"
After adcb3ec20c ("row_cache: read is not
single-partition if inter-partition forwarding is enabled") we have
noticed a regression in the results of some perf_fast_forward tests.
This was caused by those tests not disabling partition-level
fast-forwarding even though it was not needed and the commit in question
fixed an incorrect optimisation in such cases.
However, after solving that issue it has also become apparent that
mutation_reader_merger performs worse when the fast-forwarding is
disabled. This was attributed to logic responsible for dropping readers
as soon as they have reached the end of stream (which cannot be done if
fast-forwarding is enabled). This problem was mitigated with avoiding a
scan of the list and removing readers in small batches.
Fixes#4246.
Fixes#4254.
Tests: unit(dev)
"
* tag 'perf_fast_forward-fix-regression/v1' of https://github.com/pdziepak/scylla:
mutation_reader_merger: drop unneded readers in small batches
mutation_reader_merger: track readers by iterators and not pointers
tests/perf_fast_forward: disable partition-level fast-forwarding if not needed
a0b0db7915