Re: Protect syscache from bloating with negative cache entries

On Wed, Dec 21, 2016 at 5:10 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> If I thought that "every ten minutes" was an ideal way to manage this,
> I might worry about that, but it doesn't really sound promising at all.
> Every so many queries would likely work better, or better yet make it
> self-adaptive depending on how much is in the local syscache.
>
> The bigger picture here though is that we used to have limits on syscache
> size, and we got rid of them (commit 8b9bc234a, see also
> https://www.postgresql.org/message-id/flat/5141.1150327541%40sss.pgh.pa.us)
> not only because of the problem you mentioned about performance falling
> off a cliff once the working-set size exceeded the arbitrary limit, but
> also because enforcing the limit added significant overhead --- and did so
> whether or not you got any benefit from it, ie even if the limit is never
> reached. Maybe the present patch avoids imposing a pile of overhead in
> situations where no pruning is needed, but it doesn't really look very
> promising from that angle in a quick once-over.

Have there been ever discussions about having catcache entries in a
shared memory area? This does not sound much performance-wise, I am
just wondering about the concept and I cannot find references to such
discussions.
--
Michael