LCS backlog tracker uses STCS tracker for L0. Turns out LCS tracker
is calling STCS tracker's replace_sstables() with empty arguments
even when higher levels (> 0) *only* had sstables replaced.
This unnecessary call to STCS tracker will cause it to recompute
the L0 backlog, yielding the same value as before.
As LCS has a fragment size of 0.16G on higher levels, we may be
updating the tracker multiple times during incremental compaction,
which operates on SSTables on higher levels.
Inefficiency is fixed by only updating the STCS tracker if any
L0 sstable is being added or removed from the table.
This may be fixing a quadratic behavior during boot or refresh,
as new sstables are loaded one by one.
Higher levels have a substantial higher number of sstables,
therefore updating STCS tracker only when level 0 changes, reduces
significantly the number of times L0 backlog is recomputed.
Refs #12499.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Closes#12676
(cherry picked from commit 1b2140e416)
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
0c9a0faf0d