Re: Relation extension scalability

On 3/11/16 9:57 PM, Petr Jelinek wrote:
>> 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.
>>
>>
>> I agree, We can have some max limit on number of extra pages, What other
>> thinks ?
>>
>
> Well, that's what I meant with clamping originally. I don't know what is
> a good value though.

Well, 16MB is 2K pages, which is what you'd get if 100 connections were
all blocked and we're doing 20 pages per waiter. That seems like a
really extreme scenario, so maybe 4MB is a good compromise. That's
unlikely to be hit in most cases, unlikely to put a ton of stress on IO,
even with magnetic media (assuming the whole 4MB is queued to write in
one shot...). 4MB would still reduce the number of locks by 500x.
--
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