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.
regards, tom lane