Hi, On Mon, 21 Jul 2025 at 03:59, Thomas Munro <thomas.mu...@gmail.com> wrote: > > On Sun, Jul 20, 2025 at 1:07 AM Thomas Munro <thomas.mu...@gmail.com> wrote: > > On Sat, Jul 19, 2025 at 11:23 PM Tomas Vondra <to...@vondra.me> wrote: > > > The thing that however concerns me is that what I observed was not the > > > distance getting reset to 1, and then ramping up. Which should happen > > > pretty quickly, thanks to the doubling. In my experiments it *never* > > > ramped up again, it stayed at 1. I still don't quite understand why. > > > > Huh. Will look into that on Monday. > > I suspect that it might be working as designed, but suffering from a > bit of a weakness in the distance control algorithm, which I described > in another thread[1]. In short, the simple minded algorithm that > doubles on miss and subtracts one on hit can get stuck alternating > between 1 and 2 if you hit certain patterns. Bilal pinged me off-list > to say that he'd repro'd something like your test case and that's what > seemed to be happening, anyway? I will dig out my experimental > patches that tried different adjustments to escape from that state....
I used Tomas Vondra's test [1]. I tracked how many times StartReadBuffersImpl() functions return true (IO is needed) and false (IO is not needed, cache hit). It returns true ~%6 times on both simple and complex patches (~116000 times true, ~1900000 times false on both patches). A complex patch ramps up to ~250 distance at the start of the stream and %6 is enough to stay at distance. Actually, it is enough to ramp up more but it seems the max distance is about ~270 so it stays there. On the other hand, a simple patch doesn't ramp up at the start of the stream and %6 is not enough to ramp up. It is always like distance is 1 and IO needed, so multiplying the distance by 2 -> distance = 2 but then the next block is cached, so decreasing the distance by 1 and distance is 1 again. [1] https://www.postgresql.org/message-id/aa46af80-5219-47e6-a7d0-7628106965a6%40vondra.me -- Regards, Nazir Bilal Yavuz Microsoft