Re: backend suddenly becomes slow, then remains slow

On Thursday, December 27, 2012, Andrew Dunstan wrote:

> On 12/26/2012 11:03 PM, Jeff Janes wrote:
>
>>
>> Do you have a huge number of tables? Maybe over the course of a
>> long-lived connection, it touches enough tables to bloat the relcache /
>> syscache. I don't know how the autovac would be involved in that, though.
>>
>>
>>
> Yes, we do indeed have a huge number of tables. This seems a plausible
> thesis.
>

All of the syscache things have compiled hard-coded numbers of buckets, at
most 2048, and once those are exceeded the resulting collision resolution
becomes essentially linear. It is not hard to exceed 2048 tables by a
substantial multiple, and even less hard to exceed 2048 columns (summed
over all tables).

I don't know why syscache doesn't use dynahash; whether it is older than
dynahash is and was never converted out of inertia, or if there are extra
features that don't fit the dynahash API. If the former, then converting
them to use dynahash should give automatic resizing for free. Maybe that
conversion should be a To Do item?

Cheers,

Jeff