On Tue, 10 Nov 2020 at 12:49, 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.
I do happen to prefer having the separate Result Cache node (v8), so from my point of view, even if the performance was equal, I'd rather have v8. I understand that some others feel different though.
While I have interest about what caused the tiny difference, I admit that what direction
this patch should go is more important. Not sure if anyone is convinced that
v8 and v9 have a similar performance. The current data show it is similar. I want to
profile/read code more, but I don't know what part I should pay attention to. So I think
any hints on why v9 should be better at a noticeable level in theory should be very
helpful. After that, I'd like to read the code or profile more carefully.
