"
Current storage of cells in a row is a union of vector and set. The
vector holds 5 cell_and_hash's inline, up to 32 ones in the external
storage and then it's switched to std::set. Once switched, the whole
union becomes the waste of space, as it's size is
sizeof(vector head) + 5 * sizeof(cell and hash) = 90+ bytes
and only 3 pointers from it are used (std::set header). Also the
overhead to keep cell_and_hash as a set entry is more then the size
of the structure itself.
Column ids are 32-bit integers that most likely come sequentialy.
For this kind of a search key a radix tree (with some care for
non-sequential cases) can be beneficial.
This set introduces a compact radix tree, that uses 7-bit sub values
from the search key to index on each node and compacts the nodes
themselves for better memory usage. Then the row::_storage is replaced
with the new tree.
The most notable result is the memory footprint decrease, for wide
rows down to 2x times. The performance of micro-benchmarks is a bit
lower for small rows and (!) higer for longer (8+ cells). The numbers
are in patch #12 (spoiler: they are better than for v2)
v3:
- trimmed size of radix down to 7 bits
- simplified the nodes layouts, now there are 2 of them (was 4)
- enhanced perf_mutation to test N-cells schema
- added AVX intra-nodes search for medium-sized nodes
- added .clone_from() method that helped to improve perf_mutation
- minor
- changed functions not to return values via refs-arguments
- fixed nested classes to properly use language constructors
- renamed index_to to key_t to distinguish from node_index_t
- improved recurring variadic templates not to use sentinel argument
- use standard concepts
v2:
- fixed potential mis-compilation due to strict-aliasing violation
- added oracle test (radix tree is compared with std::map)
- added radix to perf_collection
- cosmetic changes (concepts, comments, names)
A note on item 1 from v2 changelog. The nodes are no longer packed
perfectly, each has grown 3 bytes. But it turned out that when used
as cells container most of this growth drowned in lsa alignments.
next todo:
- aarch64 version of 16-keys node search
tests: unit(dev), unit(debug for radix*), pref(dev)
"
* 'br-radix-tree-for-cells-3' of https://github.com/xemul/scylla:
test/memory_footpring: Print radix tree node sizes
row: Remove old storages
row: Prepare row::equal for switch
row: Prepare row::difference for switch
row: Introduce radix tree storage type
row-equal: Re-declare the cells_equal lambda
test: Add tests for radix tree
utils: Compact radix tree
array-search: Add helpers to search for a byte in array
test/perf_collection: Add callback to check the speed of clone
test/perf_mutation: Add option to run with more than 1 columns
test/perf_mutation: Prepare to have several regular columns
test/perf_mutation: Use builder to build schema
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