Re: WAL format and API changes (9.5)

On 2014-10-03 15:51:37 +0300, Heikki Linnakangas wrote:
> After a lot of experimentation, I figured out that the slowdown is already
> apparent with the *first* patch, the one that just refactors existing
> XLogInsert, without any changes to the WAL format or to the callers. I
> fiddled with that for a long time, trying to tease apart the change that
> makes the difference, and was finally able to narrow it down.
>
> Attached are two patches. These are both just refactoring, with no changes
> to the WAL format or APIs. The first is the same as the refactoring patch I
> posted earlier, with only minor changes to #includes, comments and such (per
> Alvaro's and Michael's suggestions - thanks!). With the first patch, the
> test case I've been using to performance test this becomes somewhere between
> 5% - 10% slower. Applying the second patch on top of that restores the
> performance back to what you get without these patches.
>
> Strange. The second patch moves the CRC calculation from a separate loop
> over the XLogRecDatas to the earlier loop, that also iterates through the
> XLogRecDatas. The strange thing about this is that when I tried to make the
> same change to current git master, without applying the first patch, it
> didn't make any difference. The CRC calculation used to integrated to the
> earlier loop in 9.1 and before, but in 9.2 it was moved to a separate loop
> for simplicity, because it didn't make any difference to performance.
>
> So I now have a refactoring patch ready that I'd like to commit (the
> attached two patches together), but to be honest, I have no idea why the
> second patch is so essential to performance.

Maybe a stupid question, but did you verify it's not caused by splitting
xloginsert over two translation units and/or not inlining the separate
function?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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