Re: Hybrid Hash/Nested Loop joins and caching results from subplans

On Mon, Nov 9, 2020 at 3:49 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> > Are you taking into account the possibility that generated machine code
> > is a small percent slower out of mere bad luck?  I remember someone
> > suggesting that they can make code 2% faster or so by inserting random
> > no-op instructions in the binary, or something like that.  So if the
> > difference between v8 and v9 is that small, then it might be due to this
> > kind of effect.
>
> Yeah.  I believe what this arises from is good or bad luck about relevant
> tight loops falling within or across cache lines, and that sort of thing.
> We've definitely seen performance changes up to a couple percent with
> no apparent change to the relevant code.

That was Andrew Gierth. And it was 5% IIRC.

In theory it should be possible to control for this using a tool like
stabilizer:

https://github.com/ccurtsinger/stabilizer

I am not aware of anybody having actually used the tool with Postgres,
though. It looks rather inconvenient.

-- 
Peter Geoghegan