Re: Protect syscache from bloating with negative cache entries

On Tue, Feb 12, 2019 at 02:53:40AM +0100, Tomas Vondra wrote:
> Right. But the logic behind time-based approach is that evicting such
> entries should not cause any issues exactly because they are accessed
> infrequently. It might incur some latency when we need them for the
> first time after the eviction, but IMHO that's acceptable (although I
> see Andres did not like that).
>
> FWIW we might even evict entries after some time passes since inserting
> them into the cache - that's what memcached et al do, IIRC. The logic is
> that frequently accessed entries will get immediately loaded back (thus
> keeping cache hit ratio high). But there are reasons why the other dbs
> do that - like not having any cache invalidation (unlike us).

Agreed. If this fixes 90% of the issues people will have, and it
applies to the 99.9% of users who will never tune this, it is a clear
win. If we want to add something that requires tuning later, we can
consider it once the non-tuning solution is done.

> That being said, having a "minimal size" threshold before starting with
> the time-based eviction may be a good idea.

Agreed. I see the minimal size as a way to keep the systems tables in
cache, which we know we will need for the next query.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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