if (_val == -1) {
fmt::print("key value is reset\n");
return false;
}
if (_cookie == nullptr) {
fmt::print("key cookie is reset\n");
return false;
}
if (*_cookie != 0) {
fmt::print("key cookie value is corrupted {}\n", *_cookie);
return false;
}
return true;
}
bool less(const tree_test_key_base& o) const noexcept {
return _val < o._val;
}
int compare(const int o) const noexcept {
if (_val > o) {
return 1;
} else if (_val < o) {
return -1;
} else {
return 0;
}
}
int compare(const tree_test_key_base& o) const noexcept {
return compare(o._val);
}
explicit tree_test_key_base(int nr, int cookie = 0) : _val(nr) {
_cookie = new int(cookie);
_p_cookie = new int(1);
}
operator int() const noexcept { return _val; }
tree_test_key_base& operator=(const tree_test_key_base& other) = delete;
tree_test_key_base& operator=(tree_test_key_base&& other) = delete;
private:
/*
* Keep this private to make bptree.hh explicitly call the
* copy_key in the places where the key is copied
*/
tree_test_key_base(const tree_test_key_base& other) : _val(other._val) {
_cookie = new int(*other._cookie);
_p_cookie = new int(*other._p_cookie);
}
friend tree_test_key_base copy_key(const tree_test_key_base&);
public:
struct force_copy_tag {};
tree_test_key_base(const tree_test_key_base& other, force_copy_tag) : tree_test_key_base(other) {}
tree_test_key_base(tree_test_key_base&& other) noexcept : _val(other._val) {
other._val = -1;
_cookie = other._cookie;
other._cookie = nullptr;
_p_cookie = new int(*other._p_cookie);
}
~tree_test_key_base() {
if (_cookie != nullptr) {
delete _cookie;
}
assert(_p_cookie != nullptr);
delete _p_cookie;
}
};
tree_test_key_base copy_key(const tree_test_key_base& other) { return tree_test_key_base(other); }
struct test_key_compare {
bool operator()(const tree_test_key_base& a, const tree_test_key_base& b) const noexcept { return a.less(b); }
};
struct test_key_tri_compare {
int operator()(const tree_test_key_base& a, const tree_test_key_base& b) const noexcept { return a.compare(b); }
int operator()(const int a, const tree_test_key_base& b) const noexcept { return -b.compare(a); }
};