Re: Protect syscache from bloating with negative cache entries

On 2019-Feb-05, Tomas Vondra wrote:

> I don't think we need to remove the expired entries right away, if there
> are only very few of them. The cleanup requires walking the hash table,
> which means significant fixed cost. So if there are only few expired
> entries (say, less than 25% of the cache), we can just leave them around
> and clean them if we happen to stumble on them (although that may not be
> possible with dynahash, which has no concept of expiration) of before
> enlarging the hash table.

I think seqscanning the hash table is going to be too slow; Ideriha-san
idea of having a dlist with the entries in LRU order (where each entry
is moved to head of list when it is touched) seemed good: it allows you
to evict older ones when the time comes, without having to scan the rest
of the entries. Having a dlist means two more pointers on each cache
entry AFAIR, so it's not a huge amount of memory.

> So if we want to address this case too (and we probably want), we may
> need to discard the old cache memory context someho (e.g. rebuild the
> cache in a new one, and copy the non-expired entries). Which is a nice
> opportunity to do the "full" cleanup, of course.

Yeah, we probably don't want to do this super frequently though.

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Álvaro Herrera https://www.2ndQuadrant.com/
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