Re: Streaming read-ready sequential scan code

On Sun, May 19, 2024 at 7:00 AM Alexander Lakhin <exclusion(at)gmail(dot)com> wrote:
> With blocknums[1], timing is changed, but the effect is not persistent.
> 10 query15 executions in a row, b7b0f3f27:
> 277.932 ms
> 281.805 ms
> 278.335 ms
> 281.565 ms
> 284.167 ms
> 283.171 ms
> 281.165 ms
> 281.615 ms
> 285.394 ms
> 277.301 ms

The bad time 10/10.

> b7b0f3f27~1:
> 159.789 ms
> 165.407 ms
> 160.893 ms
> 159.343 ms
> 160.936 ms
> 161.577 ms
> 161.637 ms
> 163.421 ms
> 163.143 ms
> 167.109 ms

The good time 10/10.

> b7b0f3f27 + blocknums[1]:
> 164.133 ms
> 280.920 ms
> 160.748 ms
> 163.182 ms
> 161.709 ms
> 161.998 ms
> 161.239 ms
> 276.256 ms
> 161.601 ms
> 160.384 ms

The good time 8/10, the bad time 2/10.

Thanks for checking! I bet all branches can show that flip/flop
instability in these adverse conditions, depending on random
scheduling details. I will start a new thread with a patch for the
root cause of that, ie problem #2 (this will need back-patching), and
post a fix for #3 (v17 blocknums[N] tweak affecting
fairness/likelihood, which was probably basically a bit of ill-advised
premature optimisation) here in a few days.