ZRE: Protect syscache from bloating with negative cache entries

From: Tomas Vondra [mailto:tomas.vondra@2ndquadrant.com]

0.7% may easily be just a noise, possibly due to differences in layout
of the binary. How many runs? What was the variability of the results
between runs? What hardware was this tested on?

3 runs, with the variability of about +-2%. Luckly, all those three runs (incidentally?) showed slight performance decrease with the patched version. The figures I wrote are the highest ones.

The hardware is, a very poor man's VM:
CPU: 4 core Intel(R) Xeon(R) CPU E7-4890 v2 @ 2.80GHz
RAM: 4GB

FWIW I doubt tests with such small small schema are proving anything -
the cache/lists are likely tiny. That's why I tested with much larger
number of relations.

Yeah, it requires many relations to test the repeated catcache eviction and new entry creation, which would show the need to increase catalog_cache_max_size for users. I tested with small number of relations to see the impact of additional processing while the catcache is not full -- memory accounting and catcache LRU chain maintenance.

Regards
Takayuki Tsunakawa