Re: increasing the default WAL segment size

On Wed, Aug 24, 2016 at 11:52 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2016-08-24 23:26:51 -0400, Robert Haas wrote:
>> On Wed, Aug 24, 2016 at 10:54 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> > and I'm also rather doubtful that it's actually without overhead.
>>
>> Really? Where do you think the overhead would come from?
>
> ATM we do a math involving XLOG_BLCKSZ in a bunch of places (including
> doing a lot of %). Some of that happens with exclusive lwlocks held, and
> some even with a spinlock held IIRC. Making that variable won't be
> free. Whether it's actually measurabel - hard to say. I do remember
> Heikki fighting hard to simplify some parts of the critical code during
> xlog scalability stuff, and that that even involved moving minor amounts
> of math out of critical sections.

OK, that's helpful context.

>> What sort of test would you run to try to detect it?
>
> Xlog scalability tests (parallel copy, parallel inserts...), and
> decoding speed (pg_xlogdump --stats?)

Thanks; that's helpful, too.

--
Robert Haas
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