Re: Relation extension scalability

On 3/11/16 5:14 PM, Petr Jelinek wrote:
>> I don't really understand this part about concurrent DDL.  If there
>> were concurrent DDL going on, presumably other backends would be
>> blocked on the relation lock, not the relation extension lock - and it
>> doesn't seem likely that you'd often have a huge pile-up of inserters
>> waiting on concurrent DDL.  But I guess it could happen.
>>
>
> Yeah I was thinking about the latter part and as I said it's very rare
> case, but I did see something similar couple of times in the wild. It's
> not objection against committing this patch though, in fact I think it
> can be committed as is.

FWIW, this is definitely a real possibility in any shop that has very 
high downtime costs and high transaction rates.

I also think some kind of clamp is a good idea. It's not that uncommon 
to run max_connections significantly higher than 100, so the extension 
could be way larger than 16MB. In those cases this patch could actually 
make things far worse as everyone backs up waiting on the OS to extend 
many MB when all you actually needed were a couple dozen more pages.

BTW, how was *20 arrived at? ISTM that if you have a lot of concurrent 
demand for extension that means you're running lots of small DML 
operations, not really big ones. I'd think that would make *1 more 
appropriate.
-- 
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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