RE: Protect syscache from bloating with negative cache entries

Hi

>> > I suggest you go with just syscache_prune_min_age, get that into PG
>> > 12, and we can then reevaluate what we need.  If you want to
>> > hard-code a minimum cache size where no pruning will happen, maybe
>> > based on the system catalogs or typical load, that is fine.
>>
>> Please forgive me if I say something silly (I might have got lost.)
>>
>> Are you suggesting to make the cache size limit system-defined and uncontrollable
>by the user?  I think it's necessary for the DBA to be able to control the cache memory
>amount.  Otherwise, if many concurrent connections access many partitions within a
>not-so-long duration, then the cache eviction can't catch up and ends up in OOM.
>How about the following questions I asked in my previous mail?
>
>cache_memory_target does the opposit of limiting memory usage. It keeps some
>amount of syscahe entries unpruned. It is intended for sessions on where
>cache-effective queries runs intermittently.
>syscache_prune_min_age also doesn't directly limit the size. It just eventually
>prevents infinite memory consumption.
>
>The knobs are not no-brainer at all and don't need tuning in most cases.
>
>> --------------------------------------------------
>> This is a pure question.  How can we answer these questions from users?
>>
>> * What value can I set to cache_memory_target when I can use 10 GB for the
>caches and max_connections = 100?
>> * How much RAM do I need to have for the caches when I set cache_memory_target
>= 1M?
>>
>> The user tends to estimate memory to avoid OOM.
>> --------------------------------------------------
>
>You don't have a direct control on syscache memory usage. When you find a queriy
>slowed by the default cache expiration, you can set cache_memory_taret to keep
>them for intermittent execution of a query, or you can increase
>syscache_prune_min_age to allow cache live for a longer time.
>


In current ver8 patch there is a stats view representing age class distribution.
https://www.postgresql.org/message-id/20181019.173457.68080786.horiguchi.kyotaro%40lab.ntt.co.jp
Does it help DBA with tuning cache_prune_age and/or cache_prune_target?
If the amount of cache entries of older age class is large, are people supposed to lower prune_age and 
not to change cache_prune_target?
(I get confusion a little bit.)

Regards,
Takeshi Ideriha