On Mon, Nov 9, 2020 at 3:49 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > Alvaro Herrera <alvhe...@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
