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partition_snapshot_reader: Avoid quadratic behavior with lots of range tombstones

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next_range_tombstone() was populating _rt_stream on each invocation
from the current iterator ranges in _range_tombstones. If there is a
lot of range tombstones, all would be put into _rt_stream. One problem
is that this can cause a reactor stall. Fix by more incremental
approach where we populate _rt_stream with minimal amount on each
invocation of next_range_tombstone().

Another problem is that this can get quadratic. The iterators in
_range_tombstones are advanced, but if lsa invalidates them across
calls they can revert back to the front since they go back to
_last_rt, which is the last consumed range tombstone, and if the
buffer fills up, not all tombstones from _rt_stream could be
consumed. The new code doesn't have this problem because everything
which is produced out of the iterators in _range_tombstones is
produced only once. What we put into _rt_stream is consumed first
before we try to feed the _rt_stream with more data.
This commit is contained in:
Tomasz Grabiec
2021-07-26 13:33:51 +02:00
parent 0d7b3f9463
commit e74c3c885e
1 changed files with 6 additions and 1 deletions
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7
partition_snapshot_reader.hh
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@@ -245,11 +245,16 @@ class partition_snapshot_flat_reader : public flat_mutation_reader::impl, public
const std::optional<position_in_partition>& last_row,
const std::optional<position_in_partition>& last_rts,
position_in_partition_view pos) {
if (!_rt_stream.empty()) {
return _rt_stream.get_next(std::move(pos));
}
return in_alloc_section([&] () -> mutation_fragment_opt {
maybe_refresh_state(ck_range, last_row, last_rts);
position_in_partition::less_compare rt_less(_schema);
while (has_more_range_tombstones() && !rt_less(pos, peek_range_tombstone().position())) {
while (has_more_range_tombstones()
&& !rt_less(pos, peek_range_tombstone().position())
&& (_rt_stream.empty() || !rt_less(_rt_stream.peek_next().position(), peek_range_tombstone().position()))) {
range_tombstone rt = pop_range_tombstone();
if (rt.trim(_schema,
position_in_partition_view::for_range_start(ck_range),