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This series introduces a new version of a loading_cache class.
The old implementation was susceptible to a "pollution" phenomena when frequently used entry can get evicted by an intensive burst of "used once" entries pushed into the cache.
The new version is going to have a privileged and unprivileged cache sections and there's a new loading_cache template parameter - SectionHitThreshold. The new cache algorithm goes as follows:
* We define 2 dynamic cache sections which total size should not exceed the maximum cache size.
* New cache entry is always added to the "unprivileged" section.
* After a cache entry is read more than SectionHitThreshold times it moves to the second cache section.
* Both sections' entries obey expiration and reload rules in the same way as before this patch.
* When cache entries need to be evicted due to a size restriction "unprivileged" section's
least recently used entries are evicted first.
More details may be found in #8674.
In addition, during a testing another issue was found in the authorized_prepared_statements_cache: #9590.
There is a patch that fixes it as well.
Closes #9708
* github.com:scylladb/scylla:
loading_cache: account unprivileged section evictions
loading_cache: implement a variation of least frequent recently used (LFRU) eviction policy
authorized_prepared_statements_cache: always "touch" a corresponding cache entry when accessed
loading_cache::timestamped::lru_entry: refactoring
loading_cache.hh: rearrange the code (no functional change)
loading_cache: use std::pmr::polymorphic_allocator