Use the boost::intrusive containers in order to achieve a O(1) complexity
for both "LRU list" update and to minimize the memory overhead in the hash
table item to "LRU list" item connection:
- Make the timestamped_val be both a bi::list and a bi::unordered_set
item.
- Make a bi::unordered_set be a cache backend instead of the
std::unordered_map.
As a result dropping k LRU items becomes an O(k) operation instead of
O(N log N), where N is a total number of all cached items:
- Every time a value is read - move it to the front of the "LRU list"
(O(1)).
- When we need to remove k LRU items:
- Repeat k times:
- Take an element from the back of the "LRU list". (O(1)).
- Remove it from the bi::unordered_set and dispose. (O(1)).
We use an auto-unlink configuration for bi::list, therefore
disposing an item is going to auto unlink it from the list.
* 'permissions_cache_move_to_intrusive-v1' of github.com:scylladb/seastar-dev:
utils::loading_cache: cleanup
utils/loading_cache.hh: use intrusive list to store the lru entry
utils::loading_cache: implement automatic rehashing
utils::loading_cache: make the underlying map to be an intrusive unordered_set
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