Firstly, use access time to replace the decreas time of LFU.
For function LFUDecrAndReturn,
it should only try to get decremented counter,
not update LFU fields, we will update it in an explicit way.
And we will times halve the counter according to the times of
elapsed time than server.lfu_decay_time.
Everytime a key is accessed, we should update the LFU
including update access time, and increment the counter after
call function LFUDecrAndReturn.
If a key is overwritten, the LFU should be also updated.
Then we can use `OBJECT freq` command to get a key's frequence,
and LFUDecrAndReturn should be called in `OBJECT freq` command
in case of the key has not been accessed for a long time,
because we update the access time only when the key is read or
overwritten.
It looks safer to return C_OK from freeMemoryIfNeeded() when clients are
paused because returning C_ERR may prevent success of writes. It is
possible that there is no difference in practice since clients cannot
execute writes while clients are paused, but it looks more correct this
way, at least conceptually.
Related to PR #4028.
1. Refactor memory overhead computation into a function.
2. Every 10 keys evicted, check if memory usage already reached
the target value directly, since we otherwise don't count all
the memory reclaimed by the background thread right now.
It is possible to get better results by using the pool like in the LRU
case. Also from tests during the morning I believe the current
implementation has issues in the frequency decay function that should
decrease the counter at periodic intervals.
The LRU eviction code used to make local choices: for each DB visited it
selected the best key to evict. This was repeated for each DB. However
this means that there could be DBs with very frequently accessed keys
that are targeted by the LRU algorithm while there were other DBs with
many better candidates to expire.
This commit attempts to fix this problem for the LRU policy. However the
TTL policy is still not fixed by this commit. The TTL policy will be
fixed in a successive commit.
This is an initial (partial because of TTL policy) fix for issue #2647.
To destroy and recreate the pool[].key element is slow, so we allocate
in pool[].cached SDS strings that can account up to 255 chars keys and
try to reuse them. This provides a solid 20% performance improvement
in real world workload alike benchmarks.
We start from the end of the pool to the initial item, zero-ing
every entry we use or every ghost entry, there is nothing to memmove
since to the right everything should be already set to NULL.
