Avi Kivity e7fb71020b Merge 'replica: Optimize empty_flat_reader out of the hot path' from Michał Chojnowski ...

When row_cache::make_reader() and memtable::make_flat_reader() see that the query result is empty, they return empty_flat_reader, which is a trivial implementation of flat_mutation_reader.
Even though empty_flat_reader doesn't do anything meaningful, it still needs to be created, handled in merging_reader and destroyed. Turns out this is costly.

This patch series replaces hot path uses of empty_flat_reader with an empty optional.

Performance effects:

`perf_simple_query --smp 1`
TPS: 138k -> 168k
allocs/op: 80.2 -> 71.1
insns/op: 49.9k -> 45.1k

`perf_simple_query --smp 1 --enable-cache=1 --flush`
TPS: 125k -> 150k
allocs/op: 79.2 -> 71.1
insns/op: 51.7k -> 47.2k

For a cassandra-stress benchmark (localhost, 100% cache reads) this translates to a TPS increase from ~42k to ~48k per hyperthread.

Note that this optimization is effective for single-partition reads where the queried partition is only in cache/sstables or only in memtables. Other queries (e.g. where the partition is in both cache in memtables and needs to be merged) are unaffected.

Closes #10204

* github.com:scylladb/scylla:
  replica: Prefer row_cache::make_reader_opt() to row_cache::make_reader()
  row_cache: Add row_cache::make_reader_opt()
  replica: Prefer memtable::make_flat_reader_opt() to memtable::make_flat_reader()
  memtable: Add memtable::make_flat_reader_opt()

[avi: adjust #include for readers/ split]

2022-03-14 14:07:00 +02:00

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