Pavel Emelyanov 5c0f9a8180 mutation_partition: Switch cache of rows onto B-tree ...

The switch is pretty straightforward, and consists of

- change less-compare into tri-compare

- rename insert/insert_check into insert_before_hint

- use tree::key_grabber in mutation_partition::apply_monotonically to
  exception-safely transfer a row from one tree to another

- explicitly erase the row from tree in rows_entry::on_evicted, there's
  a O(1) tree::iterator method for this

- rewrite rows_entry -> cache_entry transofrmation in the on_evicted to
  fit the B-tree API

- include the B-tree's external memory usage into stats

That's it. The number of keys per node was is set to 12 with linear search
and linear extention of 20 because

- experimenting with tree shows that numbers 8 through 10 keys with linear
  search show the best performance on stress tests for insert/find-s of
  keys that are memcmp-able arrays of bytes (which is an approximation of
  current clustring key compare). More keys work slower, but still better
  than any bigger value with any type of search up to 64 keys per node

- having 12 keys per nodes is the threshold at which the memory footprint
  for B-tree becomes smaller than for boost::intrusive::set for partitions
  with 32+ keys

- 20 keys for linear root eats the first-split peak and still performs
  well in linear search

As a result the footpring for B tree is bigger than the one for BST only for
trees filled with 21...32 keys by 0.1...0.7 bytes per key.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

2021-02-02 09:30:30 +03:00

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