Re: [v9.1] sepgsql - userspace access vector cache

On Wed, Jul 20, 2011 at 4:48 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Kohei Kaigai <Kohei(dot)Kaigai(at)EMEA(dot)NEC(dot)COM> writes:
>> I'd like to have a discussion about syscache towards next commit-fest.
>> The issues may be:
>>  - Initial bucket allocation on most cases never be referenced.
>>  - Reclaim cache entries on growing up too large.
>
> There used to be support for limiting the number of entries in a
> syscache.  It got removed (cf commit
> 8b9bc234ad43dfa788bde40ebf12e94f16556b7f) because (1) it was remarkably
> expensive to do it (extra list manipulations, etc), and (2) performance
> tended to fall off a cliff as soon as you had a few more tables or
> whatever than the caches would hold.  I'm disinclined to reverse that
> decision.  It appears to me that the security label stuff needs a
> different set of performance tradeoffs than the rest of the catalogs,
> which means it probably ought to do its own caching, rather than trying
> to talk us into pessimizing the other catalogs for seclabel's benefit.

I agree that we don't want to limit the size of the catcaches. We've
been careful to design them in such a way that they won't blow out
memory, and so far there's no evidence that they do. If it ain't
broke, don't fix it. Having catcaches that can grow in size as needed
sounds useful to me, though.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company