Pavel Emelyanov 4d2f5f93a4 memtable: Switch onto B+ rails ...

The change is the same as with row-cache -- use B+ with int64_t token
as key and array of memtable_entry-s inside it.

The changes are:

Similar to those for row_cache:

- compare() goes away, new collection uses ring_position_comparator

- insertion and removal happens with the help of double_decker, most
  of the places are about slightly changed semantics of it

- flags are added to memtable_entry, this makes its size larger than
  it could be, but still smaller than it was before

Memtable-specific:

- when the new entry is inserted into tree iterators _might_ get
  invalidated by double-decker inner array. This is easy to check
  when it happens, so the invalidation is avoided when possible

- the size_in_allocator_without_rows() is now not very precise. This
  is because after the patch memtable_entries are not allocated
  individually as they used to. They can be squashed together with
  those having token conflict and asking allocator for the occupied
  memory slot is not possible. As the closest (lower) estimate the
  size of enclosing B+ data node is used

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

2020-07-14 16:30:02 +03:00

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